Book III. THE ATMOSPHERE. 357 



Empirical Equation, though tolerably accurate in the Northern Atlantic Ocean, to 

 latitude 60, are totally irreconcileable with observations in very high latitudes ; and on 

 the meridians, from 70 to 90 W. and E. of London. The results of late arctic 

 voyages, and of Russian travels, have been satisfactorily shewn, by Dr. Brewster (Edin. 

 Phil. Tr.), to prove the existence cf tivo meridians of greatest cold in the northern 

 hemisphere ; and the mean temperature of particular countries varies, not only according 

 to the parallels of latitude, but also according to their proximity to these two cold 

 meridians. ( T". ) 



2351. Inconsiderable seas, in temperate and cold climates, are colder in winter and 

 warmer in summer than the main ocean, as they are necessarily under the influence of 

 natural operations from the land. Thus the Gulf of Bothnia is generally frozen in 

 winter, but the water is sometimes heated in the summer to 70, a state which the 

 opposite part of the Atlantic never acquires ; the Gennan Sea is five degrees warmer in 

 summer than the Atlantic, and more than three colder in winter ; the Mediterranean is 

 almost throughout warmer both in winter and summer, which therefore causes the Atlantic 

 to flow into it; and the Black Sea, being colder than the Mediterranean, flows into the 

 latter. 



2352. The eastern parts of North America, as it appears from meteorological tables, have a much colder 

 air than the opposite European coast, and fall short of the standard by about ten or twelve degrees. 

 There are several causes which produce this considerable difference. The greatest elevation in North 

 America is between the 40th and 50th degree of north latitude, and the 100th and 110th of longitude west 

 from London ; and there the most considerable rivers have their origin. The height alone will partly 

 explain why this tract is colder than it would otherwise be ; but there are other causes, and those are 

 most extensive forests, and large swamps and morasses, all of which exclude heat from the earth, and 

 consequently prevent it from ameliorating the rigour of winter. Many extensive lakes lie to the east, and 

 Hudson's Bay more to the north ; a chain of mountains extends on the south of the latter, and those 

 equally prevent the accumulation of heat; besides, this bay is bounded on the east by the mountainous 

 country of Labrador, and has many islands; from all which circumstances arise the lowness of the 

 temperature, and the piercing cold of the north-west winds. The annual decrease of the forests for the 

 purpose of clearing the ground, and the consumption for building and fuel, is supposed to have occasioned 

 a considerable decrease of cold in the winter; and if this should be the result, much will yet be done 

 towards bringing the temperature of the European and American continents to something like a level. 



2353. Continents have a colder atmosphere than islands situated in the same degree of 

 latitude ; and countries lying to the windward of the superior classes of mountains, or 

 forests, are warmer than those which are to the leeward. Earth always possessing a 

 certain degree of moisture, has a greater capacity to receive and retaiil heat than sand 

 or stones, the latter therefore are heated and cooled with more rapidity : it is from 

 this circumstance that the intense heats of Africa and Arabia, and the cold of Terra del 

 Fuego, are derived. The temperature of growing vegetables changes very gradually; 

 but there is a considerable evaporation from them : if those exist in great numbers, and 

 congregated, or in forests, their foliage preventing the rays of the sun from reaching the 

 earth, it is perfectly natural that the immediate atmosphere must be greatly aflfected by the 

 ascent of chilled vapours. 



2354. Our next object is tlie ascent and descent of water: the principal appearances of 

 this element are vapour, clouds, dew, rain, frost, hail, snow, and ice. 



2355. Vapour is water rarefied by heat, in consequence of which, becoming lighter than 

 the atmosphere, it is raised considerably above the surface of the earth, and afterwards by 

 a partial condensation forms clouds. It differs from exhalation, which is properly a 

 dispersion of dry particles from a body. When water is heated to 212 it boils, and is 

 rapidly converted into steam ; and the same change takes place in much lower 

 temperatures ; but in that case the evaporation is slower, and the elasticity of the steam 

 is smaller. As a very considerable proportion of the earth's surface is covered with water, 

 and as this water is constantly evaporating and mixing with the atmosphere in the state 

 of vapour, a precise determination of the rate of evaporation must be of very great im- 

 portance in meteorology. Evaporation is confined entirely to the surface of the water ; 

 hence it is, in all cases, proportional to the surface of the water exposed to the atmosphere. 

 Much more vapour of course rises in maritime countries or those interspersed with lakes, 

 than in inland countries. Much more vapour rises during hot weather than during 

 cold : hence the quantity evaporated depends in some measure upon temperature. The 

 quantity of vapour which rises from water, even when the temperature is the same, varies 

 according to circumstances. It is least of all in calm weather, greater when a breeze 

 blows, and greatest of all with a strong wind. From experiments, it appears, that 

 the quantity of vapour raised annually at Manchester is equal to about 25 inches of 

 rain. If to this we add five inches for the dew, with Dalton, it will make the annual 

 evaporation 30 inches. Now, if we consider the situation of England, and the greater 

 quantity of vapour raised from water, it will not surely be considered as too great an 

 allowance, if we estimate the mean annual evaporation over the whole surface of the 

 globe at 35 inches. 



2356. A cloud is a mass of vapour, more or less opaque, formed and sustained at 

 considerable height in the atmosphere, probably by the joint agencies of heat and 



Aa 3 



