FREEZING.
During congelation most of the gasi form fluids, which may have been con tained in the water, are separated in the elastic form, and exhibit bubbles in the ice, unless the congelation may have been gradually effected from the bottom, or one of the sides; in which case the bub bles are driven out, and the ice is much clearer.
Ice is considerably lighter than water, namely, about one-eighth part ; and this increase of dimensions is acquired with prodigious force, sufficient, to burst the strongest iron vessels, and even pieces of artillery. It does not arise from the ex trication of the gases ; for the refractive power of ice is less than that of water, as Dr. Hooke long ago shewed, and has since been confirmed by Wollaston.
M. Prevost observes, that congelation takes place much more suddenly than the opposite process of liquefaction ; and that, of course, the same quantity of heat must be more rapiolty extricated in freez ing, than is absorbed in thawing; that the heat thus extricated being disposed to fly off in all directions, and little of it being retained by the neighbouring bo• dies, more heat is lost than is gained by the alteration : so that, where ice has once been formed, its production is in this manner redoubled. This circumstance must occur whenever it freezes, that is, on shore, in latitudes above 35 degrees ; and it appears, from 30 degrees to the pole, the land is somewhat colder than the sea, and the more as it is farther distant from it ; and nearer the equator the land is warmer than the sea : but the process of congelation cannot, by any means, be the principal cause of the difference, and it is probable that the different capacity of earth and water for heat is materially concerned in it.
Since the atmosphere is very little heat ed by the passage of the sun's rays through it, it is naturally colder than the earth's surface ; and for this reason the most elevated tracts of land, which are the most prominent, and the most expos ed to the effects of the atmosphere, are always colder than situations near the level of the sea.
The northern hemisphere is somewhat warmer than the southern, perhaps be cause of the greater proportion of land that it contains, and also in some measure , on account of the greater length of its summer than that of the southern ; for although, as it was long ago observed by Simpson, the different distance of thelmn compensates precisely for the different velocity of the earth in its orbit, with re spect to the whole quantity of heat re ceived on either side of the equinoctial points, yet M. Prevost has shewn that, in
all probability, the same quantity of heat must produce a greater effect when it is more slowly applied ; because the por tion lost by radiation from the heated bo dy is greater as the temperature is high er. Since, therefore, on account of the eccentricity of the earth's orbit, the north pole is turned towards the sun seven or eight days longer than the south pole, the northern winters must be milder than the southern ; yet the southern summers, though shorter, ought to be somewhat warmer than the northern ; but, in fact, they are colder, partly perhaps from the much greater proportion of sea, which in some degree equalizes the temperature, and partly for other reasons. The com parative intensity of the southern sum mer and winter is not exactly known ; but in the island of New Georgia, the summer is said to be extremely cold.
The northern ice extends about 9° from the pole; the southern 18° or 20°; in some parts even 3U° ; and floating ice has occasionally been found in both he mispheres 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 conge lation is three miles above the surface at the equator, where the mean heat is at Teneriffe, in latitude 28°, two miles ; and in the latitude of London, a little more than a mile; and in latitude 80° north only 1,200 feet. At the pole, ac cording to the analogy deduced by Mr. Kirwan, from a comparison of various ob servations, the mean temperature should be 31°. In London, the mean tempera ture is 50° ; at Rome, and at Montpelier, a little more than 60°; in the island of Madeira 70° ; and in Jamaica 80°.