The setting of the movable jaw at an incline enables the large stone to be first cracked, the movable jaw then opens, and as the stone falls lower between the more contracted jaws, it is broken finer, until it is finally crushed or pulverized and falls through at the bottom. The movable jaw is adjustable and can be set to crush stones to a certain size. As the rock drill made a revolution in blasting and tunnelling, so the Blake crusher revolutionised the art of road making. “Road metal,” as the supply of broken stones for roads is now called, is the fruit of the crusher. Hundreds of tons of stone per day can be crushed to just the size desired, and the machine may be moved from place to place where most convenient to use. Other crushers have been invented, formed on the principle of abrasion. The stones, or ore, fall between two great revolving disks, having corrugated steel faces, which are set the desired distance apart, and between which the stones are crushed by the rubbing action. In this style of machine the principle of a gradual breaking from a coarse to a finer grade, is maintained by setting the disks farther apart at the centre where the stone enters, and nearer together at their peripheries where the broken stone is discharged. Large smooth or corrugated rollers, conical disks, concentric rollers armed with teeth of varying sizes, and yet so arranged as to preserve the feature of the narrowing throat at the bottom or place of discharge, have also been devised and extensively used. A long line of inventions has appeared especially adapted to break up and separate coal into different sizes. To view the various monstrous heaps of assorted coals at the mouth of a coal mine creates an impression that some great witch had imposed on a poor victim the gigantic and seemingly impossible task of breaking and assorting a vast heap of coal into these separate piles within a certain time--a task which also seems to have been miraculously and successfully performed within such an exceedingly short time as to either satisfy or confuse the presiding evil genius. Modern civilisation has been developed mostly from steam and coal, and they have been to each other as strong brothers, growing more and more mutually dependent to meet the demands made upon them. The mining of coal, and its subsequent treatment for burning, before the invention of the steam engine, were long, painful, and laborious tasks, and the steam engine could never have had its modern wants supplied if its power had not been used to supplement, with a hundredfold increased effect, the labour of human hands.