128 ROYAL SOCIETY OF CANADA 



suspended in th.e air read — 15-0° Fahr. The difference diminished 

 rapidly when clouds appeared on the horizon, and vanished when the 

 sky was completely covered. Wilson also was the first to show that 

 the radiation of bodies towards the sky is practically the same at all 

 temperatures of the air. 



In 1794, Six showed that the radiation from the grass on a 

 meadow during calm and clear nights was sufficient to cool a ther- 

 mometer below one suspended five or six feet above it. In one case 

 he reports a difl:erence of 7-5° J^^ahr. 



About the beginning of 1800, Dr. Wells made a careful study of 

 the earth's radiation at night. He placed thermometers in contact 

 with the ground and leaves- of plants as well as enveloping the bulbs 

 in wool or cotton. These thermometers placed at a small distance 

 from the earth's surface gave a fall of l-S" Fahr., and even 7-8° Fahr. 

 below a similar thermometer uncovered and suspended at a height of 

 four feet. 



Difterences of 8° and 9° Fahr. were obtained by Pouillet, in 1844^ 

 Avho repeated Wells' experiments. 



In 1847 the experiments of ]\Ielloni M^ere performed. On account 

 of his classical researches in the domain of radiant heat he was well fitted 

 to investigate the problem of terrestrial radiation. 



He came to the conclusion that the cooling of a black thermometer 

 is owing to radiation, and that the radiation from a metallic ther- 

 mometer was too small to be observed. 



Nearly all of our knowledge of the radiation of heat has been 

 detained from researches with high temperature sources where the pro- 

 portion of short waves and visible rays is very much larger than from 

 low temperature sources. 



The diathermancy of many substances for rays from incandescent 

 bodies was studied by Meiloni and his table is familiar to everyone. 

 The same investigator extended his study to the radiation from a 

 copper ball heated to 400° Cent., and to that from the face of a Leslie 

 cube containing boiling water. 



It was foimd that many substances, which allowed a considerable 

 proportion of the high temperature radiation, including the light rays 

 to penetrate, were opaque to the rays of a low temperature source. 



In general we find that the higher the temperature the more of 

 the short waves are produced and the less of the long waves. A study 

 cf the very long heat waves is a matter of great interest on account 

 cf the relation which these waves have to electrical waves. 



A good deal of attention has been devoted of late to separating 

 out the long waves from a source of radiation and determining the 

 properties of these waves. 



