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 however, by refinements in workmanship, etc., efforts are made to keep the torque required so low as to be within the power of the operator. Usually a revolving scale round the edge of the field shows the direction of the view. If the whole instrument moves, the operator also moves round with it in the boat, and knows at once in which direction he is looking.

Owing to proximity to the magnetic compass the whole of the tube must be non-magnetic. High-strength bronze was used in the earlier practice in the British navy. A special nickelchrome steel was manufactured and machined by Messrs. Krupp for use for the outer tube of the German navy periscope used before the war, and a similar steel was developed and used in the British service, but it is costly and more difficult to machine to the required accuracy than is the case with bronze.

The use of aircraft for anti-submarine work led to the demand for a periscope which could be used for looking overhead. In the sky- searching periscope the upper prism can be rotated by mechanism inside the periscope, so that aerial observations can be readily made before the submarine " breaks surface."

To enable a distant ship or other object to be examined more closely it is possible in some periscopes to change the magnification from a normal power of 1.5 to a power of 6. This, and the sky search- ing previously mentioned, means increased internal gearing and a larger upper tube. As a rule every submarine has at least two peri- scopes, one unifocal with a small upper tube and the other bifocal and sky-searching with a larger upper tube.

Whilst in the British service sky searching up to right overhead was arranged for, German periscopes as a whole are limited to 20 above the horizontal. In each case 10 depression is allowed for to follow the roll of the submarine.

For special purposes other features are added, such as range- finding attachments, etc. A " night " periscope for use at dusk has been developed. It is much shorter than the typical instruments described, so that the maximum brightness of image is obtained.

The periscope when installed in. the submarine is used for two purposes: (a) general observation for submerged navigation; (b) for correctly aligning the submarine when firing a torpedo at a target. In connexion with (a) the principal requirement is clearness of field. Continuous use of a periscope is very trying for the observer's eyes, and for use in bright weather light-filter screens are provided to reduce the glare. It has also been found that in foggy and misty weather suitable colour screens are of assistance. These screens are usually embodied in the eyepiece. For purposes of torpedo attack the periscope is used as a range-finder to determine the distance the target is away, and also in connexion with tables to determine the correct time to fire the torpedo, allowing for the speed of the enemy, course, etc. Officers of submarines have devised various mechanical devices to avoid calculations, and these have been added to the periscope. Although two periscopes are provided when attacking, one only would be shown for short periods to get check observation so as to prevent the wash of the upper tube revealing the proximity of the submarine.

PERKIN, WILLIAM HENRY (1860–), English chemist, was born at Sudbury, England, in 1860, eldest son of Sir William Perkin, founder of the aniline dye industry. He received his general education at the City of London School, and his scientific education at the Royal College of Science, South Kensington, and at the universities of Würzburg and Munich. During 1883–6 h; held the position of Privatdocent in the university of Munich. In i83; he returned to England and became professor of chemistry at the Heriot-Watt College, Edinburgh. In 1892 he accepted the chair of organic chemistry at the Victoria University, Manchester, which he held until 1912. During this period his stimulating teaching and brilliant researches attracted stu- dents from all parts, and he formed at Manchester a school of organic chemistry famous throughout Europe. In 1912 he succeeded Prof. Wm. Odling as Waynflete professor of chemistry at Oxford. He soon made his influence felt there—new and more extensive laboratories were built, and for the first time in England a period of research became a necessary part of the academic course in chemistry for an honours degree. Prof. Perkin was president of the Chemical Society from 1913 to 1916. He was awarded the Longstaff medal of the Chemical Society in 1900, and the Davy medal of the Royal Society in 1904. The main results of his work are embodied in a very numerous and brilliant series of papers in the Transactions of the Chemical Society. The earlier papers deal chiefly with the properties and modes of synthesis of cloud chain hydrocarbons and their derivatives. This work led naturally to the synthesis of many terpenes and members of the camphor group; also to the investigation of various alkaloids and natural colouring matters. In addition to purely scientific work Prof. Perkin always kept in close touch with chemical industry. His text-books on practical chemistry, inorganic and organic chemistry, written in conjunction with Prof. Kipping, are in general use.

