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incompressible, the machine is locked. Thus all hydraulic machines; possess an inherent brake; indeed, many of them are used solely as brakes.

Hydraulic power transmission does not possess the flexibility of electricity, its useful applications being comparatively limited, but the simplicity, efficiency, durability and reliability of typical hydraulic apparatus is such that it must continue to occupy an important position in industrial development.

Sometimes a much higher pressure than 700 ℔ or 1000 ℔ per square inch is desirable, more particularly for heavy presses and for machine tools such as are used for riveting, for punching, shearing, &c. The development of these applications has been largely due to the very complete machinery invented and perfected by R. H. Tweddell. One of the principal installations of this kind was erected in 1876 at Toulon dockyard, where the machines are all connected with a system of mains of 21/2-in. bore and about 1700 yds. long, laid throughout the yard, and kept charged at a pressure of 1500 ℔ per square inch by engines of 100 h.p. with two large accumulators. Marc Berrier-Fontaine, the superintending engineer of the dockyard, stated that the economy of the system over the separately-driven geared machines formerly used is very great. But while pressures so high as 3 tons per square inch (as in the 12,000-ton Armstrong-Whitworth press) have been used for forging and other presses, it is not desirable, in the distribution of hydraulic power for general purposes, that 1000 ℔ per square inch should be much exceeded; otherwise the rams, which form the principal feature in nearly all hydraulic machines, if proportioned to the work required, will often become inconveniently small, and other mechanical difficulties will arise. The cost of the machinery also tends to become greater. In particular cases the working pressure can be increased to any desired extent by means of an intensifier (fig. 8).

Fig. 8.

An important application of hydraulic power transmission is for ship work, the system being largely adopted both in H.M. navy and for merchant vessels. Hydraulic coal-discharging machinery was fitted by Armstrong as early as 1854 on board a small steamer, and in 1868 some hopper barges on the Tyne were supplied with hydraulic cranes. A. Betts Brown of Edinburgh applied hydraulic power to ship work in 1873, and in the same year the first use of this power for gunnery work was effected by G. M. Rendel on H.M.S. “Thunderer.” The pressure usually employed in H.M. navy is 1000 ℔ per square inch. Accumulators are not used and the engines have to be fully equal to supply directly the whole demand. The distance through which the power has to be transmitted is, of course, very short, and the high velocity of 20 ft. per second is allowed in the main pipes. The maximum engine-power required under these conditions on the larger ships is very considerable. A recent development of hydraulic power on board ship is the Stone-Lloyd system of closing bulkhead doors. In hydraulic transmission of power it is usually the pressure which is employed, but there are one or two important cases in which the velocity of flow due to the pressure is utilized in the machine. Reference has already been made to the use of turbines working at 750 ℔ per square inch at Antwerp. The Pelton wheel has also been found to be adapted for use with such high pressures. Another useful application of the velocity due to the head in hydraulic transmission is in an adaptation of the well-known jet pump to fire hydrants. The value of the system of hydraulic transmission for the extinction of fire can hardly be overestimated where, as in London and most large towns, the ordinary pressure in the water mains is insufficient for the purpose.

Authorities.—Armstrong, Proc. Inst. C.E. (1850 and 1877), Proc. Inst. Mech. E. (1858 and 1868); Blaine, Hydraulic Machinery (1897); Davey, Pumping Machinery (1905); Dunkerley, Hydraulics (1907); Ellington, Proc. Inst. C.E. (1888 and 1893), Proc. Inst. Mech. E. (1882 and 1895), Proc. Liverpool Eng. Sic. (1880 and 1885); Greathead, Proc. Inst. Mech. E. (1879); Marks, “Hydraulic Power,” Engineering (1905); Parsons, “Sanitary Works, Buenos Aires,” Proc. Inst. C.E. (1896); Robinson, Hydraulic Power and Hydraulic Machinery (1887); Tweddell, Proc. Inst. C.E. (1883 and 1894), Proc. Inst. Mech. E. (1872 and 1874); Unwin, Transmission of Power (1894), Treatise on Hydraulics (1907).  (E. B. E.) 

III.—Pneumatic

Every wind that blows is an instance of the pneumatic transmission of power, and every windmill or sail that catches the breeze is a demonstration of it. The modern or technical use of the term, however, is confined to the compression of air at one point and its transmission to another point where it is used in motors to do work. The first recorded instance of this being done was by Denis Papin (b. 1647), who compressed air with power derived from a water-wheel and transmitted it through tubes to a distance. About 1800 George Medhurst (1759–1827) took out patents in England for compressing air. He compressed and transmitted air which worked motors, and he built a pneumatic automobile. William Mann in 1829 took out a patent in England for a compound air compressor. In his application he states: “The condensing pumps used in compressing I make of different capacities, according to the densities of the fluid to be compressed, those used to compress the higher densities being proportionately smaller than those previously used to compress it to the first or lower densities,” &c. This is a very exact description of the best methods of compressing air to-day, omitting the very important inter-cooling. Baron Van Rathen in 1849 proposed to compress air in stages and to use inter-coolers between each stage to get 750 ℔ pressure for use in locomotives. For the next forty years inventors tried without success all manner of devices for cooling air during compression by water, either injected into the cylinder or circulated around it, and finally, with few exceptions, settled down to direct compression with no cooling worthy of mention. Only in the last ten years of the 19th century were the fundamental principles of economical air compression put into general practice, though all of them are contained in the patent of William Mann and the suggestion of Van Rathen. The first successful application of compressed air to the transmission of power, as we know it, was at the Mont Cenis Tunnel in 1861. The form of compressor used was a system of Water rams-several of them in succession-in which water was the piston, compressing the air upwards in the cylinder and forcing it out. Although the air came in Contact with the water, it was not cooled, except slightly at the surface of the water and around the Walls of the cylinders. The compressors were located near the tunnel, and the compressed air was transmitted through pipes to drilling machines working at the faces in the tunnel. Rotary drills were tried first, but were soon replaced by percussion drills adapted from drawings in the United States Patent Office, copied by a French and Italian commission from the patent of J. W. Fowle of Philadelphia. H. S. Drinker (Tunneling, Explosive Compounds and Rock Drills, New York, 1893) states positively that the first percussion drill ever made to work successfully was patented by J. J. Couch of Philadelphia in 1849. Shortly afterwards Fowle patented his drills, in which the direct stroke and self-rotating principle was used as We use it now. The first successful drill in the Hoosac Tunnel was patented in 1866 by W. Brooks, S. F. Gates and C. Burleigh, but after a few months was replaced by one made by Burleigh, who had bought Fowle’s patent and improved it. Burleigh made a compressor, cooling the air during compression by an injected spray of water in the cylinders. The successful work in the Mont Cenis and Hoosac Tunnels with the percussion drilling machines caused the use of compressed air to spread rapidly, and it was soon found there were many other purposes for which it could be employed with advantage.

The larger tunnels and metal mines were naturally the earliest to adopt pneumatic transmission, often using it for pumping and hoisting as well as drilling. In Paris and Nantes, in Berne and in Birmingham (England), street tramways have been operated by pneumatic power, the transmission in these, however, being in tanks rather than, pipes. Tanks on the cars are filled at the central loading stations with air at very high pressure, which is used in driving the motors, enough being taken to enable the car to make a trip and return to the loading station. Several attempts in pneumatic street traction were made in America, but failed owing to financial troubles and the successful introduction of electric traction. It is used Very successfully, however, both in Europe and in America., in underground mine haulage, being especially adapted to coal mines, where electricity would be dangerous from its sparks. The copper smelting Works at Anaconda, Montana., U.S.A., uses twelve large pneumatic