1848] WRITINGS OF JOSEPH HENRY. 283 



ON HEAT, AND ON A THERMAL TELESCOPE.* 

 (Silliman's American Journal of Science, Januar}', 1848, vol. r, pp. 113, 114.) 



Professor Henry showed the analogy between light and 

 heat, by stating that as two rays of light might be so opposed as 

 to produce darkness, so two rays of heat might be so opposed 

 as to produce cold. The facts with regard to heat as well as 

 light therefore show that the theory of undulation is not an 

 imagination, but the expression of a law. The minimum 

 of heat, as proved by his experiments with the thermo- 

 electric pile, does not correspond with the minimum of 

 light. Among flames there are many which give but little 

 light, but which give great heat; as for example the flame 

 of hydrogen. The amount of radiant heat and radiant light 

 were found to be about the same. 



The spots on the sun are colder than the surrounding 

 surface; and its surface is variously heated. This result 

 he obtained by a very simple experiment of throwing the 

 disc of the sun on a screen, and placing the very sensitive 

 thermo-electric pile before its diff'erent parts. He had not 

 yet concluded his experiments on the sun, and had not 

 measured the comparative heating powers of the centre and 

 circumference, from the results of which observations very 

 important consequences would be drawn. 



This apparatus he fitted to a common pasteboard tube, 

 covered with gilt paper externally, and blackened inter- 

 nally, with which he measured the heat of distant objects. 

 He could detect the heat of a man's face a mile off"; that of 

 a house five miles off". He thus discovered that the coldest 

 spot of the sky is at the zenith. One day, on directing his 



* [An abstract of a paper read before the Association of American Geolo- 

 gists and Naturalists, at its eighth annual meeting, held at Boston, Septem- 

 ber, 1847. The only published report however of the proceedings of this 

 meeting of the Association appears to be that given in Silliman's Ameri- 

 can Journal of Science, above quoted. At this meeting, it was agreed by 

 the body to resolve itself into the "American Association for the Advance- 

 ment of Science; " and the new organization held its first meeting at Phila- 

 delphia on the following year, September 20, 1848.] 



