(t(9"8 lecture xv«. 



neighbonring places. It is obvious that, at any individual j)lace, tlie climate ia 

 summer must approach in some degree to the equatorial climate, the sun's al- 

 titude 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. Pr6vost has very ingeniously deduced the proportion of the sun's 

 beat 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 

 verj-^ 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 di- 

 urnal variation of temperature is 6°, in March £0°, in July 10°, and in Sep- 

 tember, 18°. Hence, says Mr. Kirwan, we may understand the reason of 

 the great frequency of colds in spring and in autumn.- 



Some philosophers have supposed the earth to become progressively warmer 

 in the course of ages, while others have imagined that its heat is exhausted. 

 Both these opinions appear in general improbable. The greater heat the 

 earth receives by day, the more it throws off, both by day and by night; so 

 that in the course of a few ages the heat must probably have attained its maxir 

 mum. Local changes may indeed arise from local circumstances; thus, the 

 climate of America is said to have become considerably warmer, since a large 

 part of its surface has been cleared from its dense forests by human labour: 

 and to judge from the descriptions of the ancients, it appears that even in 

 Europe the winters were formerly much colder than they are at present. If, 

 however, Dr. Herschel's opinion of the variation of the heat of the sun be 



