700 LECTUKE LVI. 



and for this reason, the most elevated tracts of land, which are the most 

 prominent, and the most exposed to the effects of the atmosphere, are always 

 colder than situations nearer the level of the sea. The northern hemisphere 

 is somewhat warmer than the southern, perhaps because of the greater pro- 

 portion 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 the sun com- 

 pensates precisely for the different velocity of the earth in its orbit, with 

 respect to the whole quantity of heat received on either side of the equinoctial 

 points, yet Mr. Provost has shown, that in all probability the same quantity 

 of heat must produce a greater effect when it is more slowly applied; because 

 the portion lost by radiation from the heated body is greater, as the tempera- 

 ture is higher. Since, therefore, on account of the eccentricity of the earth's 

 orbit, the north pole is turned towards the sun 7 or 8 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 equalises the temperature, and 

 partly for other reasons. The comparative intensity of tlie southern summer 

 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 IB" or 

 S0° ; in some parts even 30°; and floating ice has occasionally teen 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 6o° 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 fe8°, two miles; in the latitude of 

 London, a little more than a mile; and in latitude 80° north, only 1200 feet. 

 At the pole, according to the analogy deduced by Mr. Kirwan, from a com- 

 parison of various observations, the mean temperature should be 31°. In 

 London the mean temperature is 50"* ; at Rome and at iNlontpelier, a little 

 more than 60° ; in the island of Madeira, 70°; and in Jamaica, 80°. 



There are frequently some local causes of heat and coldM'hich are independ- 



