518 Prof. Tyndall on the Action of Free Molecules on 



loped by every successive film of moisture condensed upon 

 its blades is instantly wasted by radiation. Those who are 

 accustomed to work with the thermopile know how rapidly 

 the associated galvanometer-needle falls from a high deflection 

 to zero when the heat incident upon the pile is suddenly cut 

 off. A similar rapidity of waste would assuredly occur during 

 the slow formation of dew. The heat of condensation could 

 not, for this reason, be housed in the manner supposed by 

 Wells *. The true explanation I hold to be that already in- 

 dicated — the checking of radiation by vapour, the abundance 

 of which was indicated by the copious deposition of dew. 



If the experiments of Wilson could be made in an atmo- 

 sphere still colder than that in which he worked — on a larger 

 plain, for instance, and in a country remarkable for the dryness 

 of its air — Wells considered that a difference of at least 30° 

 would be observed on serene nights between the air and a 

 downy substance placed on the earth. And as Six had found 

 the air-temperature at an elevation of 220 feet to be 10° higher 

 than at 7 feet, these 10° being added to the 30° would make 

 the surface at least 40° colder than the air at the height of 

 200 or 300 feet. With all this I agree. I would go even 

 further, and reiterate here a statement made by me nineteen 

 years ago, that the withdrawal of the aqueous vapour of our 

 atmosphere, for a single cairn night, would kill every plant 

 in England capable of being killed by a freezing temperature. 



Pictet, I believe, was the first to notice that the temperature 

 of the air near the earth's surface on serene nights diminished 

 as the surface was approached, the sequence of the day tem- 

 peratures being thus inverted. To account for the chilling 

 of the air, say, at 10 feet above the earth's surface beyond that 



* Wells Himself observed in grass a fall of temperature of 7° in twenty 

 minntes. This gives us some notion of the rapidity with which a radiant 

 so powerful as water would dispose of its heat (Essays, p. 157). 



At the instance of my friend Mr. Francis Galton, and with the kind 

 sanction of the Meteorological Council, the following instructive ob- 

 servations, showing the temperatures recorded by two thermometers — the 

 one placed on cotton-wool resting on the earth, and the other hung at 

 a height of four feet in the air — were recently made by Mr. Whipple at 

 Kew: — 



Time. Air. Wool. 



4.20 p.m. . . 



. . 34-8 



33-2 



4.25 „ . . 



. . 32-5 



27-6 



4.30 „ . . 



. . 32-4 



25-7 



4.35 „ . . 



. . 32-4 



23-4 



4.40 „ . . 



. . 32-2 



21-7 



4.45 „ . . 



. . 32-2 



20-7 



The rapidity of radiation is well shown by these observations, an ex- 

 posure of twenty-five minutes sufficing to establish a difference of 11 0, 5. 



