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DEVONPORT 341 DEWAR calcareous slates, limestones, etc. They are divided into the Lower, Middle, and Upper Groups, all containing fossils; but in the middle division, corresponding practically to the Hamilton of New York, organic remains are especially abundant and include corals, crinoids, brachiopods, mollusks, and crustaceans. Devonian rocks occupy a large area in central Europe, as well as in the United States, eastern Canada, and Nova Scotia. In the United States they are found in New York and Pennsylvania, and include sand and limestone, used as building ma- terial, and are classed under the names of Oriskany, the oldest term, Corniferous or Upper Helderberg, Hamilton, and Chemung. Devonian rocks appear in some regions of the Appalachian Moun- tains. In the middle part of Michigan they surround the coal basin; and they are also found in Kentucky, Indiana, Ohio, Illinois, eastern Iowa, and Nevada. In Maine they are in a metamorphic con- dition. In the Upper Groups of the De- vonian System there are carbonaceous shales, which by mutual distillation give much of the petroleum and natural gas found in the sandstones of Pennsylvania and eastern Ohio. DEVONPORT (before 1824 called Plymouth Dock), a parliamentary and municipal borough, maritime town, and naval arsenal, in the S. W. of Devonshire, England; on the E. shore of the estuary of the Tamar, two miles W. N. W. of Plymouth. It stands on high ground, and is separated from its suburbs of Stoke and Morice Town by the glacis of its fortifications, once important, but now dismantled. The streets are regular, and the footpaths of marble. Devonport is supplied with water from Dartmoor by a circuitous route of 30 miles. It owes its existence to the dockyard established here by William III. in 1689, and is one of the chief naval arsenals in Great Britain, Pop. about 85,000. DEVONSHIRE, a county of England in the S. W. part. It has an area of 2,604.9 square miles, of which three- fourths are pasture land, or cultivated area. The north coast is steep and rocky. The south coast is lined with cliffs and indented with several bays. The general surface of the county is hilly. There are important agricultural industries and also considerable mining, manufacturing, and fishing. The chief cities are Exeter, the county town, Plymouth and Barnsta- ple. Population, about 460,000. DE VRIES, HUGO, a Dutch botanist, born in 1848 at Harlem. He was edu- cated at Leyden and German universi- ties, and in 1871 joined the staff of the University of Amsterdam as a lecturer. He afterward became professor of botany at that institution. He devoted special attention to the development of the theory of mutation and made important contributions to that branch of evolution- ary science. His researches resulted in a change of the method of studying evolu- tion from observation to experimental work. He wrote several books, including "Plant Breeding" (1907). DEW, a deposition of water from the atmosphere on the surface of the earth in the form of minute globules. During the day the earth both absorbs and emits heat, but after sunset its supply of warmth is cut off, while it still continues to radiate heat into the surrounding space. Grass, flowers, and foliage being good radiators, lose after sunset the heat which has previously been absorbed by them, without receiving any in return, and their temperature consequently falls considerably below that of the atmos- phere. From the proximity of these cold substances the particles of vapor in the adjoining air are condensed and deposited on their surfaces in the form of dew, or of hoar-frost where the temperature of the earth is below 32°. When the sky is clouded the heat abstracted from the earth's surface by radiation is restored by the clouds, which, being good radia- tors, send back an amount of heat equal to what they receive; and a balance of temperature being thus maintained be- tween the earth and the surrounding at- mosphere, no dew is formed. Horizontal surfaces, and those which are exposed to a wide expanse of sky, re- ceive a greater supply of dew than shel- tered or oblique surfaces, where circum- stances diminish the amount of radia- tion. The radiation from the earth's sur- face is one of these happy provisions for the necessities of living beings with which nature everywhere abounds. The heavy dews which fall in tropical regions are in the highest degree beneficial to vegetation, which, but for this supply of moisture, would, in countries where scarcely any rain falls for months, be soon scorched and withered. In cold climates the earth, being cold and suf- ficiently moist, requires little dew; ac- cordingly the clouds, which are so com- mon in damp and chilly regions, prevent the radiation of heat; the surface is thus preserved warm, and the deposition of dew is, in a great measure, prevented. DEWAR. SIR JAMES, a British scientist. He was bom in Scotland in 1842, and was educated at Dollar Acad- emy and Edinburgh University. He became assistant to Lord Playfair when Professor of Chemistry at Edinburgh