Chapter I: The Walls. Stone and Stone-Work

The material mainly employed in the construction of buildings depends partly on the purpose for which the buildings are intended, and still more, perhaps, on the prevailing geological character of the surrounding country. In such a place as London, where there is an immense mass of tenacious clay beneath the vegetable soil, and where solid stone is not to be had, except by bringing it, at a great expense, from a distance of many miles, clay seems to be the natural material for dwellings; and thus we find that almost all the London houses are built of brick formed of clay. In other parts of Great Britain, such as Glasgow or Edinburgh, the case is very different; for, in those places, clay is scarce, and stone is plentiful. There are quarries not far from Edinburgh, and others within the very precincts of Glasgow, where an abundant supply of good building-stone is obtained at a very low rate. Hence it follows as a natural consequence, that the houses in those two cities exhibit a large proportion of stone structures; so much so, indeed, that an inhabitant of London, who is accustomed to see stone appropriated only to large important public buildings, is apt to imagine that the houses in the two northern cities must necessarily be very costly. This is by no means certain, however, for the builders in each city make use of those materials which may be most available.

Whether stone form the main portion of the walls of a house, as in the cases just named, or whether it is only used in smaller degree, as in London houses, the operations by which it is worked and fitted are pretty much the same; and we will therefore devote this chapter to a brief description of the principal kinds of building-stone, followed by an outline of the _Mason’s_ operations.

Principal Varieties of Building-stone.

_Granites_ are rocks which have been formed by the union of three different minerals in a state of fusion; these, on cooling, have crystallized and become distinct from each other in the mass. It is on the varied proportions in which these three constituents are combined, that the colour, hardness, durability, and beauty of the various granites depend. The light-red and rose-coloured granites contain the felspar in greatest abundance and in the largest crystals; but this mineral varies in hue from the purest white to nearly black; it is the ingredient most acted on by the atmosphere; the rock, therefore, which abounds in it, though it may be more beautiful to the eye, and more easily worked at first, is not so durable as that which contains it in smaller crystals, and with a larger proportion of _quartz_. It is to this last-named mineral that granite owes the sparkling appearance which it presents when the sun shines on it; quartz is the hardest and most imperishable of the three minerals which form the granite-rock. The third, _mica_, is distinguishable from the other two by its satiny, shining, dark hue, and is very apparent in the coarse-grained, handsome stone of our own country, brought from Cornwall.

When the felspar is replaced by another mineral called _hornblende_, the stone is of a dark-greenish hue, and the component parts are in a finer form and less distinguishable from each other. The Aberdeen granite is an example of this kind, which is more durable than the former, though not so pleasing to the eye.

Granite occurs in all the larger mountain-ranges, and in isolated masses in every country; not being a stratified rock, and being excessively hard, it is difficult to quarry and get out in manageable masses. Blasting with gunpowder is the mode usually employed in this country; the pieces detached by this means are hewn roughly into form on the spot by a small pickaxe. Aberdeen granite is quarried by cutting a deep line some yards long, and placing strong iron wedges at equal distances in this line; these wedges are struck in succession by heavy hammers till the mass splits down. This, or analogous modes, may always be employed when the rock approaches a slaty or stratified structure, as is the case with some nearly related to granite. Another method of detaching masses of rock, is by driving wooden wedges into a deep fissure, either natural or artificial; the wedges are then wetted, and the consequent expansion of the wood bursts the rock asunder.

As granite has always to be brought from a great distance to the spot where it is wanted, because its natural localities are far from the places where edifices are usually constructed, and also on account of its hardness, this rock is only used for important buildings, such as bridges, markets, churches, &c., and not commonly even for these. London and Waterloo bridges, Covent Garden and Hungerford markets, and the York column in Pall Mall, are instances of its use in London.

