(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.