516 Prof. Tyndall on the Action of Free Molecules on 



on the other hand, submit that such experiments are necessary 

 to rescue their science from empiricism. What could Wells 

 have done with dew had he not been preceded by Leslie and 

 Eumford ? His whole theory is an application of results ob- 

 tained in the laboratory *. 



What I have stated regarding Wilson applies also to Six, 

 who concluded from his experiments " that the greatest dif- 

 ferences at night in point of temperature, between bodies on 

 the surface of the earth and the atmosphere near it, are those 

 which take place in very cold weather."" This is quoted from 

 Wells t 3 who, in his essay on Dew, recurs more than once to 

 the subject. He signalizes, but does not explain, " the 

 greater difference which takes place in very cold weather, if it 

 be calm and clear, between the temperatures of the air and of 

 bodies on the earth at night, than in equally calm and clear 

 weather in summer " \ . A considerable n amber of observations 

 bearing upon this point are scattered through the essay. The 

 radiant power of the air being practically nil, it retains for a 

 considerable time the warmth imparted to it during the day, 

 while, when it is dry, the rays from the surface of the earth 

 pass unimpeded through it. Hence the relative refrigeration 

 of the surface §. 



In regard to the action of water-vapour Magnus considered 

 experiment superfluous, as the phenomenon of dew sufficed to 

 prove me wrong. If the vapour possessed the power which 

 I ascribed to it, he contended that dew could not be formed. 

 It is not difficult to dispose of this objection. The formation 

 of dew and superficial refrigeration are connected, not by 

 coincidence, but by opposition. I would venture to predict 

 that where the one is great the other, in general, will be small. 

 " Very little dew," says Wells, " appeared on the two nights 

 of the greatest cold 1 have ever observed on the surface of 



* " Its complete theory," says Wells, " could not possibly, in my 

 opinion, have been attained, before the discoveries on heat were made, 

 which are contained in the works of Mr. Leslie and Count Itumfurd " 

 (Essays, p. 191). 



t Essays, p. 176. Wells thus generously refers to the' labours of 

 Wilson: — "Indeed, several of my experiments upon dew were onlv imi- 

 tations of some which had been previously made on hoar frost, by that 

 ingenious and worthy man " (Essays, p. 151). 



% Ibid. p. 188. 



§ It ouglit to be stated that, contrary to Six and to Wells, Mr. Glaisher 

 has found that " the differences between the temperature of tbe air and 

 of bodies on the earth at night, in equally clear and calm weather, were 

 the same at every period of the year" (Phil. Trans. 1647, p. 126). He 

 moreover records differences considerably in excess of those observed by 

 Wilson and by Wells. Keeping the action of aqueous vapour in view, 

 the elaborate paper of Mr. Glaisher might repay further discussion. 



