34 MR JOHN AITKEN ON DEW. 



But os the eye gives us no information about the greater proportion of the 

 radiant energy, its indications are of no value in determining the radiating and 

 absorbing powers of different surfaces. 



Experiment shows that different surfaces have different absorbing powers 

 for different rays. Melloni, for instance, found that white lead absorbed only 

 about half as much heat from a Locatelli lamp as lamp black did, while it 

 absorbed as much as lamp black when the source of heat was copper at 100° C. 

 It is evident from this, that we cannot take the result of experiments made in 

 the laboratory, and apply them to surfaces exposed to the temperature of 

 the sky on a clear night. It may be possible that the radiating and 

 absorbing powers of different surfaces may bear the same proportion to each 

 other when the temperature is 0°, and they radiate into space, as when their 

 temperature is 100°, and they are exposed to surfaces at the ordinary tem- 

 perature of the laboratory. This may be so, but till it is proved we can- 

 not apply these laboratory experiments to the cooling effect of radiation at 

 night. 



Some experiments on the radiating power of different substances exposed 

 to a clear sky were made by Daniell. He used for his purpose two similar 

 parabolic reflectors. In the focus of each was placed the bulb of a ther- 

 mometer. In experimenting he turned the reflectors to the sky, and coated 

 the bulbs of the thermometers with the substances to be tested. Comparing 

 garden mould with black wool, his measurements show, from the average of 

 three readings given by him, that while the black wool fell 9° below the 

 temperature of the air, the mould fell only 6°. The difference between the 

 radiating powers of chalk and black wool, as given by him, was not quite so 

 great. There seems to be an objection to this method of experimenting. 

 The different surfaces here lose more heat by radiation into space than they 

 receive. To supply this loss, they receive more heat by radiation from the 

 reflector than they give, and they also receive heat from the surrounding air, 

 conveyed to them by connection currents. Now in the experiment as arranged 

 by Daniell, the two surfaces will not receive the same amount of heat from 

 the latter source. The wool surface will not have such a free circulation of air 

 over it as the other one ; it will therefore not receive so much heat, and its 

 temperature will thus tend to fall lower. 



It appeared that something more might be done in this direction, and on con- 

 sideration it was thought that the radiation thermometers, described by me in 

 a previous paper, might be suitable for the purpose. It may be remembered 

 that the principle on which these radiation thermometers is constructed is, 

 that a large surface is more highly heated than a small one by radiation during 

 the day, on account of the absorbed heat being more slowly taken away by 

 the passing air from the former than from the latter ; and for a similar reason 



