260 



SCIENCE OF GARDENING 



Part II. 



Now the force of vapors at 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 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 preced- 

 ing observations. 



1251. The mean monthly and annual quantities of rain at various places, deduced from 

 the average for many years, by Dal ton, is given in the following Table : 



1252. 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 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, 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. 



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 therefore 

 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. 



1253. 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. 



1254. Snow is formed by the freezing of the vapors in the atmosphere. It differs from 

 hail and hoar frost, in being as it were crystallised, which they are not. As the flakes 

 fall down through the atmosphere, they are continually joined by more of these radiated 

 spicula, and they increase in bulk like the drops of rain or hailstones. The lightness ot 

 snow, although it is firm ice, is owing to the excess of its surface in comparison to the 

 matter contained under it: as gold itself may be extended in surface till it will rule 

 upon the least breath of air. The whiteness of snow is owing to the small particles into 

 which it is divided ; for ice when pounded, will become equally white. 



1255. Snow is of great use to the vegetable kingdom. Were we to judge from appearance 

 only, we might imagine, that so far from being useful to the earth, the cold humidity ot 

 snow would be detrimental to vegetation. But the experience of all ages asserts the con- 



