In such fields as those of the cheapening of light and motive power, the utilization of electricity, the production of metals, it would seem that the day of the solitary researcher or inventor is drawing to a close. To-day the man of original ideas, of combining faculty, of uncommon deftness, of rare visual accuracy, is mated with his peers for a group attack on a many-sided problem where each man’s resources will find their special play. In untiring labor at the bench and lathe, at the muffle and the test tube, one experiment follows another, all duly compared, judiciously varied and advanced as indication may suggest. Thus the fences which extreme specialization have set up are surmounted, each worker supplements the deficiencies of his fellows, and all join hands to take by assault a citadel that might forever defy single attack. CHAPTER XXVIII COMPRESSED AIR An aid to the miner, quarryman and sculptor . . . An actuator for pumps . . . Engraves glass and cleans castings . . . Dust and dirt removed by air exhaustion . . . Westinghouse air-brakes and signals. Some recent noteworthy advances of invention have been due to co-operation by many workers, not however on such lines of definite group attack as have just been remarked. Among these advances may be chosen for rapid survey the applications of compressed air, of plain and reinforced concrete, the economy of power-production and of fuel for whatever purpose employed. Let us begin with compressed air. Compressed Air. In Effect Cold Steam for Driving Hammers, Drills, and Picks. Hammers, drills, and picks, all working by percussion, are among the most effective tools. They may be attached to a steam piston, as are Nasmyth hammers and common quarry drills, yielding a much cheaper product than does hand labor. In many places where it is not feasible to use steam in this direct and most economical way, it is best to employ compressed air which works much as steam does, so that a motor or a drill with no change of build may be operated by one or other motive power at will. Compressed air, unlike steam, may be taken long distances without condensation; in tight receivers it may be kept without any loss as long as we like, and used in mines and tunnels where steam heat would be a nuisance, or where electricity would be unsafe. Electrical drills and cutters, moreover, are liable to have their insulation harmed by working shocks, and by surrounding grit, sand or chips. In mines after a blast of gunpowder, a direct current from the main pipe quickly freshens the air; at all times the cool, pure breeze from the exhaust pipe is a welcome aid to ventilation. Steam, one of the chief servants of industry, must be kept and used hot. When its energy is used to compress air we have at command a substance with all the working quality of steam, without having to keep it warm. As it toils at common temperatures, we can imagine compressed air to be, in effect, cold steam.