Illustration: FIG. 2. (‡ CHISEL AND SCRAPER.)
Some stones, as “Bath stone” can be cut with a common toothed saw, and are but little harder than chalk; others, as marble, Portland stone, &c., require to be cut with a flat blade of iron stretched in a frame and having a supply of sand and water. A man sits in a shed having this heavy saw suspended from two poles, and balanced by a piece of stone swung over a pulley; he alternately pushes and pulls the frame, allowing the water to trickle into the seam as it forms, the sand being rubbed between the edge of the saw and the stone as the saw is moved backwards and forwards, slowly cutting the stone (see illustration). By this slow and tedious process building stones are cut into squares, slices, slabs, or any other form required. Granite is too hard for even this slow process, and after the pieces are chosen as nearly as can be got of the size and shape required, they are worked with a heavy iron pick (fig. 1), which at each blow strikes off a little piece not bigger than a pea, by which method the stone is shaped into the form required. Smaller cuttings of stone for building purposes, such as carvings, &c., are formed by the mallet and chisel (fig. 2), the work being finished with a rasp or steel scraper.
Illustration: BEAM OF STEAM ENGINE, WOOLWICH ARSENAL.
The great improvements in machinery--whether for looms, locomotive engines, or steam-ships, for forging anchors, boring cannon, rolling out and rivetting iron plates together for tubular bridges and boilers, or any other kind of work--are chiefly owing to the wonderful ease with which these machines can be driven by the power of steam. It matters not whether the object to be wrought is the head of a pin, or the crank of a steam-ship, it is done with both delicacy of touch and power of arm, a hundredfold beyond what could be effected by hand in the same time. The motive power of steam is derived from the property which water has of being expanded into vapour when heated to a certain degree, and of again resuming the form of water when cooled; this moreover takes place in the most easily manageable manner, and either by degrees or suddenly, according as the heat and pressure balance each other; moreover water, being easily obtained, and in sufficient quantity for the purpose, in all places where machinery is required, can always be applied. Before the use of steam, wind, water, horse, and hand power were chiefly in use; water-mills were, of course, only erected in those situations where a good supply of water could be obtained, and this even often failed in dry weather; windmills also depended on that uncertain element. Horse and hand powers are limited in their extent, and are moreover very expensive. The first attempts at a steam-engine were those in which the steam was only used that by its condensation a vacuum might be formed in a cylinder under a piston, so that the weight of the air should cause this to descend with considerable force--15 lbs. on the square inch. The piston was balanced by a weight, so that the steam might raise it with scarcely any pressure; the steam beneath the piston being condensed by a stream of cold water, the weight of the air again forced down the piston into the vacuum. This therefore was not a steam but an air-engine, as all the power exerted was derived from the weight of the air, and the steam merely used to procure a vacuum. After this came the low-pressure or condensing engine, and then the high-pressure or non-condensing engine, both of which are now used, the former in marine engines and the latter in locomotives.