ROLLERS. Cylindrical rollers are used for very many purposes in the arts, especially where two are employed in contact. If one roller is rotating on its axis, and if another be brought close to it, surface to surface, with the axes parallel, the second roller will be made to rotate in an opposite direction from the first; this property is often made use of in trans mitting and reversing motion. In cylindd printing, at the cotton print works, the pass ing of the cloth between two cylinders, one of which is engraved and supplied with colour suffices to print the cloth. In bookbinding or rather in pressing the sheets for the book binders, a roller-press affords an immense power: this consists, in principle, of two rollers rotating nearly in contact, and pressing be tween them the quire of leaves. In all kinds of metal working the pressure of two rollers is employed to flatten a mass of metal into slabs, sheets, or ribbons. If the surface of one or both rollers be grooved in a direction trans verse to the axis, any substance passed between the rollers will assume a form corresponding with those grooves. It is in this way that railway bars are made ; the iron is first passed between rollers grooved so as to produce a square bar, and then through other rollers so grooved as to give the contour of a railway bar. All such modes of compression have
a tendency to increase the length of bars according as the width and thickness are di minished.
In agricultural matters the roller is often used singly, and generally as a weight. In the common garden and road roller a heavy cylindrical mass is employed to press the earth or gravel of a path smooth or level; it is sometimes made of stone ; but sometimes a hollow iron cylinder, weighted, if necessary, with stones. A roller is sometimes used over corn fields, whose surface is covered with teeth. Such is Crosskill's Patent Serrated Roller and Clod Crusher. This roller is very heavy, and requires three horses abreast to work it over a newly ploughed field; the surface of the roller presents angular teeth in every part, and these teeth tend to breakup the clods of earth which have been turned up by the plough. It also tends to give solidity to loose soil, one of the advantages ordinarily produced by sheep treading.