-1859] WRITINGS OF JOSEPH HENRY. 147 



even the heat of the stars may so accumulate as sensibly 

 to contribute to the temperature of the earth. Though at 

 first sight it may appear that the effect from this source must 

 be exceedingly feeble, yet when we reflect that the heat of 

 the stars comes from every part of the whole concave 

 of the heavens, while that of the sun proceeds from a disk 

 which occupies only the five-millionth part of the whole sky, 

 we may be inclined to attribute to the stellar radiation a 

 much greater importance than without this reflection we 

 should ascribe to it. 



M. Pouillet, of Paris, has made a series of very ingenious 

 researches on the subject of the temperature of space, and 

 has arrived at very unexpected results. He employed in 

 his observations an instrument to which he gave the name 

 of " actinometer," or ray-measurer. It consisted of a cylin- 

 drical box of polished silver, about eight inches in diameter,, 

 and five in height, enveloped in swan's-down, and enclosed 

 in an outer cylinder, so as to prevent as much as possible the 

 effect of the temperature of the circumambient air. The 

 box was filled with several layers of swan's-down, so sup- 

 ported as not to press upon each other. In the centre of 

 the upper surface of the open box was placed the bulb of a 

 thermometer, the stem projecting horizontally. A cylindri- 

 cal border was raised round the edge of the box, to cut off 

 the lateral rays, and at such a height that two-thirds of the 

 whole sky could be seen by an eye at the point occupied by 

 the bulb. The thermometer thus enclosed was turned dur- 

 ing the night to the zenith, and exposed to the radiation 

 from the clear sky. The temperature of this thermometer 

 and one exposed to the air at four feet from the ground was 

 observed hourl3^ 



If the heat of the surrounding air were entirely excluded 

 from the enclosed thermometer, it is evident that it would 

 onl}' be affected by the radiation from celestial space, and 

 from the atoms of the air in the column between it and the 

 top of the atmosphere. 



Of these two sources of radiation one, namely that of celes- 

 tial space, would be constant and remain the same during 



