PLYMOUTH SOUND                                1508                                                                PO

other Wyoming-Valley towns, and claims the distinction of sending the first coal to market. Population 16,996.

Plymouth Sound, on the southwest of Devonshire, England, is a deep inlet of the English Channel into which the Tamar falls from the west and the River Plym from the east. In 1812 was begun the construction of Plymouth Breakwater, an insulated mole of stones, a mile in length, stretching across the middle of the sound and thoroughly protecting all vessels anchored within. It was not fully completed till 1841, and cost nearly $7,500,000. Plymouth Sound has been said to rivai the Bay of Naples in beauty.

Pneumatics (nŭ-măt'īks), is that part of physics which treats of the properties of gases. It has to do with investigating their weight, pressure, elasticity, condensation etc. and with a iarge number of machines and appliances based upon the behavior of gases under pressure. Few studies in science have yielded more important results than has the study of the nature and properties of gases. Almost the entire structure of modern chemistry rests upon our knowledge of gases; and by the study of gases physicists have been led to a clear and simple explanation of the phenomena of heat and many other phenomena forming important chapters in modern physics. The applied sciences, too, owe much to our knowledge of gases. A knowledge of the properties of compressed air has led to many mechanical devices of the greatest economic value. A large variety of portable tools, percussion and rotary; the air-gun; the air-pump; and the diving-bell owe their origin to a knowledge of gases. Compressed air is used extensively in building bridge-piers, tunnels and other foundations, where water and quicksand are to be excluded. It has been used also for motor-power for street-railways, though not very economically. But for the transmission of mail and small parcels at high speed compressed air has been used to very great advantage, and its use in this direction is only beginning to be developed. The first application of pneumatic transmission to the conveying of mail and parcels was made in London in 1863, but the United States Government decided to give the system a trial only in 1892. The Western Union Telegraph Company has used it for many years, and it has been largely employed in commercial establishments for conveying change and goods. But it has not reached the development in this direction in the United States that it has in Europe. Its use in the United States for mechanical appliances, as carpenters', machinists' and other tools, is more developed.

Pneumatic Tires were first invented by R. W. Thompson, an English civil engineer, in 1843, for use on road-machines. In the

United States a patent was granted in 1847. But it was not until these tires came to be used for bicycles that they acquired real importance. In 1889 Dr. J. B. Dunlop fitted his son's bicycle with a tire of rubber hose, with such success that a patent was obtained, although this patent covered certain details rather than the broad principle of the pneumatic tire. A patent of Mr. Tillinghast of Rhode Island led to the universal use of the pneumatic tire on bicycles within five or six years. The pneumatic tire is also employed universally on automobiles and widely for other vehicles. Its great advantage is that it sinks into small depressions instead of jumping over them, thus reducing both the jolt and the friction of motion and increasing speed. Its only defect is the liability to puncture, when the tire becomes useless until patched or renewed.

Pneumatic Tool, a mechanical tool operated by compressed air. In tunneling and mining pneumatic drills are used, and similar tools are used for stone-carving. The power is applied by pressure of compressed air against a piston in a cylinder, and by a suitable automatic valve the blow can be repeated several times a minute. Pneumatic riveting-machmes and hoists are two other pneumatic tools used very extensively. Pneumatic tools are generally simple to operate, but not so economical as either steam or electricity.

Pneumatic Tube, a tube or pipe in which packages and written messages are sent by the use of compressed air or by normal atmospheric pressure acting against a vacuum. The packages are inclosed in carriers of proper shape or size to go into the tube and are forced along the tube by pressure behind them. Pneumatic dispatch is used very extensively in large stores, in connection with cash-systems. In newspaper and telegraph offices it is used to transmit messages. In New York City, London and several other large cities it is used to send the mails between the main postoffice, the substations and the railroad stations. In Berlin and several continental cities it has been used for rapid delivery of local letters in connection with the postoffice. Lead-pipes are ordinarily used for messages, but steel pipes are used for large packages. See Pneumatics.

Po, the largest river of Italy, rises on Monte Viso, one of the Cottian Alps, at an altitude of 6,000 feet, close to the French frontier. About 60 miles from its mouth, above Ferrara, it begins to form its delta, 60 miles wide from north to south. The delta is rapidly growing in area; Ravenna, once a city on the seashore, is now four miles inland. The entire length of the Po is 360 miles, and it drains an area of 28,900 square miles. It has always been a difficult river to cross on account of its width and