1819.] Prof. Leslie on Heat and Climate. 27 



As the great commerce of air between the poles and the equa- 

 tor, tempers, as much as possible, the inequalities of climates 

 and seasons, so the circulation between the higher and lower 

 strata preserves the balance of day and night. And how much 

 more complete the operations of the latter motion than those of 

 the former, may be judged from this circumstance, — that in our 

 latitudes there is smaller difference of temperature between day 

 and night than between summer and winter, though in the 

 one case the sun is entirely withdrawn from us, though he 

 sheds some light in the other ; not to mention the vast dis- 

 proportion of the times in which those different effects are pro- 

 duced. 



When the sun shoots his rays vertically through a bright sky, 

 a very large share of them will reach the surface of the earth, 

 and there generate a heat which will greatly exceed the quantity 

 diffused by the air. But, in the evening, the circulation which 

 had been so violently excited by the meridian heats will restore 

 the balance, and even produce an inclination to the opposite 

 extreme. It is, hence, that the sultry days of the tropical coun- 

 tries are generally succeeded by evenings comparatively cool ; 

 a circumstance which proves so dangerous to new-settlers. 

 Upon the whole, however, the difference between the heat gene- 

 rated at the surface, and that at a certain height in the air, is not 

 so great as might be apprehended. If, in clear weather, the 

 ground be much heated by a profusion of rays, it is likewise 

 cooled by the evaporation, which is then most considerable. 

 When an extensive tract affords not moisture, the accumulation 

 of heat, on the surface, will be greater. Such are the sandy 

 plains of Arabia Deserta. On the other hand, the condensation 

 of vapours, which takes place in the lower regions of the atmo- 

 sphere, must evolve much heat in the vicinity ; and the clouds, 

 absorbing almost the whole of the sun's light, become imme- 

 diately warmed, and diffuse their influence all round. We are 

 sensible of this fact * * * * 



[Two or three concluding sentences of the Manuscript are 

 unfortunately lost.] 



Article III. 



Analysis of a Specimen of Water, taken out of a Boiling Spring, 

 uniting with the Sea in the Harbour of' Milt o, one of the Gre- 

 cian Islands in the Archipelago. By T. Thomson, M.D. F.R.S. 



The bottle of water which I have subjected to analysis was 

 put into my hands about two years ago, by the Rev. Mr. Holuae, 



