ON THE SOURCES OF HEAT WHICH INFLUENCE CLIMATE. 
459 
the short period of six days at the autumnal and vernal 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 1? 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 equator, 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 formulae which have been con¬ 
structed by Meyer, Sir D. Brewster and others for climate, to ascertain the 