PERNERSTORFER, ENGELBERT (1850–1918), Austrian politician, was born on April 27 1850 at Vienna, the son of a small master tailor. While still a lad at the Gymnasium he had to earn his living by giving lessons. At the Schollengymnasium he struck up a close friendship with Viktor Adler, and became interested in the Pan-German political movement. While still at the Gymna- sium he gave courses of lectures at the Workmen's Education Union. At the university he came into contact with Schonerer, to whose intimate circle he belonged. He collaborated in the preparation of the so-called Linz programme of the Left National party, and for a quarter of a century, from 1 88 1 onwards, he edi ted the periodical Deutsche Worle. He separated from Schonerer as the latter adopted an increasingly reactionary and anti-Sem- itic attitude. He was also the inspirer and one of the founders of the German School Union. In 1885 he was elected to the Aus- trian Parliament as independent candidate for the manufactur- ing centre of Wiener-Neustadt. From that time, with the excep- tion of the electoral period 1897 to 1901, he sat in Parliament until his death, and from 1907 onwards was its vice-president. In 1907 he became president of the parliamentary Social Dem- ocratic party, which had in the meantime increased in number to 87 in consequence of the adoption of universal suffrage. He died, after a rather long illness, in Vienna on Jan. 7 1918.

PERRY, JOHN (1850–1920), British mathematician, was born in Ulster Feb. 14 1850 and educated at Queen's College, Belfast. Though he took a post as a schoolmaster in 1870, he also qualified as an electrical engineer and devoted much of his time to turning mathematics to practical account. He served for a time as assistant to Lord Kelvin. Later he was associated with Prof. Ayrton and together they were responsible for many inven- tions in electrical apparatus (see 3.76, 8.782 and 783, 9.236, etc.). In 1881 he became professor of engineering and mathematics at the City and Guilds of London Technical College and in 1896 professor of mathematics and mechanics at the Royal College of Science, retiring in 1914. He published many books on applied mathematics and did much to further scientific engineering, especially by his lectures to operatives and by such works as The Steam Engine (1874), Spinning Tops (1890), The Calculus for Engineers (1897), etc. During the World War he was an ad- viser on gyroscopic compasses. He died in London Aug. 41920.

PERSHING, JOHN JOSEPH (1860- ), American soldier, was born near Laclede, Mo., Sept. 13 1860. He studied at the Kirksville (Mo.) Normal School (B.A. 1880); graduated from the U.S. Military Academy in 1886; was commissioned second-lieutenant and immediately assigned to the 6th Cavalry in a campaign against the Apaches in Arizona. His conduct won the praise of General Nelson A. Miles, and in 1890, during an uprising of the Sioux, he was sent to Dakota, in charge of the Indian scouts.. In 1891 he was appointed military instructor at the university of Nebraska, remaining there four years. He entered the law school and received the degree of LL.B. in 1893, having been made first-lieutenant the preceding year. In 1897 he was appointed instructor in tactics at the U.S. Military Academy, but on the outbreak of the Spanish-American War (1898) asked to be assigned to active duty. He served in Cuba through the Santiago campaign, was appointed chief of ordnance with the rank of major of volunteers, and in June 1899 assistant adjutant-general. He organized in Cuba the Bureau of Insular Affairs of which he was head for several months. In Nov. 1899 he was sent to the Philippines as adjutant -general of the Department of Mindanao, and in 1901 was honourably discharged from volunteer service. The same year he was made captain in the regular army and later conducted a campaign against the Moros, which he completed with success in 1903. The same year he returned to America and was appointed a member of the General Staff. In 1905 he went to Japan as military attache to the American embassy, and during the Russo-Japanese War spent several months as military observer with the Japanese army in Manchuria. As a reward for his success in the Philippines President