The ice at each pole of the earth forms an immense cupola, the arch of which extends some thousand miles over the continents; the thickness of which, beyond the 60th degree of latitude, is several hundred feet. Navigators have assigned to detached masses, which are met with floating at sea, an elevation of from 1,500 to 1,800 feet.[80] There can be no doubt but that the thickness of these cupolas of ice is much greater nearer the poles; for astronomy sometimes presents in the heavens so vast an image of them, that the rotundity of the earth seems to be considerably affected thereby. Captain Cook could never approach nearer the south pole, where there is no land, than the 70th degree of latitude; that is, no nearer than 1,500 miles; and it was only under the favor of a bay, that he was permitted to advance even so far.[81] All the results of observations made by navigators, concur in proving that the temperature of the sea decreases according to the depth; and that the deepest gulfs are continually covered with ice, even under the equator. From a late memoir by M. Perron, some say, there is reason to believe that these mountains of ice at the poles, which have hitherto impeded the progress of European navigators, have been detached from the depths of the sea to float at the surface.[82]
When water is converted into ice, it is lighter[83] than when in a fluid state, which is a circumstance of great importance. Galileo was the first who observed this. Ice consequently floats upon water, its specific gravity being to that of water as eight to nine. This rarefaction seems to be owing to the air-bubbles produced in water by freezing; and which, being considerably larger in proportion to the water frozen, render the body so much specifically lighter: these air-bubbles, during their production, acquire a great expansive power, so as to burst the containing vessels though ever so strong.
[The specific weight of ice is known to be less than that of water. Our author assigns a reason not entirely satisfactory. We must admit that the freezing of the upper stratum of water, although it may _include_ the air which was in the water frozen, yet, _it does not expel the air from the subjacent volumes of water_. Hence the air in the water below will balance the effects of the air included in the ice.