Mr. E. f oote on the Heat in the Sun's Rays. 169 



to my surprise the effect rose to 115°. The experiment was 

 many times repeated with similar results, although varying some- 

 what in amount from the different degrees of brightness in the sun. 

 The change in the temperature of the air was still to be added, 

 and the conclusion seemed to be irresistible, that the sun's rays 

 in passing into the heated room acquired a temperature that 

 they did not derive from the sun. 



The experiment was next repeated with different temperatures 

 of the room, and it was found that the intensity of the rays de- 

 pended upon the heat of the room . Indeed in the coldest weather 

 in winter I could impart to them a power which belonged to a 

 summer's sun. 



At a later period, when the circumstances were changed and 

 the heat on the outside had become greatest, the indications of 

 the instrument were reversed. The high temperature of the 

 summer rays was in a great measure lost or dissipated on enter- 

 ing into a cool room. There they had no greater power than 

 had been found at similar temperatures in the winter. 



For the purpose of a more accurate investigation of the sub- 

 ject, I prociu-ed a glass shade or receiver about 10 inches in dia- 

 meter and 22 in height. A copper base was adapted to it with 

 a groove around the outer edge, into which the receiver fitted; 

 and when it was filled with dry ashes, the point was thereby 

 rendered sufficiently air-tight. It was supported by legs so high 

 that a spirit-lamp could be placed under it, and any required 

 temperature given to the air within. 



A second receiver of the same size was sometimes used for the 

 purpose of simultaneous comparison. The air within it was 

 cooled by inserting a tin canister filled with frigorific mixtures. 

 The thermometers were supported within the receivers, and thus 

 at the same time the same rays could be tested in the opposite 

 extremes of temperatures. 



I subjoin, as an example, the following Table (p. 170), containing 

 the results of an experiment made in February last at eight o'clock 

 in the morning. It was a clear day, and the sun shone through 

 a window into the room where the instruments were placed. 



The first observation was the temperature of the room and in 

 the sun upon a mercurial thermometer. The lamp was placed 

 under the receiver, and as the temperature of the air was gra- 

 dually increased, the effect was noted until the heat in the sun 

 had attained the highest limit of the thermometer. The fourth 

 column contains the differences between the thermometer in the 

 shade and the one in the sun. The fifth column shows the true 

 relative heat of the sun's rays at the different temperatures. It 

 was obtained, as before explained, by adding to the differences 

 the increase in the temperature of the air. Several observations 

 may be made in regard to the results in the Table. 

 Phil. Mag. S. 4. Vol. 13. No. 85. March 1857. N 



