CATARACTS, AND INUNDATIONS. 113 



in every climate, consider the lowest degree of heat as approaching 

 nearest to tiie mean temperature ; and therefore we cannot con- 

 clude the mean temperature at Brighthelmstone to be more than 

 50. The mean temperature of London is computed about 52; 

 Brighthelmstone is nearly 50 miles farther south than London, and 

 is immediately on the sea, and must therefore be at least as warm 

 as London. It is evident that the observations from which the 

 mean is taken, must generally contain more of the extremes of 

 beat than of cold, as the former happen in the day-time, and the 

 latter in the night, in consequence of which they will often escape 

 notice. There is a table, next following this paper, constructed by 

 Dr. Heberden, expressing the heat in London for every month in 

 the year, from a mean often years, beginning with 1/63, and end- 

 ing with 1772. The mean temperature is given both at 8 A. M. 

 and 2 P. M. There is further in the table, a column of the mean 

 of the greatest monthly colds in the night, observed during the 

 $ame 10 years by Lord Charles Cavendish, in Marlborough-street. 

 There will not probably be any great error in considering the heat 

 observed at 2 P. M. as the greatest daily heat; and taking a mean 

 between the greatest heats of the day, and greatest colds of the 

 night, they give 49. 196 for an annual mean, which is much lower 

 than is commonly supposed. At the house of George Glenny, Esq. 

 near Bromley, there is a well 75 feet deep, which in November 

 \vas 49^. M. de Mairan has given a table of the greatest heats 

 and greatest colds observed at Paris for 56 years, beginning from 

 1/01; and a mean of them is 10 above freezing, or 1010 of 

 Reaumur's scale*. The temperature of the cave of the Observa- 

 tory where those observations were made, is 10J* above freezing, 

 by the same scale of Reaumur. There appears not therefore any 

 necessity for an internal heat; on the contrary, it is matter of de- 

 monstration, that were there any source of heat in the earth 

 \vhich was not equally in the air, the heat of the interior parts 

 ought to be higher than a mean : and if the central heat bore as 

 high a proportion to that of the sun as M. de Mairan alledges, the 

 heat of the earth itself ought to be a great deal above the mean tem- 

 perature of the air, which from observation there is no ground for 



* Mem, de I'Acad. d Sconces, An, 1765, p. 202. 

 VOL. III. I 



