502 MEMORIAL OP JOSEPH HENRY. 



by General Duane, in wliich a fog-signal is only heard during the 

 occurrence of a northeast snow-storm. Certainly this phenomenon 

 cannot be explained by any peculiarity of the atmosphere as to 

 variability of density, or of the amount of vapor which it may 

 contain." 



Henry's services to the Light-House Board were of great value 

 to the country. The fact that his investigations showed that lard 

 oil when heated to about' 250° Fahrenheit is superior in fluidity 

 and illuminating power to sperm oil, caused the substitution of the 

 former for the latter; and thus was saved a dollar on each gallon 

 of illuminating material purchased. This amounted to about one 

 hundred thousand dollars a year in favor of the Government. 



In light and heat Henry made several interesting investigations 

 which, reluctantly, we are obliged to pass over. One however 

 holtts so important a place in the history of science that it cannot be 

 omitted from any discourse which would treat of Henry as a dis- 

 coverer. I refer to his application of the thermopile to the deter- 

 mination of the distribution of heat on the optical images of distant 

 objects. It occurred to Henry that images in the foci of mirrors 

 and lenses are formed not alone by converging pencils of light coming 

 from corresponding points of the objects placed before these mirrors 

 and lenses, but that images are also formed by the convergence of 

 rays which liave no effect on the optic nerve, such as the rays of 

 heat. Indeed Henry looked upon the image as having, on a small 

 scale, the same distribution of physical actions as exists on the sur- 

 face of the large object, of which this image is the optical repro- 

 duction. 



He applied this conception in a bold and wonderful experiment; 

 which was no other than to study the distribution of heat on the 

 surface of, the sun. In 1845,' in company with his brother-in-law, 

 Professor Stephen Alexander, he formed an image of the sun by 

 pointing a telescope to that body and then drawing out the eye-tube 

 of the instrument till the solar image was clearly defined on a screen. 

 In this screen was cut a small aperature, closed by the surface of a 

 thermopile. By motion of the telescope any part of the solar 

 image could be brought on to the surface of the pile. A solar spot 

 of considerable magnitude being then present, he brought it on to 



