The methods of obtaining the impor tant object of diminishing the friction are of two sorts, viz. either by the inter position of particular unctuous or oily substances between the contiguous mov ing parts, or by particular mechanical contrivances. Olive-oil is the best, and perhaps the only substance that can be used in small works, as in watches and clocks, when metal works against, metal. But in large works the oil is liable to drain off, unless some method is adopt ed to confine it. Therefore, for large works, tallow is mostly used, or grease of any sort, which is useful for metal, as well as for wood. In the last case tar is also frequently used. The mechanical con trivances which have been made, and are in use, for the purpose of diminishing the effects of friction, consist either in avoid ing the contact of such bodies as produce much friction, or in the interposition of rollers, viz. cylindrical bodies, between the moving parts of machines, or between moving bodies in general. Such cylin ders derive, from their various size and application, the different names of rollers, friction wheels, and friction rollers. Thus in mill-work, and other large machines, the wooden axis of large wheelsterminate in iron gudgeons, which turn in wood, or more frequently in iron or brass, which construction produces less friction than the turning of wood in wood. In the
finest sort of watch-work the holes are jewelled, viz. many of the pivots of .the wheels, &c. move in holes made in rubies, or topazes, or other hard stone, which, when well finished, are not liable to wear, nor do they require much oil. In order to understand the nature of rollers, and the advantage with which their use is attended, it must he considered, that when a body is dragged over the surface of another body, the inequalities of the surfaces of both bodies meet, and oppose each other, which is the principal cause of the friction or obstruction ; but when one body, such as a cask, a cylinder, or a ball, is rolled upon another body, the surface of the roller is not rubbed against the other body, but is only successively ap plied to, or laid on, the other, and is then successively lifted up from it. There fore, in rolling, the principal cause of friction is avoided, besides other advan tages: hence a body may be rolled upon another body, when the shape admits of it, with incomparably less exertion than that which is required to drag it over the surface of that other body. In fact, we commonly see large pieces of timber, and enormous blocks of stone, moved upon 1.ollers that are laid between them and the ground with ease and safety, when it would be almost impossible to move them otherwise.