The principal kinds of stone used in building are the LIMESTONES, or _calcareous rocks_ of the geologist; of these it would be useless to describe or enumerate more than a few. In our own country, the _Portland stone_, so called from its principal quarries being in Portland Island, in Dorsetshire, holds the first rank, and is that almost exclusively used in London for building, and for the ornamental parts of edifices. It unites the qualities of being easily sawn and worked, when lately quarried, and of subsequently hardening by exposure to the air; it is close and even in its texture, admitting of being wrought into delicate work, and receiving a very smooth surface, which it will retain for a considerable period, though it is surpassed in durability by many other rocks. It is said that the Banquetting-house, Whitehall, was the first building in London in which this stone was employed. St. Paul’s, Westminster and Blackfriars’ bridges, Newgate, and, indeed, most of the public buildings of the metropolis, are examples of its use.

_Bath-stone_, so called from its being entirely used in the neighbourhood of that city, is softer and far less durable than the preceding. When recently quarried, it may be sawn with a toothed saw, like timber, and can be carved with the greatest facility into the richest ornaments; hence it is often employed, and, if sheltered from the weather, is well adapted for such purposes, from its rich, even cream colour; but though it hardens considerably by exposure, it is acted upon, after a time, by the air, so as to render it very perishable. The restoration of Henry the Seventh’s Chapel, Westminster, is, unfortunately, made with this stone.

The two preceding, and many others, distinguished by names according to the principal localities, as _Oxford_-stone, _Ketton_-stone, &c., belong to what geologists term the _Oolitic_ formation, from the resemblance of some kinds of the rock to fishes’ roe, which is observable in that we have last mentioned. They all agree in their principal qualities.

_Purbeck-stone_, also from Dorsetshire, is used for steps, paving, door-sills, and copings; it is coarser, harder, and less uniform in texture than the foregoing, and not, therefore, calculated for fine buildings, except for the purposes we have specified.

_Yorkshire-stone_ resembles the last; it is used for the same purposes, but especially for paving. The greatest part of the foot-paths in the streets of London are laid with this or the preceding.

_Rag-stone_ is obtained from quarries on the banks of the Thames, Medway, &c. It was the stone chiefly used for building in ancient London, and a great deal is still used for paving.

The lower _chalk_, which is of a grey colour, and contains masses of flint, was formerly much employed for building in the south-western counties of England; its good qualities are proved by the perfect state of many old churches in that part of the kingdom, which are known to be from seven to nine hundred years old. It is now only sparingly used in farm-building and cottages, but it is consumed in vast quantities to burn into lime for mortar and other purposes, and as a manure.

Belonging to the same family of calcareous rocks, and next in utility to those we have just enumerated, though far surpassing them in beauty and value, stand the endless varieties of _Marbles_, essentially characterized by their crystalline texture, superior hardness, and by the absence of shells or organic remains found so abundantly in all other limestones. The name of marble is, however, popularly given to many stones not possessing these characters, but which are hard enough to be susceptible of a high polish, and are ornamental when so treated. In this country the finer kinds of real marble are only sparingly employed in the decorative departments of architecture, such as, for chimney-pieces, slabs, hearths, capitals of columns in halls, saloons, monuments, &c. The secondary kinds are also employed for similar purposes, but more abundantly. The cold white statuary marble is not adapted for out-of-door use in our foggy and cloudy climate, under the influence of which it would soon become dingy and disagreeable, as is proved by the total failure in the effect of the little triumphal arch erected before Buckingham Palace. In Italy many ancient and modern edifices are faced with white marble, and in that clear and pure atmosphere they retain the beauty of the material for ages. The use to which the finest marbles of Greece and Italy are applied in sculpture, is familiar to every one.

The last class of rocks employed in building, in those localities where they occur, are the _Sandstones_, silex, or flint, in finely-comminuted particles agglutinated together, being their principal ingredient; they constitute excellent building-stone, and are abundantly used as such in the West of England.

On Quarrying Stone.

A quarry is an excavation made in the ground, or among rocks, for the purpose of extracting stone for building, or for sculpture. The name appears to have originated in the circumstance that the stones, before they are removed to a distance, are first _quadrated_, or formed into rectangular blocks.

