Mr. Codrington[39] gives the result of breaking whinstone in a 16-inch by 9-inch Hope machine, from which it appears that the total cost, including wages, coal, oil, cottonwaste, etc., wear and tear of machinery, and, I presume, interest on first cost of machine, was about 1_s._ per cube yard. This effected a saving of 10_d._ per cube yard as compared with the same stone broken by hand, and the machine broke 40 tons of stone per diem. To make a stone-breaking machine pay, it is necessary: (1.) To give it nearly constant work. (2.) That the stone to be broken shall be too tough to break economically by hand. (3.) That the machine shall be at the quarry, so as to save the expense of much handling. (4.) To exercise care in feeding, to give it a sufficient supply without allowing an undue quantity of stone to pass in at one time. (5.) As about 20 per cent. of grit or dust is produced, this must be used for foot-paths, or as a binding material for roads, or in asphalte or tar paving. In addition to the grit which is produced, a great many long and thin pieces of stone pass through the machine, which have to be again broken by it before they could be used as road metal; and having once taken this form, they will frequently pass several times through the machine before they get properly broken. The wear and tear of a stone-breaking machine is very considerable, as can be easily imagined; it has been known to reach as high as 62·5 per cent.[40] of the first cost of the machine in one year. The objections to stone-breaking by machinery are principally: (1.) In some districts labour can be successfully employed in this manner. (2.) Hand-broken stone is sharper in fracture, as it is done by a blow and not by gradual pressure, whereas machine-broken stone is often flaky or with rounded edges, and frequently each stone may be cracked and shaken by the pressure. (3.) Want of uniformity in the size of the stones. The smaller the stone is broken the heavier a cubic yard of it will weigh, as the percentage of vacant space between each stone will be less. It has been found by experiment, however, that 55 per cent. of ordinary road metal is solid, so that the weight of a cubic yard of it can easily be ascertained in the following manner.[41] Multiply the weight of a cubic foot of any stone by 27 to bring it to a cubic yard, and then multiply this by 0·55: the result will be the weight of a cubic yard of the same stone when broken for metalling. A cubic yard of Guernsey granite broken to pass through a 2¹⁄₂ inch ring has been weighed, and gives an average of 1 ton 3 cwt. 2 qrs. tons cwt. qrs. A cube yard of flint weighed 1 1 3 „ „ pit gravel weighed 1 4 3 „ „ limestone weighed 1 3 0 A cubic yard of ordinary broken road metal will, when properly spread, cover an area of about 30 square yards of surface of a roadway.