Book III. OF THE ATMOSPHERE. 361 



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. 



2574. 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. 



2375. Huil is generally defined as frozen rain ; it differs from it in that the hailstones 

 for the most part 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 sometimes 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 or 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. 



2376. Snow is formed by the freezing of the vapours in the atmosphere. It differs 

 from hail and hoar frost, in being as it were crystallised, while 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 with 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. 



2377. 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 o e 

 them contains 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 he 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. Different vegetables 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, pro- 

 vided 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. 



2378. 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, sufficient 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 neighbouring 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. 



2379. The northern ice extends during summer about 9° from the pole ; the southern 

 1S° 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 1 250 feet. At the pole, according to the analogy deduced by Kirwan, from 

 Mayer's Formula, and which is not however found to agree very exactly with what takes 

 place, from a comparison of various observations, the mean temperature should be 31°. 



