UO'O 



SCIENCE OF AGRICULTURE. 



Paut II. 



2368. Phenomena of rain. Rain never descends till the transparency of the air ceases, and the invisible 

 vapours liecome vesicular, when clouds form, and at length the drops fall : clouds, instead of forming 

 gradually at once throughout all parts of the horizon, generate in a particular spot, and imperceptibly in- 

 crease till the whole expanse is obscured. 



2369. The cause of rain is thus accounted for by Hutton and Dalton. If two masses 

 of air of unequal temperatures are, when saturated with vapour, intermixed by the 

 ordinary currents of the winds, a precipitation ensues. If the masses are under satu- 

 ration, then less precipitation takes place, or none at all, according to the degree. Also 

 the warmer the air, the greater is the quantity of vapour precipitated in like circumstances. 

 Hence the reason why rains are heavier in summer than in winter, and in warm countries 

 than in cold. 



2370. The quantity of raiiiy taken at an annual mean, is the greatest at the equator, and 

 it lessens gradually to the poles ; at which there are fewer days of rain, the number in- 

 creasing in proportion to the distance from them. From north latitude 1 2^ to 43" the 

 mean number of rainy days is 78 ; from 43 to 46 the mean number is 103 ; from 46 

 to 50, 134; and from 51 to 60, 161. Winter often produces a greater number of 

 rainy days than summer, though the quantity of rain is more considerable in the latter 

 than in the former season ; at Petersburgh rain and snow fall on an average 84 days of 

 the winter, and the quantity amounts to about five inches ; on the contrary, the summer 

 produces eleven inches in about the same number of days. Mountainous districts are 

 subject to great falls of rain ; among the Andes particularly, it rains almost incessantly, 



. while the flat country of Egypt is consumed by endless drought. Dalton estimates the 

 quantity of rain falling in England at 3 1 inches. The mean annual quantity of rain for 

 the whole globe is 34 inches. 



2371. The cause why less rainfalls in the first six months of the year than in the last sir 

 months is thus explained. The whole quantity of water in the atmosphere in January 

 is usually about three inches, as appears from the dew point, which is then about 32 ; 

 now the force of vapours of that temperature is 0*2 of an inch of mercury, which is equal 

 to 2*8 or three inches of water. The dew point in July is usually about 58 or 59, cor- 

 responding to 0-5 of an inch of mercury, which is equal to seven inches of water. Thus 

 it is evident that, in the latter month, the atmosphere contains four inches of water more 

 than in the former month. Hence, supposing the usual intermixture of currents of air 

 in both the intervening periods to be the same, the rain ought to be four inches less in 

 the former period of the year than the average, and four inches more in the latter period, 

 making a difference of eight inches between the two periods, which nearly accords with 

 the preceding observations. 



2372. The mean monthly and annual quantities of rain at various places, deduced from 

 the average for many years, by Dalton, is given in the following Table ; 



2373. Frost, being derived from the atmosphere, naturally proceeds from the upper parts 

 of bodies downwards ; so the longer a frost is continued, the thicker the ice becomes 

 upon the water in ponds, and the deeper into the earth the ground is frozen. In about 

 16 or 17 days' frost, Boyle found it had penetrated 14 inches into the ground. At 

 Moscow, in a hard season, the frost will penetrate two feet deep into the ground ; and 

 Captain James found it penetrated 10 feet deep in Charlton Island, and the water in the 

 same island was frozen to the depth of six feet. Scheffer assures us, that in Sweden the 

 frost pierces two cubits (a Swedish ell) into the earth, turning what moisture is found 

 there into a whitish substance like ice ; and into standing water three ells or more. The 

 same author also mentions sudden cracks or rifts in the ice of the lakes of Sweden, nine 

 or ten feet deep, and many leagues long ; the rupture being made with a noise not less 



