ON CLIMATES AND WINDS. 545 



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

 directions, 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 variations 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 the earth's surface are unquestionably 

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

 equator, where the sun is always nearly vertical, any given part of the 

 surface receives a much greater quantity of light and heat, than an equal 

 portion near the poles ; and it is also still more affected by the sun's verti- 

 cal 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 the same as that of the meridian heat 

 of a vertical sun, to the heat derived, at the altitude 23, in the middle 

 of the long annual day at the poles. But the difference is rendered still 

 greater, by the effect of the atmosphere, which interrupts a greater portion 

 of the heat at the poles than elsewhere. Bouguer 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 neigh- 

 bouring places. It is obvious that, at any individual place, the climate 

 in summer must approach in some degree to the equatorial climate, the 

 sun's altitude being greater, and in winter to the climate of the polar 

 regions. 



While the earth is becoming warmer at any particular spot, the heat 

 thrown off by radiation into the atmosphere, and thence into the empty 

 space beyond it, together with that which is transmitted to the internal 

 parts of the earth, must be less than the heat received from the sun ; and 

 when the earth is growing colder, more heat must pass off than is received : 

 but whenever the heat of the surface is stationary, neither increasing nor 

 diminishing, as at the times of the greatest and least heat, it is obvious 

 that the heat received from the sun must be precisely equal to the heat 

 which is thrown off. Now this quantity may be estimated by the degree 

 of refrigeration in the night ; and hence Mr. Prevost* has very ingeniously 

 deduced the proportion of the sun's heat arriving at the surface of the earth 

 in the latitude of Geneva, in July, and in December ; which he finds to be 

 as 7 or 8 to 1 ; and this result agrees very well with a calculation deduced 

 from the length of the day, the sun's altitude, and the interception of his 

 rays by the atmosphere. 



In London the temperature generally varies, in the course of the day 

 and night, somewhat more than 5, and less than 20. In January, the 

 mean diffrnal variation of temperature is 6, in March 20, in July 10, 

 * Jour, de Phy. xlii. 81. 

 2 N 



