Book III. 



OF THE ATMOSPHERE. 



353 



to 50, liH; and from 51 to &^, 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 falls on an average 84 days of the 

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

 duces eleven inches in about the same number of days. Mountainous districts are sub- 

 ject 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 31 inches. The mean annual quantity of rain for the whole 

 globe is 34 inches. 



2304. The cause why less rainfalls in the first six months of the year than in the last six 

 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 vapors 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 ; the 

 difference is four inches of water, which the atmosphere then contains more than in the 

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

 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 tlie two periods, which nearly accords with the pre- 

 ceding observations. 



2305. T/ie 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. 



2306. Frost, being derived from the atmosphere, naturally proceeds from the upper parts 

 of bodies downwards, as the water and the earth ; so the longer a frost is continued, the 

 thicker the ice becomes upon the water in ponds, and tlie 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, and turns what 

 moisture is found there into a whitish substance, like ice ; and standing water to 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 loud than if many guns were discharged together. By such means 

 however the fishes are furnished with air, so that they are rarely found dead. 



2307. The history of frosts furnishes very extraordinary facts. The trees are often scorched and burnt up, 

 as with the most excessive heat, in consequence of the separation of water from the air, which is tlierefore 

 very drying. In the great frost in 1683, the trunks of oak, ash, walnut, and other trees, were miserably 

 split and cleft, so that they might be seen through, and the cracks often attended with dreadful noises like 

 the explosion of fire-arms. 



2308. Hail is generally defined as frozen rain, it differs from it in that the hailstones 

 are not formed of single pieces of ice, but of many little spherules agglutinated together; 

 neither are those spherules all of the same consistence ; some of them being hard and 

 solid, like perfect ice ; others soft, and mostly like snow hardened by a severe frost. 

 Hailstone has a kind of core of this soft matter ; but more frequently the core is solid 

 and hard, while the outside is formed of a softer matter. Hailstones assume various 

 figures, being sometimes round, at other times pyramidal, crenated, angular, thin, and 

 flat, and sometimes stellated with six radii, like the small crystals of snow. Natural 

 historians furnish us with various accounts of surprising showers of hail in which the 

 hailstones were of extraordinary magnitude. 



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