The following may be taken as an example of the general operations of quarrying building-stones. If the stone be vertically below the surface of the ground, the quarrymen first remove the earth and surface soil, and then dig a perpendicular shaft, or pit, to afford access to the stone; but if, as frequently happens, the stone be within the flank of a hill, or mountain, the quarrymen excavate horizontal galleries into the hill, leaving pillars here and there to support the superincumbent mass. Supposing a large quarry about to be opened beneath the soil, the earth is first removed, and then a sort of inferior stone called “rag,” which generally lies between the soil and the good stone beneath. Large masses of available stone generally consist of distinct strata lying close together in a kind of cemented bulk, and the contiguous surfaces forming _cleavages_, greatly assist the quarrymen in detaching blocks from the mass. The block is always more easily separated in a direction parallel to these planes of cleavage than in any other direction, and the operations are, therefore, guided by this circumstance. The workmen drive a series of iron wedges into the mass of stone parallel to the cleavage-planes; and, after a few blows, a portion of the mass becomes separated in that direction. They then measure off a portion equal to the intended length and breadth of the stone, and drive their iron wedges similarly in these directions, by which the piece is entirely severed from the rocky mass. The cleavage-planes vary interminably in direction, so that the quarrymen have to work in various positions, according to the direction of stratification. The operations are more easily conducted when the cleavage-planes are vertical, than in any other direction. After the blocks have been severed, they are brought to an irregularly square shape, by means of a tool called a _kevel_; and are finally hoisted by cranes on to low trucks, and conveyed on tram-ways out of the quarry; or else are hoisted to the surface of the quarry at once, if the depth render that plan necessary.

In quarrying sandstone, and those rocks which consist of regular layers, the pick, the wedge, the hammer, and the pinch, or lever, are the chief tools. But for many kinds of limestone, and for greenstone and basalt, recourse is had to the more violent and irregular effects of gunpowder. Indeed, some of the primitive rocks, such as granite, gneiss, and sienite, could scarcely be torn asunder by any other means. The great objection to blasting by gunpowder is, that the blocks are broken irregularly, and much of the stone is wasted; but it has the advantage of being simple in its application, and powerful in its effects. The grains of powder are suddenly converted into a permanently elastic air, occupying about four hundred and seventy-two times more space than their own bulk. The elastic fluid expands with a velocity calculated at the rate of about ten thousand feet per second; and its pressure or force, when thus expanding, has been estimated as equal to one thousand atmospheres, that is, one thousand times greater than the atmospheric pressure upon a base of the same extent. By applying this product to a square inch, upon which the atmosphere exerts a pressure of about fifteen pounds, the elastic fluid of the gunpowder will be found, at the moment of the explosion, to exert a force equivalent to six tons and a half upon the square inch of surface exposed to it; and that with a velocity which the imagination can hardly follow.

In boring a rock preparatory to blasting, it is necessary to consider the nature of the stone, and the inclination or dip of the strata, in order to decide upon the diameter, the depth, and direction of the hole for the gunpowder. The diameter of the hole may vary according to the nature of the rock, from half an inch to two and a half inches; and the depth from a few inches to as many feet; the direction may vary to all the angles from the perpendicular to the horizontal. The tools used in this operation are very simple. The chisel, or _jumper_, as it is called, varies in size according to the work to be performed, and its edge is more or less pointed to suit the hardness of the rock to be bored. If the hole is to be small and not deep, it may be bored by a single person; with one hand he manages the chisel, which he turns at every blow so as to cross the previous cut, and with the other hand he strikes it with a hammer of six or eight pounds’ weight, occasionally clearing out the hole by means of a _scraper_. But when the hole is large and deep, one man in a sitting posture directs the jumper, pours water into the hole, and occasionally cleans it out, while two or three men, with hammers of ten or twelve pounds’ weight, strike successive blows upon the jumper, until the rock is perforated to the required depth. To prevent annoyance to the workmen, a small rope of straw or hemp is twisted round the jumper, and made to rest in the orifice of the hole. When the holes are to be made to a greater depth than about thirty inches, it is common to use a chisel from six to eight feet in length, pointed at both ends, having a bulbous part in the middle for the convenience of holding it; it thus becomes a kind of double jumper, and is used without a hammer, with either end put into the hole at pleasure. The workmen holding this jumper by the bulbous part, lift it, and allow it to drop into the hole by its own weight, and by this simple operation, a hole to the depth of five feet and upwards is perforated with ease and expedition. When the boring is completed, the fragments are carefully removed, and the hole is made as dry as possible, which is done by filling it partially with stiff clay, and then driving into it a tapering iron rod, called the _claying bar_, which nearly fills it. This, being forced in with great violence, drives the clay into all the crevices of the rock, and secures the dryness of the hole. Should this plan fail, tin cartridges are used: these are furnished with a stem or tube, as shown in the following figure, through which the powder may be ignited. When the hole is dry, the powder is introduced, mixed sometimes with quicklime, which, it is said, increases the force of the explosion. A long iron or copper rod, called the _pricker_, is then inserted amongst the powder, and is afterwards withdrawn, when the priming powder is introduced. The hole is filled up with burnt clay, pounded brick, stone, or any other substance not likely to produce a spark during the ramming. This is called the _tamping_. In filling up the hole, the chief danger is the production of a spark among the materials, a circumstance which has occasioned the most fatal and distressing accidents to quarriers. Prickers and rammers of copper, or of bronze, have been employed, but their greater expense, and liability to twist and break, have prevented their general introduction.

