26 Pi'tf' Leslie on Heat and Climate. [Jul,*, 



in our climate, amount to a degree, on an average, for every 

 220 feet.* But this uniform progression will not extend to con- 

 siderable elevations. It is hence easy to perceive that the 

 Tule for barometrical measurements, which directs the whole 

 intervening column of air to be reckoned at the medium tem- 

 perature of the two stations, must be inaccurate; and the more 

 so, as the difference of a degree produces a variation of near 

 _L^th on the whole result. If it were not a digression from the 

 main subject, I would proceed here to deduce the proper cor- 

 rection from our formula. The most exact method, however, 

 would be to make observations with the barometer and thermo- 

 meter at different stages, in ascending and descending the moun- 

 tain. I cannot, on this occasion, help expressing my surprise 

 that no attempts, as far as I recollect, have been made to 

 estimate the effects of humidity, which must often be very 

 great. I have lately inferred from theory, and verified it by ex- 

 periment, that the air, in dissolving water, expands so much as 

 to become specifically lighter. It would, therefore, be highly 

 expedient to introduce the hygrometer in barometrical measure- 

 ments. But no instrument of that kind, yet offered to the 

 public, is constructed upon principles that are unexceptionable .f 

 I shall probably resume the subject at another opportunity. 



In applying the formula to ascertain the cold of elevated con- 

 tinents, some abatement should be made ; for, in these cases, 

 the communication between the higher and lower strata of the 

 atmosphere, being necessarily distant and circuitous, must be 

 imperfect. The climate of such territories will evidently partake 

 of that which they would enjoy if depressed to the level of 

 the sea. The heat derived immediately from the sun, which 

 acts principally near the surface, being dispersed with unusual 

 slowness, must continue to accumulate, till this very ascendency 



Eroduce an increased circulation of the atmosphere sufficient 

 enceforth to maintain the equilibrium. Upon the same prin- 

 ciples, the air upon the top of a mountain must be rather warmer 

 than at tfae same height in the open plain. In like manner, a 

 spot enclosed on all sides by lofty precipices, but yet accessible 

 to the sun's rays, will become extremely hot : not because the 

 light is reflected and concentrated by the rocks, as some have 

 strangely supposed, but because the circulation of the air at the 

 narrow basin is obstructed, and, consequently, the dispersion 

 of the heat. Such are the delightful vales scattered among the 

 Alps and Appennines. If these recesses of nature be encircled by 

 naked rocks, the effects will be still more remarkable ; for 

 evaporation not only produces cold, but, by dilating the air, it 

 prjmotes the diffusion of that influence. 



* This estimate is rather too large. It will be found nearer the truth, to reckon, 

 1° of cold, on Fahrenheit's scale, for every hundred yards of ascent. — A. 



+ I could not then anticipate that 1 should have the good fortune to invent aa 

 hygrometer, of a construction at once simple and correct.— A. 



