ON CLIMATES AND WINTJS, 657 



is usuall)' filled with spirit of wine, which is in contact with a portion of mer- 

 cury occupying the lower part of the tube; and this is succeeded by a second 

 portion of spirit. The mercury carries on each of its surfaces an index, which 

 is retained in its remotest situation by means of a weak spring; and conse- 

 quently shows the greatest degree of heat or of cold that has happened since 

 the last observation. The indexes are of iron or steel, and may be brought 

 back to the surface at pleasure by means of a magnet; they are carried up by 

 the mercury, more by its capillary action, than by the difference of the specific 

 gravities. A similar effect is obtained in Rutherford's arrangement of a pair 

 of thermometers, one with mercury, the other with spirit of wine, placed in a 

 horizontal position; one index being without the surface of the mercury, the 

 other within that of the spirit: the thermometers being in contrary direc- 

 tions, both indexes may be brought back to their places, by merely raising the 

 end of the instrument. Self registering thermometers have also sometimes 

 been constructed, for keeping a still more accurate account of all the varia- 

 tions of temperature that have occurred, by describing a line on a revolving 

 barrel, which shows the height for every instant during the whole time of 

 their operation. (Plate XLI. Fig. 579, 580.) 



The climates of different parts of tlie earth's surface are unquestionably 

 owing in great measure to their position with respect to the sun. At the equa- 

 tor, where the sun is always nearly vertical, any given part of the surface re- 

 ceives a much greater (juantity of light and heat, than an equal portion near 

 the poles; and it is also still more affected by the sun's vertical rays, because 

 their passage through the atmosphere is shorter than that of the oblique rays. 

 As far as the sun's mean altitude only is concerned, it appears from Simpson's 

 calculations, that the heat received at the equator in the whole year, is nearly" 

 twice and a half as great as at the poles; this proportion being nearly tlie 

 same as that of the meridian heat of a vertical sun, to the heat derived, at the 

 altitude 234^°, in the middle of the long annual day at the poles. Hut the 

 difference is rendered still greater, by the effect of the atmosphere, which in- 

 terrupts a greater portion of the heat at the poles than elsewhere. IJouguer 

 has calculated, upon the supposition of the similarity of the affections of heat 

 and light, that in latitude 45°, 80 parts out of 100 are transmitted at noon 

 in July, and 55 only in December. The heat intercepted by the atmosphere 

 is perhaps not wholly, but very nearly, lost with respect to the climate of the 