The quarrier is, of course, accustomed to suppose that the more firmly he rams in the powder, the greater will be the resulting effect. It is, however, a curious property of sand, that it fills up all the void spaces in the tube or hole, and for some rocks entirely supersedes the necessity of ramming and pricking.

When the hole is fully charged with the powder and wadding, the pricker is withdrawn, and the small tubular space, or vent-hole, which it leaves, is sometimes filled up with powder; but, for the sake of economy, it is more common to insert straws filled with powder, and joined together, so as to reach the required depth. The lower straw is one terminating in the root part, where a natural obstruction occurs, or it is artificially stopped with clay to prevent the powder from being lost. The lower part of the priming straw is pared quite thin, so as to insure the inflammation of the charge of powder in the hole. Sometimes the fire is conveyed by means of the large and long green rushes, which grow in marshy ground. A slit is made in one side of the rush, along which the sharp end of a bit of stick is drawn, so as to extract the pith, when the skin of the rush closes again by its own elasticity. This tube is filled up with gunpowder; it is then dropped into the vent-hole, and made steady with a bit of clay. This being done, a slow match, called a _smift_, consisting generally of a bit of soft paper, prepared by dipping it into a solution of saltpetre, is carefully applied to the priming powder. When this match is about to be fired, the quarriers usually blow a horn or ring a bell, to give notice to all around them to retire. The explosion commonly takes place in about a minute; the priming first explodes, attended only with flame; a short interval of suspense commonly ensues; the eyes of the bystanders being anxiously directed towards the spot; the rock is instantly seen to open, when a sharp report or detonating noise takes place, and numerous fragments of stone are observed to spring into the air, and fly about in all directions, from amidst a cloud of smoke. The quarrier then returns with alacrity to the scene of his operations.

The accompanying figure shows the plan of blasting the rock, and a section of the hole ready prepared for firing. The portion of the rock to be dislodged by the explosion is that included between A and B. The charge of powder is represented as filling the bore to C, from which point to the top, the hole is filled up with _tamping_. The _smift_ is represented at D.

In the year 1831, a patent was taken out by Mr. Bickford, of Tucking Mill, Cornwall, for an invention called “the Miner’s Safety Fuse.” It consists essentially of a minute cylinder of gunpowder, or other suitable explosive mixture, inclosed within a hempen cord, which is first twisted in a peculiar kind of machine, then overlaid to strengthen it; afterwards it is varnished with a mixture of tar and resin to preserve the powder from moisture, and finally is coated with whitening to prevent the varnish from sticking to the fingers, or the fuses to one another. These fuses are said to have been used with good effect, and to have greatly diminished the number of accidents.

The application of Electricity to the Blasting of Rocks.

