The _pneumatic sand-jet_ is used for a variety of purposes: for cleaning off old paint, or the weathered surface of stonework; for polishing up castings and forgings after they have been brazed. At the cycle factory you will find the sand-jet hard at work on the joints of cycle frames, which must be cleared of all roughness before they are fit for the enameller. The writer, a few days before penning these lines, watched a jet removing London grime from the face of a large hotel. Down a side street stood a steam-engine busily compressing air, which was led by long pipes to the jet, situated on some lofty scaffolding. The rapidity with which the flying grains scoured off smoke deposits attracted the notice of a large crowd, which gazed with upturned heads at the whitened stones. A peculiarity about the jet is that it proves much more effective on hard material than on soft, as the latter, by offering an elastic surface, robs the sand of its cutting power.
After merely mentioning the _pneumatic rammer_ for forcing sand into foundry moulds, we pass to the _pneumatic sand-papering_ machine, which may be described briefly as a revolving disc carrying a circle of sand-paper on its face revolved between guards which keep it flat to its work. The disc flies round many hundreds of times per minute, rapidly wearing down the fibrous surface of the wood it touches. When the coarse paper has done its work a finely-grained cloth is substituted to produce the finish needful for painting.
Have you ever watched carefully a steam-roller's action on the road when it is working on newly laid stones? If you have, you noticed that the stones, gravel, etc., in front of the roller moved with a wave-like motion, so that the engine was practically climbing a never-ending hill. No wonder then that the mechanism of such a machine needs to be very strong, and its power multiplied by means of suitable gearing.
Again, suppose that an iron-tyred vehicle, travelling at a rapid pace, meets a large stone, what happens? Either the stone is forced into the ground or the wheel must rise over it. In either case there will be a jar to the vehicle and a loss of propulsive power. Do not all cyclists know the fatigue of riding over a bumpy road--fatigue to both muscles and nerves?