350 SCIENCE OF AGRICULTURE. Pakt II. 



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. 



2285. The eastern parts of North America, as a^T^eaxs 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 Lon- 

 don ; and there the most considerable rivers have their origin. The height alone is sufficient to make this 

 tract colder than it would otherwise be ; but there are other causes, and those are most extensive forests, 

 and large swamps and morasses, each of which ;exclude heat from the earth, and consequently prevent it 

 from ameliorating the rigor 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 accu- 

 mulation 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. 



2286. 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 cer- 

 tain degree of moisture, has a greater capacity to receive and retain heat than sand or 

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

 cumstance tliat 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 congre- 

 gated, or in forests, their foliage preventing the rays of the sun from reaching the earth, it 

 is perfectly natural that the imiinediate atmosphere must be greatly affected by the ascent 

 of chilled vapors. 



2287. Our next object is the ascent and descent of water : the principal appearances of 

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



2288. Vapor 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 dis- 

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

 pidly converted into steam ; and the same change takes place in much lower temper- 

 atures ; 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 

 vapor, a precise determination of the rate of evaporation must be of very great import- 

 ance in meterology. 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 vapor of course rises in maritime countries or those interspersed with lakes, 

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

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

 quantity of vapor 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 vapor raised annually at Manchester is equal to about 25 inches of rain. If 

 to tliis we add five inches for the dew, with Dalton, it will make the annual evapor- 

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

 of vapor 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. 



2289. A cloud is a mass of vapor, more or less opaque, formed and sustained at con- 

 siderable height in the atmosphere, probably by the joint agencies of heat and electricity. 

 The first successful attempt to arrange the diversified form of clouds, under a few general 

 modifications, was made by Luke Howard, Esq. We shall give here a brief account of 

 his ingenious classification. 



2290. The simple modifications are thus named and defined : 1 . Cirrus, parallel, 

 flexuous, or diverging fibres, extensible in any or in all directions (Jig. 248 a) ; 



2. Cumulus, convex or conical heaps, increasing upwards from a horizontal base (b) ; 



3. Stratus, a widely-extended, continuous, horizontal sheet, increasing from below (c). 



2291. The intermediate modifications which require to be noticed are, 4. Cirro-cumulus, 

 small, well-defined, roundish masses, in close horizontal arrangement {d) ; 5. Cirro-stratus, 

 horizontal or slightly inclined masses, attenuated towards a part or the whole of their 

 circumference, bent downward or undulated, separate or in groups consisting of small 

 clouds having these characters (e). 



2292. The compound modifications are, 6. Cumulo-stratus, or twain cloud ; the cirro-' 

 stratus, blended with the cumulus, and either appearing intermixed witli the heaps of the 