Perhaps the greatest modern improvement that has been made in blasting rocks has been by the introduction of the galvanic battery. It is well known that by closing the circuit of a voltaic current by means of thin platinum wire, or by fine iron or steel wire, the platinum becomes red-hot, and the iron or steel becomes instantly fused. All, therefore, that is necessary is to connect the two terminal wires of a voltaic battery by means of a fine wire of platinum or iron, and to bury this in gunpowder contained in a tin canister, or a fuse connected with a deposit of gunpowder. This was the method adopted by Colonel Pasley in removing the Royal George, which lay sunk at the bottom of the water at Spithead. Canisters of gunpowder, sometimes to the extent of three thousand pounds’ weight, were employed, and securely deposited in the sunken vessel, by workmen who descended in the diving-bell; the terminal wires of the battery, connected as above stated, having been previously inserted in the canisters, and these wires being extended to a great distance, the explosion took place the instant they were connected with the voltaic battery. After the vessel was thus blown to pieces by repeated explosions, divers descended to clear away the wreck, and to attach guns, &c., to chains let down from a ship above, and which were then hauled up by means of a crane.

Mr. Morgan, in the _American Journal of Science_, describes a fuse or cartridge which he has used with success in connexion with the voltaic battery. This cartridge is prepared by joining two pieces of clean copper wire to the ends of a fine steel wire, about one quarter of an inch in length, by means of waxed silk; a thin piece of wood is then spliced to both copper wires, to protect the steel wire from accidents, and to enable the maker to introduce it easily into a quill or small paper tube, which is to form the cartridge. This tube is filled with fine gunpowder, and made air and water-tight. Another piece of wood is then attached to this arrangement, and one of the copper wires is bent over so as to form an angle with the straight wire.

When it is required to use this cartridge, the copper wires are rubbed with sand-paper, and twisted round the wires of the voltaic battery. The cartridge is then placed deep in the hole made to receive the gunpowder, and the charge is fired from any distance.

Mr. Morgan found this arrangement very useful in removing stumps of trees; but one of his applications of it was curious and novel: he exploded some powder in a pond at the depth of ten feet, with the battery at the distance of two hundred and ten feet; the explosion, which was instantaneous, had the effect of killing a large eel; and “I have no doubt,” says Mr. Morgan, “that wild-fowl will yet be killed by means of shells placed at low-water on the banks where they feed; and by means of long connecting wires, the shells can be made to explode simultaneously among the birds.”

But the grandest application of gunpowder and the voltaic battery to the blasting of rocks, was made in the month of January, 1843, at Dover. It was determined by these means to attempt the removal of an enormous mass of the cliff facing the sea, which formed an obstruction to the line of railroad. A portion of the cliff which was penetrated by the tunnel made through Shakspeare’s Cliff gave way, about two years previously. About fifty yards of the tunnel were carried away, and a clear space was thus formed for the line of railroad, with the exception of a projecting point, which, prior to the slip alluded to, was the extremity of the part of the cliff pierced by the tunnel, and to remove which was the object of the operation in question.

To clear away this mass by the tedious process of manual labour, would have cost above twelve thousand pounds; and this consideration, as well as the time that would have been lost, induced Mr. Cubitt, the engineer, to try the bold expedient of blowing it away with gunpowder. “It cannot be denied,” remarks Captain Stuart, whose account of this great engineering operation we follow, “that there was apparent danger in the undertaking, for the weight of the mass to be removed was estimated at two million tons, and the quantity of powder used was more than eight tons, or eighteen thousand pounds. The quantity used in blowing up the fortifications of Bhurtpore was twelve thousand pounds, and this is said to have been the greatest explosion that had ever previously taken place for any single specific object.”

The front of the projection was about one hundred yards wide; this front was pierced with a tunnel about six feet in height, and three in breadth; three shafts, equidistant from each other, and from the entrances to the tunnel, were sunk to the depth of seventeen feet, and galleries were run, one from each shaft, parallel with each other, and at right angles with the line of the tunnel. These galleries varied in length, the longest having been twenty-six feet, and the shortest twelve feet, and at their extremities chambers were excavated in a direction parallel with the tunnel. This description will be the better understood by reference to the following figure. 1. The tunnel. 2. The shafts. 3. The galleries. 4. The chambers.

