320 MEMORIAL OF JOSEPH HENRY. 



a resume of the electrical phenomena exhibited by the Leyden jar, 

 and their true interpretation, he remarked that "for the last three 

 and a half years, all his time and all his thoughts had been given 

 to the details of the business of the Smithsonian Institution. He 

 had been obliged to withdraw himself entirely from scientific 

 research; but he hoped that now the Institution had got under 

 way, and the Regents had allowed him some able assistants, that he 

 would be enabled in part at least to return to his first love the 

 investigation of the phenomena of nature." * 



Thermal Telescope. Shortly after his establishment at Washing- 

 ton, he continued a series of former experiments with the " thermo- 

 galvanic multiplicator " devised by Nobili and Melloni in 1831; 

 and by some slight but significant modifications of the apparatus, 

 he succeeded in imparting to it a most surprising delicacy of action. 

 With the thermo-electric pile carefully adjusted at the focus of a 

 suitable reflector, his " thermal telescope" when directed to the 

 celestial vault, indicated that the heat radiated inward by our 

 atmosphere when clear, is least at the zenith, and increases down- 

 ward to the horizon ; as was to have been inferred from its increas- 

 ing mass : when directed to clouds, they were found to diifer very 

 widely accordingly as they were condensing or being dissipated; 

 some even indicating a less amount of radiation than the surround- 

 ing atmosphere. When directed to a horse in a distant field, its 

 animal heat concentrated on the pile, was distinctly made manifest 

 on the galvanometer needle. Even the heat from a man's face at 

 the distance of a mile could be detected ; and that from the side of 

 a house at several miles distance.f These and many similar obser- 

 vations demonstrated to sense the inductions of reason, that there 

 is a constant and universal exchange by radiation in straight lines 

 from every object in nature, following the same laws as the palpable 

 emanation from incandescent bodies; and that even when the 

 amplitude of the thermal vibrations (equivalent to the square root 

 of their dynamic energy) is reduced a million fold, its existence 

 may still be distinctly traced. 



* Proceed. Am. Assoc. 4th Meeting, New Haven, Aug. 1850, p. 378. 

 fSilliman's Am. Jour. Sci. Jan. 1848, vol. v. pp. 113, 114. 



