Mr. John Aitken on Deiv. 367 



takes place in one country what will take place in the other, 

 unless the conditions are alike ? 



All my experiments show that in our climate dew never 

 falls on the earth — though of course Mr. Tomlinson, if he is 

 consistent, must think otherwise — and dew is only deposited 

 on plants and other bodies not in good heat communication 

 with the ground. But it would be rash from this to conclude 

 that in arid countries, where the air is highly diathermatons, 

 and the ground dry and probably a bad conductor of heat, 

 the surface of the earth is never cooled by radiation below 

 the dew-point. But on this point I repeat, " I wait for 

 further information before forming any opinion as to what 

 takes place in other and unknown conditions/'' I observe 

 that Mr. Tomlinson does not observe the same caution, as he 

 states distinctly that, after passing through arid regions, 

 "long before the travellers reached any considerable body 

 of water, nocturnal dews were abundant, and they were de- 

 posited from the air, and did not rise out of the ground." Now 

 on what does he found this last statement which I have put 

 in italics? The fact that the dews were deposited out of the 

 air in no way proves that vapour did not rise from the ground 

 while the dews were being deposited. 



With regard to the bearing of the experiments of the Flo- 

 rentine Academicians, of Robert Boyle, and Le Roi, my critic 

 says, " These early observers proved that the moisture which 

 forms dew and hoar-frost exists in the air, and does not exhale 

 from the ground." It is a self-evident fact that the vapour 

 must be in the air before it condenses on the different surfaces ; 

 but, as has been already said in my previous letter, this fact 

 has " no bearing on the subject," as it neither proves nor dis- 

 proves that vapour " does not exhale from the ground." 



Mr. Tomlinson is welcome to any consolation he can get 

 by shielding himself behind the word " abstract " to account 

 for his misconception of the essential conditions of the crucial 

 experiment made by weighing the turfs before and after "dew- 

 fall." As, however, my last letter gave no further information 

 on the subject, and as Mr. Tomlinson seems now to under- 

 stand the conditions of this test, it may be presumed that the 

 abstract was complete enough on that point and did not give 

 rise to the misunderstanding. 



I may say I have carefully considered the observations of 

 Melloni, referred to by Mr. Tomlinson, and do not find any- 

 thing in them that affect the conclusions I have arrived at. 

 To enter, however, into a detailed examination of his work 

 would extend the limits of this letter to an undue length. 



To many this discussion must have appeared extremely 