In the chambers, the powder was deposited in three nearly equal quantities; it was done up in fifty-pound bags, and the proportion in each chamber was contained in a wooden case, nearly as large as the chamber itself. Ignition was communicated by means of a voltaic battery; the conducting wires, one thousand feet in length, were passed over the cliff, one to each chamber, and the electricity was communicated in a shed built for the purpose on the top of the cliff, about fifty yards from the edge. The explosion was conducted by Lieutenant Hutchinson, R.E., who was engaged with General Pasley in blowing up the wreck of the Royal George. The time appointed for the explosion to take place, was two o’clock P.M., 26th January, 1843, the tide being then at its lowest ebb. The arrangements, to preserve order and prevent danger, were good. A space was kept clear by a cordon of artillery, and the following programme was issued:--

“Signals, January 26, 1843.

“1st. Fifteen minutes before firing, all the signal flags will be hoisted.

“2nd. Five minutes before firing, one gun will be fired, and all the flags will be hauled down.

“3rd. One minute before firing, two guns will be fired, and all the flags (except that on the point which is to be blasted) will be hoisted up again.”

These signals were given exactly at the specified times, and when the expected moment arrived, a deep subterranean sound was heard, a violent commotion was seen at the base of the cliff, and the whole mass slid majestically down, forming an immense _débris_ at the bottom. Tremendous cheers followed the blast, and a royal salute was fired.

The remarks of different intelligent observers, as to the effects of this explosion, would of course differ according to their position with respect to the scene of explosion. One observer states that “the earth trembled to the distance of half a mile; a stifled report, not loud, but deep, was heard; the _base_ of the cliff, extending on either hand to upwards of five hundred feet, was shot as from a cannon, from under the superincumbent mass of chalk seaward; and in a few seconds not less than a million tons of chalk were dislodged by the shock, and settled gently down into the sea below.”

But the most eminent observer who has described the effects of this explosion is Sir John Herschel, from whose letter to the _Athenæum_ we gather the following particulars. His position was on the summit of the cliff, next adjoining the scene of operations, to the southward, the nearest point to which access was permitted.

Sir John Herschel was particularly struck with “the singular and almost total absence of all those tumultuous and noisy manifestations of power, which might naturally be expected to accompany the explosion of so enormous a quantity (19,000lbs.) of gunpowder.” He describes the noise which accompanied the immediate explosion as “a low murmur, lasting hardly more than half a second, and so faint, that had a companion at my elbow been speaking in an ordinary tone of voice, I doubt not it would have passed unheeded.”

The fall of the cliff, the ruins of which extended over no less than eighteen acres of the beach, to an average depth of fourteen feet, was not accompanied with any considerable noise. “The entire absence of smoke was another and not less remarkable feature of the phenomenon. Much dust, indeed, curled out at the borders of the vast rolling and undulating mass, which spread itself like a semi-fluid body, thinning out in its progress; but this subsided instantly; and of true smoke there was absolutely not a vestige. Every part of the surface was immediately and clearly seen--the prostrate flagstaff (speedily re-erected in the place of its fall)--the broken turf, which a few seconds before had been quietly growing at the summit of the cliff--and every other detail of that extensive field of ruin, were seen immediately in all their distinctness. Full in the midst of what appeared the highest part of the expanding mass, while yet in rapid motion, my attention was attracted by a tumultuous and somewhat upward-swelling motion of the earth, whence I fully expected to see burst forth a volume of pitchy smoke, and from which my present impression is, that gas, purified from carbonaceous matter in passing through innumerable fissures of cold and damp material, was still in progress of escape; but whether so or not, the remark made at the moment is sufficient to prove the absence of any impediment to distinct vision.”

The amount of tremor experienced by Sir John Herschel at the point where he was standing was so slight, that he thinks he has felt it surpassed by a heavy waggon passing along a paved street. “The impression, slight as it was, was single and brief, and must have originated with the first shock of the powder, and not from the subsequent and prolonged rush of the ruins.” We have already noticed the remark of one observer, that “the earth trembled to the distance of half a mile;” but this seems to be a mistake; the writer fancied that it must have been so, and that he should be suspected if he were to state it otherwise. It is to be regretted that people do not endeavour to describe what they see and hear, without the embellishment of the imagination.

