262 



Professor Tyndall 



[March 16, 



the surface would have been in this case greater ; for you can readily 

 understand that even a light wind sweeping over the surface, and 

 mixing the chilled with the warmer air, must seriously interfere with 

 the refrigeration. 



Hind Head, Elevation, 850 feet. 

 Course of Temperature, March 4tli, 1883. 



Glacial wind from north-east. Stars very bright. 



Various circumstances may contribute to lessen, or even abolish, 

 the difference between the two thermometers. Haze, fog, cloud, rain, 

 snow, are all known to be influential. These are visible impediments 

 to the outflow of heat from the earth ; but my position for some time 

 has been that a very powerful obstacle to that outflow exists which 

 is entirely invisible. The pure vapour of water, for example, is a 

 gas as invisible as the air itself. It is everywhere diff'used through 

 the air ; but, unlike the oxygen and nitrogen of the atmosphere, it is 

 not constant in quantity. We have now to examine whether meteoro- 



