sliding friction at the axle is far less than it would be at the point of contact with the ground, were the wheel locked so that it could not turn. But it is not necessary to have any sliding friction at the axle if we revert to the old roller system that prevailed before the day of the wheel. The axle may be considered the load and the axle bearing the road. We can then put rollers between load and road. Because the road is a circular one that travels with the load we can line it with rollers throughout its length, and the load will never lack for rollers to roll upon. Thus we have the roller bearing which is so widely used in modern vehicles. Ball bearings operate on the same principle except that the balls furnish less contacting surface and are not so suitable for supporting heavy loads, as are rollers. Sliding friction is almost completely eliminated and unless heavily loaded a wheel on ball bearings will not heat even when the bearings are not lubricated. REDUCING ROAD FRICTION While there is little friction between a wheel and the roadway upon which it travels, the roughness of the road is a very important factor. Every time a wheel goes over a stone the entire vehicle must be lifted; this represents so much wasted energy. The advantage of the pneumatic tire lies in the fact that it absorbs small inequalities without making it necessary for the entire vehicle to rise over them. This means less load lifted and hence less work done. Large wheels are better than small ones because they do not sink so deeply into depressions and because they surmount small obstacles more readily. On a steel track the size of the wheel does not make so much difference as it does on a road because the track surface is smooth and is but little depressed by the wheel. It has been estimated that a horse can pull ten times as great a load on rails as on an ordinary macadamed road. Some years ago broad steel tracks were laid in some of our city streets for the use of horse-drawn trucks. They served very well as far as the vehicles were concerned. The road friction was reduced considerably, but the fact was overlooked that the horses needed a good friction surface under their feet. Two horses could not pull a truck along the track without walking on the track and the smooth steel made such slippery footing that the tracks had to be torn out and replaced with common paving. INCREASING TRACK FRICTION The difference between sliding and rolling friction is well illustrated in a locomotive. The driving wheels are turned around by steam power. They must either roll or slide on the track and the load they will pull without slipping is a measure of the excess of sliding friction over rolling friction. To increase the traction or the adhesion of the locomotive to the track it is provided with a number of driving wheels. In some of our largest locomotives driving wheels are placed under the tender so as to obtain a maximum of traction.