KAILWAY RAIN 185 ted States and Canada reference should be made to the manuals published annually in New York by H. V. and H. W. Poor and Edward Vernon; and for methods of con- struction to the various works in English, French, and German upon railroad engineering. RAILWAY, Atmospheric. See PNEUMATIC DES- PATCH, and PNEUMATIC RAILWAY. RAIMONDI, Marc' Antonio, an Italian engraver, born in Bologna about 1480, died there subse- quent to 1539. He was instructed in design by Francesco Francia, some of whose pictures he engraved as early as in his 15th year. Sub- sequently he imitated Albert Dilrer, and while on a visit to Venice made facsimile copies on copper of his set of 36 woodcuts representing the life and passion of the Saviour, and of another set of 17, representing the life of the Virgin ; and the imitation was so exact that Raimondi's prints sold for originals. Diirer was obliged to visit Venice to procure redress, but only succeeded in preventing the use of his monogram. Raimondi soon went to Rome, where he was employed in engraving the choice works of Raphael, and afterward of Giulio Ro- mano. He is said to have excited the wrath of Pope Clement VII. by executing a set of obscene prints, for which he was thrown into prison; but he appeased him by an admira- ble engraving after Bandinelli's picture of the "Martyrdom of St. Lawrence." The sack of the city by the army of the constable de Bour- bon in 1527 reduced him to poverty, and he returned to Bologna. For purity of outline, correct expression, and drawing, he was one of the best engravers on record. The British museum has 500 of his choicest productions. RAIN, the moisture of the atmosphere con- densed into drops large enough to fall with perceptible velocity to the earth. The water thus precipitated is quite pure, except in so far as it absorbs a slight quantity of air, carbonic acid, ammonia, or nitric acid, from the atmos- phere. The formation of rain is in general a continuation of the processes of the formation of clouds, dew, and fog. The deposition of moisture depends upon the cooling of the at- mosphere, as was first recognized by Dalton (1787), but concerning the precise process by which that cooling is effected erroneous views have been widely entertained. In general it may be said that the temperature of a given mass of warm moist air is lowered in the ordinary course of atmospheric phenomena by one or another of the following four process- es : 1, by radiation to the cold sky, according to the views first developed by Fourier (1812) and Wells (1818) ; 2, by radiation to neighbor- ing masses of cold air, or the cold ground ; 3, by mixture with cooler air, a view suggested by Hutton (1787), the slight importance of which was demonstrated by Espy (1833, &c.) ; 4, by the absorption of heat in the expansion of ascending air, a view first developed by Espy. Radiation takes place especially at night du- ring the winter, when the upper regions of the atmosphere contain but little moisture to hinder the free radiation of heat, but the re- sult is more frequently snow or cloud than rain ; its importance has been well shown by Tyndall (1859, &c.). The third process de- pends for its effect upon the principle that the density of saturation increases faster than the temperature, so that if we mix two equal vol- umes of saturated air having different tem- peratures, the resulting mixture will have a temperature somewhat less than the average of the two, and the quantity of moisture then present will be slightly in excess ; but the con- densation of even a slight portion of this moist- ure into cloud evolves latent heat sufficient to elevate the temperature above the point of con- densation. The fourth of the above processes is doubtless by far the most efficient of all in lowering the temperature and producing rain. Its precise importance in the economy of the atmosphere has been well shown by the re- searches of Espy (1833, &c.), Thomson (1862), Peslin (1868), Him (1870), Reye (1872), and Hann (1874). According to Hann, the ascent of dry air to higher altitudes must, by reason of its expansion under the lower pressure there prevailing, be from this cause alone attended with a uniform diminution of temperature at the rate of 0-9907 C. per 100 metres of ascent, a rate that becomes 0'9751 when the moisture in the air gives it a relative humidity of 60 per cent. But so soon as by this cooling the air is brought to its point of saturation and the forma- tion of cloud or rain, snow, or hail begins, the evolution of latent heat largely reduces the rate of diminution of temperature. As the saturated air ascends in the form of cloud, its temperature no longer diminishes uniformly, but at a decreasing rate, so that a point may be ultimately reached where its rate of diminution becomes zero. In general, therefore, clouds thus formed are warmer than the adjacent clear air. Doubtless it rarely happens but that rainfall is produced by the concomitant action of two or more of our four principles. Such attempts as have been made to show that elec- tricity has an influence in causing rain must at present be considered "wholly unsatisfactory ; yet it is acknowledged that the electrical dis- plays which so frequently accompany rain, and especially hail, are but very imperfectly under- stood. Of other phenomena attending the for- mation of rain, the most important is the gen- eral elevation of temperature on the earth's surface, which is largely due to the great amount of sensible heat thrown into the at- mosphere by the condensation of vapor into rain. The other important factor in this ele- vation of temperature is probably the protec- tion afforded by the clouds against radiation, so that any heat which emanates from the surface of the earth is retained under the cov- ering of clouds. The distribution of rain over the surface of the globe is of importance to the interests of mankind both as regards its quan- tity and its frequency. Aa regards the quan-
