Book II. OF THE ATMOSPHERE. 261 



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 ram 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 die 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 or 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 tiiey 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. 



1 256. 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, name- 

 ly, about one eighdi part ; and this increase of dimensions is acquired with prodigious 

 force, sufficient to burst the strongest iron vessels, and even pieces of artillery. Congel- 

 ation 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 directions, 

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

 by the alternation : so that where ice has once been formed, its production is in this manner 

 redoubled. 



1257. 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 niean temperature should be 31. In London 

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

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



1 258. 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, 

 particularly in July and August ; the north-east prevails during January, March, April^ 

 May, and June, and is most unfrequent in February, July, September, and December 

 the north-west occurring more frequently from November to March, and less so in 

 September and October than in any other months. 

 Near Glasgow, the average is stated as follows : 



Winds. Days. Winds. Daus 



South-west - - 174 North-east - . 104 



North-west 40 J South-east - - 47 



In Ireland, the prevailing winds are the west and south-west. 



1 259. The different degrees of motion of wind next excites our attention : and it seems al- 



S 3 



