af54 SCIENCE OF AGRICULTURE. Part II. 



2309. 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 of 

 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 ride 

 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. 



2310. 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 of 

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

 trary. Snow, particularly in those northern regions where the ground is covered with it 

 for several months, fructifies the earth, by guarding the corn or other vegetables from 

 the intenser cold of the air, and especially from the cold piercing winds. It has been 

 a vulgar opinion, very generally received, that snow fertilises the land on which it falls 

 more than rain, in consequence of the nitrous salts which it is supposed to acquire 

 by freezing. But it appears from the experiments of Margraaf, in the year 1731, that 

 the chemical difference between rain and snow-water, is exceedingly small; that the 

 latter contains a somewhat less proportion of earth than the former ; but neither of 

 them contain either earth, or any kind of salt, in any quantity which can be sensibly 

 efficacious in promoting vegetation. The peculiar agency of snow as a fertiliser, in 

 preference to rain, may be ascribed to its furnishing a covering to the roots of vegetables, 

 by which they are guarded from the influence of the atmospherical cold, and the 

 internal heat of the earth is prevented from escaping. The internal parts of the earth 

 are heated uniformly to the fifty-eighth degree of Fahrenheit's thermometer. This degree 

 of heat is greater than that in which the watery juices of vegetables freeze, and it is pro- 

 pagated from the inward parts of the earth to the surface, on which the vegetables grow. 

 The atmosphere, being variably heated by the action of the sun in different climates, and 

 in the same climate at different seasons, communicates to the surface of the earth, and to 

 some distance below it, the degree of heat and cold which prevails in itself. Different ve- 

 getables are able to preserve life under different degrees of cold, but all of them perish 

 when the cold which reaches their roots is extreme. Providence has, therefore, in the 

 coldest climates, provided a covering of snow for the roots of vegetables, by which they are 

 protected from the influence of the atmospherical cold. The snow keeps in the internal 

 heat of the earth, which surrounds the roots of vegetables, and defends them from the 

 cold of the atmosphere. 



2311. Ice is water in the solid state, during which the temperature remains constant, 

 being 32 degrees of the scale of Fahrenheit, Ice is considerably lighter than water, 

 namely, about one-eighth part ; and this increase of dimensions is acquired with prodi- 

 gious force, suflScient to burst the strongest iron vessels, and even pieces of artillery. 

 Congelation takes place much more suddenly than the opposite process of liquefaction ; 

 and of course, the same quantity of heat must be more rapidly extricated in freezing than 

 it is absorbed in thawing ; the heat thus extricated being disposed to fly off in all direc- 

 tions, and little of it being retained by the neighboring bodies, more heat is lost than 

 is gained by tlie alternation : so that where ice has once been formed, its production is 

 in this manner redoubled. 



2312. The northern ice extends about 9 from the pole ; the southern 18 or 20 ; in 

 some parts even 30 ; and floating ice has occasionally been found in both hemispheres 

 as far as 40 from the poles, and sometimes, as it has been said, even in latitude 41 or 

 42. Between 54 and 60 south latitude, the snow lies on the ground, at the sea-side, 

 throughout the summer. The line of perpetual congelation is three miles above the 

 surface at the equator, where the mean heat is 84 ; at Teneriffe, in latitude 28, two 

 miles ; in the latitude of London, a little more than a mile ; and in latitude 80 north, 

 only 1250 feet. At the pole, according to the analogy deduced by Kirwan, from a 

 comparison of various observations, the mean temperature should be 31. In London 

 the mean temperature is 50 ; at Rome and at Montpellier, a little more than 60 ; in 

 the island of Madeira, 70 ; and in Jamaica, 80. 



2313. Wind. Were it not for this agitation of the air, putrid effluvia arising from the 

 habitations of man, and from vegetable substances, besides the exhalations from water, 

 would soon render it unfit for respiration, and a general mortality would be the conse- 

 quence. The prevailing winds of our own country, which were ascertained by order of 

 the Royal Society of London, at London, are, 



The south wind blows more upon an average in each month of the year than any other, 