This grand experiment was no less grand from the absence of noise, smoke, earthquake, and fragments hurled to vast distances through the air. “I have not heard of a single scattered fragment flying out as a projectile in any direction”--continues Sir John Herschel--“and altogether the whole phenomenon was totally unlike anything which, according to ordinary ideas, could have been supposed to arise from the action of gunpowder. Strange as it may seem, this contrast between the actual and the expected effects, gave to the whole scene a character rather of sublime composure than of headlong violence--of graceful ease than of struggling effort. How quietly, in short, the gigantic power employed performed its work, may be gathered from the fact, that the operators themselves who discharged the batteries were not aware that they had taken effect, but thought the whole affair a failure, until reassured by the shout which hailed its success.”

Sawing the Stones for the Mason.

Whatever may be the purpose to which the stone is to be applied, the larger blocks obtained from the quarry must be cut into smaller and more manageable pieces; this is done by _sawing_. The saw used is a long blade of steel without teeth, fixed in a heavy wooden frame, similar in principle to that which holds the finer spring-saws employed by cabinet-makers. The stone-saw, from its great size, however, requires a more powerful contrivance for drawing it to the proper degree of tension: this consists of a long screw-bolt fixed to a piece of chain, which hooks over one of the upright arms of the frame; a similar chain from the other carries a swivel-joint with a screw-nut to receive the screw: by turning the swivel by a lever, the nut on the screw draws up or tightens the chains, and that draws the blade tight, which is contained between the other ends of the arms.

These huge saws are worked by one or two men, who, in London stone-yards, sit in watch-boxes, in order to be sheltered from the sun and rain. Barrels filled with water, which is allowed to drop out at a tap, are mounted on the block of stone, so that the water may drip into the cut and facilitate the motion of the saw by removing some of the friction, as well as prevent it becoming hot, and so losing its temper by the same cause.

In some large establishments, the sawing is effected by machinery. The block is fixed in a proper position, and a group of saws brought to act on it. These saws are all arranged parallel, according to the thickness of the pieces into which the stone is to be cut; and a steam-engine being brought to bear on the whole group, the cutting is effected with great rapidity.

The Processes of Stone-Masonry.

When the stone is sawed to the proper size, the surfaces which are exposed to view, have to be made smooth and even. The tools used by the mason for this purpose consist of iron chisels of different widths, and principally of a sharp-pointed one called a _pointer_; these chisels are struck with a mallet made of a conical-formed lump of hard wood, fixed to a short handle.

The _pointer_ is used for chipping off the principal roughnesses on the face and edges, and for working the whole face over to bring it level, the workman trying his work by applying a _straight-edge_ occasionally to it. When the front and edges are made _true_, the face is sometimes _tooled_ over, so as to leave regular furrows in it, according to certain forms, by which the different kinds of work are distinguished. But this practice is going out of use, now that soft free-stone is so much employed in building. In old edifices, such as St. Paul’s, Whitehall, &c., &c., the stone will be found to be wrought on its face in the manner alluded to.

Stones in buildings are not only fixed with mortar, as bricks are, but are further secured in their places by being clamped together with iron clamps. These are short iron bars, from seven to twelve inches long, one and a half wide, and half an inch thick, according to the size of the stone; the ends of the clamps being turned down a little, to afford a better hold. A channel is cut in the two contiguous stones deep enough for the clamp to lie in, and the ends of the channel are sunk deeper, to receive the turned-down ends of the clamp; when this is put into the channel, molten lead is poured in to fill up the interstices, to keep the clamp in its place, and to prevent it from rusting.

From the expense of carrying and working stone, the walls of buildings at a distance from a quarry, such for example as those in London, are seldom now built of solid stone, but a facing of this material is applied only on the external surface of the wall, which is built of brick. This kind of work is called _ashler_ work, and both the brick and stone-work must be executed with considerable care, to enable a wall composed of two materials to preserve its perpendicularity; it being obvious, that if the brick part yielded to the weight, it must, from its construction, do so more than the stone facing, and, therefore, the wall would bend inwards and become crippled.

