ON THE SOURCES OF HEAT WHICH INFLUENCE CLIMATE. 459 



the short period of six days at the autumnal and vernai equinox. In June and 

 July the sun, being nearly vertical over the extreme limit of the northern tropic, 

 gives to that latitude the greatest temperature the solar rays are capable of 

 imparting. This necessarily elevates the snow line to its highest point above 

 the level of the sea. On the Andes, at a distance of 17 degrees from the equator, 

 it is found at an elevation of 17,000 feet ; directly under the equator it is said to 

 be 1,000 feet lower, but this appears to me to want confirmation ; for though the 

 limits of the tropics have nearly two months or more of vertical sun's rays than 

 the equatoi - , it is to be remembered that for all the rest of the year countries 

 under the equator have the solar rays less obliquely than those near the tropics. 

 Since high table-lands have a more elevated temperature than isolated mountains 

 of the same height, it requires caution in the choice of the situations for deter- 

 mining by measurement the snow line. The Himalaya mountains afford a 

 remarkable instance of this on the southern side ; where we should expect the 

 sun's rays to have far the more influence, we find the snow line at an elevation 

 3,000 feet lower than on the northern side, where, in addition to the slope of the 

 ground being most unfavourable for the reception of the solar heat, the station is 

 a degree further from the equator than that of the southern side. An immense 

 table-land of 10,000 feet elevation, stretching from the northern basis of the 

 Himalaya range, thus appears comparatively to reduce the altitude of places on 

 this side of the mountains. 



Some observations made on the Andes would certainly seem to support the 

 belief that at the equator the snow-line is depressed below the elevation assigned 

 to it nearer the tropics ; but since we have, as yet, no proof that the mean tem- 

 perature of the year is greater at the limits of the equinoctial regions than under 

 the equator, I think we cannot safely assign a depression of 1,000 feet to the 

 snow-line as it crosses this parallel. — After leaving the southern tropic, we find 

 it descend much more rapidly towards the earth than in the northern hemisphere. 

 Those parts of New Zealand corresponding in latitude to Bordeaux, do not enjoy 

 a warmer climate than the north of England, while Terra del Fuego, though as 

 distant from the South Pole as the country which lies between London and 

 Edinburgh is from the North, has the climate of Norway. The snow-line is 

 elevated only 3,000 or 3,500 feet above the level of the sea, while with us it is 

 not to be found at an elevation under 5,000 feet. The statements of our 

 mariners also prove the severity of southern winters, as they frequently expe- 

 rience difficulty and detention while doubling Cape Horn, from fields of ice 

 extending as far south as the 54th parallel. 



It was my wish, from some of the empirical formula? which have been con- 

 structed by Meyer, Sir D. Bkewster and others for climate, to ascertain the 



