264 



Professor Tijndall 



[March 16, 



intrusion of a perfectly invisible vapour was competent to check tlie 

 radiation, while its passing away re-opened a doorway into space. 



It is well to bear in mind that the difference between the two 

 thermometers on the evening here referred to varied from 4° to 9°, the 

 latter being the maximum. 



Such observations might be multiplied, but, with a view to 

 saving space, I will limit the record. On the evening of January 

 30th, the atmosphere was very serene ; there was no moon, but the 

 firmament was powdered with stars. At 7.15 p.m. the difference 

 between the two thermometers was 6° ; while at 9.30 p.m. it was 4°, 

 the wool thermometer being in both cases the colder of the two. On 

 February 3rd observations were made under similar conditions of 

 weather, and with a similar result. At 7.15 p.m. the difference 

 between the thermometers was 6° ; wdiile at 8.25 p.m. it was 4°. On 

 both these evenings the sky was cloudless, the stars were bright, while 

 the movement of the air was light, from the south-west. 



In all these cases the air passing over the plateau of Hind Head 

 had previously grazed the comparatively warm surface of the Atlantic 

 Ocean, where it had charged itself with aqueous vapour to a degree 

 corresponding to its temperature. Let us contrast its action with 

 that of air coming to Hind Head from a quarter less competent to 

 charge it with aqueous vapour. We were visited by such air on the 

 10th of last December, when the movement of the wind was light 

 from the north-east, the temperature at the time, moreover, was very 

 low, and hence calculated to lessen the quantity of atmospheric 

 vapour. Snow a foot deep covered the heather. At 8 . 5 a.m. the two 

 thermometers were taken from the hut, having a common temperature 

 of 35°. The one was rapidly suspended in the air, and the other laid 

 upon the wool. I was not prepared for the result. A single minute's 

 exposure sufficed to establish a difference of 5° between the ther- 

 mometers ; an exposure of five minutes produced a difference of 13° ; 

 while after ten minutes' exposure the difference was found to be no 

 lefcs than 17°. Here follow some of the observations. 



December 10th. — Deep snow; low temperature ; sky clear ; light 



north-easterly air. 



During these observations, a dense bank of cloud on the opposite 

 ridge of Blackdown, virtually retarded the rising of the sun. It had, 