The width of the courses of ashlers must, therefore, be made equal exactly to a certain number of courses of bricks with the intervening mortar, and the brick-work must be executed with such care, that this number of courses may be everywhere of the same width in the whole height of the wall. In every course of ashler there must be solid stones laid quite, or nearly quite, across the width of the wall to form a _bond_ to the stone facing, and all the stones of the ashler must be fixed with iron cramps to one another and to these bond-stones. But, however carefully a faced wall may be executed, it is never so firm or durable as one built entirely of either material; indeed, if well executed, of good materials, and of competent thickness in proportion to its height, a brick wall is the most durable, light, and efficient structure that can be erected.

When stone is to be cut into cornices, mouldings, &c., the blocks having been sawed, the ends, top and bottom, are worked very true and parallel, or perpendicular to each other, and one edge or _arris_ cut to a perfectly straight line; a thin wooden mould of the section of the cornice is then applied to each end, and the profile of the mouldings marked out on the stone. The workman being guided by this figure, cuts away the stone down to the general surface of the mouldings, and then proceeds to get the flat fillets of the mouldings perfectly straight and true by the rule; these again guide him in working the curved mouldings, such as _ovolos_, _cavettos_, _cyma rectas_, and _ogees_; when these are cut nearly to their profile, and perfectly straight on the _bed_ line, they are finished off by being rubbed down smooth by thin long straight-edges of stone.

Foliage and carved work is executed by a better kind of workman, possessing some of the taste of an artist, and he works on the same general principles as a sculptor when executing a statue; it would be foreign to our present object, therefore, to dwell on this branch of the mason’s art.

It often, or even most commonly occurs, that the distance between two columns of a portico, is of greater length than a stone can be obtained, and if the architrave, or that part of the _entablature_ immediately over the capitals of the columns, be looked at attentively, a stone will be perceived between the columns apparently unsupported, for neither end rests on the column, and the joints of those ends are upright, not presenting any character of a voussoir-stone or arch. The contrivance by which such an architrave stone is supported deserves to be described.

The stone in question has a projecting part, wrought at each end, of the form shown in the annexed figure; this projection is received into a corresponding cavity, cut in the end of the stone supported by the column, and the joint is thus really an arched or wedge-shaped one, though the bevel line is concealed, and the two stones, when put together, present only a vertical joint.

The mason uses _squares_, _levels_, _plumb-lines_, and _straight-edges_ to set out his work, and trowels and mortar to set the stones with; but the latter is rather used to make the joints water-tight than to keep the stones together, this being effected by their weight or by iron clamping. Formerly the mason required far more accurate and extensive knowledge of geometry than is possessed by persons of the trade at present; this was when he was called on to construct groined and vaulted roofs, enriched with carved work and pendent corbels, where the nicest workmanship was required, to ensure the stability of the light and graceful columns and vaulting of a Gothic cathedral. It was this possession of superior skill and knowledge that caused the establishment of the Society of Freemasons, which dates its rise from the tenth or eleventh century.

Marble, from its costliness, and the difficulty of working it, is seldom, if ever, used in solid pieces in buildings; thin facings of it are set upon stone _backings_, much as rare woods are used in _veneering_ by the cabinet-maker. The marble is sawn into thin slabs, like other stone, and the face is polished by rubbing on it the surface of another piece, fine sand, mixed up with water, being used to cause abrasion.

Various contrivances are resorted to for cutting marble, and building-stones generally, into _curved_ forms. In some cases a lever is made to work at one end on a pivot, while at the other end is attached a curved piece of sheet-iron, which passing backwards and forwards over the stone, cuts it in a circular form. In other cases a cylinder of sheet-iron is formed; and this being allowed to fall vertically on the surface of the stone, and rotated rapidly, cuts out a piece of stone of the diameter of the cylinder. Sometimes, when a large circular piece of stone is required, a kind of wheel is employed, furnished on its under surface with four curved cutting-irons, and these cutters, when the wheel revolves, cut the stone. By a modification of the arrangements, an oval instead of a circular curve may be given to the piece of stone.