574 Hil and cold Currenis in the Sen. 



much below the mean annual temperature of the earth in the latitudes where it has been 

 found, and which of courfe mull have been brought from colder latitudes. 



The mean annual temperature in the latitude oi 6-]" has been determined by Mr. Kirwan, 

 in his excellent ireatife on the temperature of diiVcrent latitudes, to be 39^ ; but Lord MuU 

 grave found on the zo.th of June, when the temperature of the air was 48: ^ that the tem- 

 perature of the fca he depth of 4680 feet was 6 degrees below freezing, or 26'* of 

 Fahrenheit's ihetmometer. 



Oh the 3itt of Auguft in tlie latitude of 69", where the annua! temperature is about 

 3<i'=, the temperature of the fca at the depth of 4038 feet was 32° ; the temperature of the 

 atmofpherc (and probably that of the water at the furface of the fea) being at the fame 

 time at 59^. 



But a ftill more flriking and incontrovertible proof of the exidcnce of currents of cold 

 water at the bottom of the fea, fetting from the poles towards the equator, is the very 

 remarkible difference that has been found to fublift between the temperature of the fea at 

 the furface, and at great depth at tl>e tropic, though the temperature of the atmofphere 

 there is fo conftant, that the greatell changes produced in it by the feafons feldoin 

 amount to more than live or fir. degrees ) yet the diiference between the heat of the water 

 at the furface of the fea, and that at the depth of 3600 feet, has been found to amount to 

 no lefs than 31 degrees; the temperature above, or at the furface, being 84, and at the 

 given depth below, no more than 53 *. 



It appears to tlie Count to be extremely dilTicult, if not quite impolTible, to account for 

 this degree of cold at the bottom of the fea in the torrid zone, on any other fuppofition than 

 that of cold currents from the poles ; and the utility of thefe currents in tempering the 

 exceffive heats of thofe climates is too evident to require any illurtration. 



Thefe currents arc produced, as we have already feen, in confequencc of the difference la 

 the fpccific gravity of the fea water at different temperatures : their velocities muft there- 

 fore be in proportion to the change produced in the fpecific gravity of water by any given 

 change of temperature ; and hence we fee how much greater they muft be in fait water than 

 they could poffibly have been had the ocean been compofed of frelli water. 



It is not a little remarkable, that the water of all great lakes is frefli, and nearly fo in all 

 inland feas (like the Baltic) in cold climates, which communicate with the ocean by 

 narrow channels. We fliall find reafon to conclude, that this did not happen without de- 

 fvgn, when we confider what confequences would probably enfue, fliould the waters of a 

 large lake in an inland fituation in a cold country (fuch as the lake Superior for inftance in 

 North America) become as fait as the fea. 



Though the cold winds which blow over the lake in the beginning of winter would be 

 more warmed, and the temperature of the air on the fide of the lake oppofite to the quarter 

 from whence thefe winds arrive, would be rendered fomewhat mililer than it now is ; 

 yet, as the water of the lake would give offanimmenfe quantity of heat before a covering 

 of ice could be formed on its furface for its prote£lion, it would on the return of fpriiig be 

 found to be extremely cold ; and as it would require a long time to regain, from the influ- 

 ence of the returning fun, the enormous quantity of heat loft during the winter, it would 



* Philofoiiliical Tranfaflions m.dcc.lii. 



