382 PHILOSOPHICAL TRANSACTIONS. [aNNO 1788. 



take the teinperature of the air more or less : for any change of temperature com- 

 municated at the surface will, from the fluidity of the water, be readily diffused 

 through the whole. It is probably owing to this cause, that the wells in the 

 neighbourhood of Brighthelmstone vary from 50° to 52°, for those were the 

 highest that had most water in them. The observations were made in summer. 

 These wells are of various depths, from 15 to 150 feet. That which is always 

 found the coldest is not more than 22 feet deep ; its heat was never greater than 

 50°. It is near the beach, and is a tide well, that is, the water in it rises and 

 falls, and yet does not correspond exactly with the tides, but follows them with 

 an interval of about 3 hours. At the lowest there is not more than a foot of 

 water in it ; and it may be considered as a subterraneous spring running through 

 the bottom of the well. There are in fact numerous springs that break out on 

 the sand, a few feet above the low-water mark, which are doubtless the same 

 that supply the wells. As we are not acquainted with any cause that produces 

 cold in the bowels of the earth, we must necessarily, in every climate, consider 

 the lowest degree of heat as approaching nearest to the mean temperature ; and 

 therefore we cannot conclude the mean temperature at Brighthelmstone to be 

 more than 50°. The mean temperature of London is computed about 52° ; but 

 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 heat 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 of 10 years begin- 

 ning with 1763, and ending 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 same 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°.! 96 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 was 49^°. M. de Mairan has given 

 a table of the greatest heats and greatest colds observed at Paris for 56 years, 

 beginning from 1701 ; and a mean of them is 10° above freezing, or 1010°, of 

 Reaumur's scale*. The temperature of the cave of the Observatory where those 



* Mem. de I'Acad. des Sciences, An. 17^5, p. 202. 



