After the blocks have been severed from the mass, they are reduced as nearly as possible to a rectangular form; • and this is done by means of a tool a keret, pointed at one end and flat at the other, with which the irregular parts aro knocked off. The blocks are then usually, by means of cranes which are capable of being moved from place to place, raised upon trucks or low carriages; and these are drawn, generally on iron railways, to the quays or wharfs where the stone is put on ship-board. A curious account is given by Vitruvius of an unsuccessful attempt which was mode by an engineer named Paconius to bring from a quarry a mass of stone containing about 576 cubic feet. For this purpose, two broad cylinders of wood, 15 feet in diameter, were constructed; and the stone, being raised from the ground, was made to rest between them, the line of its length being in a horizontal position in the direction of their common axis, then a rope being wound round the circumference of the united cylinders in the middle of the length of the latter, and one end of it being made fast to the harness of the oxen, it was intended that, by the motion of the animals, the cylinder should revolve on the ground, and the stone be brought up to its place. The contrivance appears to have failed; but had the oxen been attached to the two ends of the cylinder instead of its middle, there is no reason to doubt that it would have succeeded.
So far as concerns our own country, granite is the hardest stone quarried in any considerable quantity. The largest operation in Cornish granite is believed to have been the detaching and working of a block weighing 33 tons, for an obelisk. The granite of Brown Willy, sent down to Wadebridge for shipment ; that of the Cheesewring, shipped at Liskeard ; that of the Par Valley, shipped at Par Harbour ; and that of Penrhyn, shipped at Falmouth,—are well-known examples of Cornish granite. In some of these quarries the stone is of beautiful texture, and glistens brightly in the sun ; so hard is the stone, that the blows of the picks against it give forth a real musical sound, vary ing in pitch according to the size of the pick. We have said that the use of gunpowder is rather limited in quarries of stone for building, because the substance is apt to be too much shattered by this explosive force. We may, however, refer to BLASTING for further information
as to this process—especially in those instances where an electric cur rent is employed to ignite the powder. Enormous masses of Holyhead mountain are now being quarried in this way, to furnish materials for a new breakwater in the harbour of refuge; a railway extends from the place of detachment to the place of deposition, thereby greatly facilitating the operations.
At the Aberdeen meeting of the British Association, in 1S59, Mr. Gibb gave an account of the Aberdeenshire granite quarries, from which it appears that those quarries have been worked about two centuries and a half. The operations were on a small scale until about the year 1741, when an extensive fire at Aberdeen led to a municipal order that the fronts of houses should be built of stone, instead of wood as heretofore. Aberdeen granite was first used in the London streets in 1764; but Waterloo Bridge was the first great metropolitan work constructed in this stone. The quarries are about twenty in number, and present five kinds of granite, technically known as blue, red or Peterhead, light red, soft gray, and white. It exhibits irregular masses, and a columnar structure. A good deal of blasting is neces sary in the operations. The best stone is now found at some depth beneath the surface. Dressing machines have occasionally been tried to bring the blocks of granite into form, but they have not been satis factory, because they acted like a plane ; whereas granite requires a kind of percussive action to bring down the surface. There are about 500 workmen and 50 horses at the quarries. The produce is now about 50,000 tons annually, of which 30,000 tons are exported.
It is right to observe that builders generally consider it advantageous to dispose the stones in the lower part of an edifice in the same posi tion as they had when lying in the quarry ; it being understood that they are then beat capable of supporting the weight of the super structure.