1397. The northern ice extends during summer about 9ï¾¦ from the pole; the southern 18ï¾¦ or 20ï¾¦; in some parts even 30ï¾¦; and floating ice has occasionally been found in both hemispheres as far as 40ï¾¦ from the poles, and sometimes, as it has been said, even in latitude 41ï¾¦ or 42ï¾¦. Between 54ï¾¦ and 60ï¾¦ 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 84ï¾¦; at Teneriffe, in latitude 28ï¾¦, two miles; in the latitude of London, a little more than a mile; and in latitude 80ï¾¦ north, only 1250 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 31ï¾¦. In London the mean temperature is 50ï¾¦; at Rome and at Montpelier, a little more than 60ï¾¦; in the island of Madeira, 70ï¾¦; and in Jamaica, 80ï¾¦.