164 BIOGRAPHICAL MEMOIR OF JOSEPH HENRY. 



by the terminal sparli: produced in a ^Yire surrounding a bundle of iron 

 wires. Since the publication of this invention various other plans 

 founded on the same principle have been introduced into practice. 



X. Another series of experiments was in regard to the relative heat 

 of different parts of the sun's disk, and especially to that of the spots 

 on the surface. These were made in connection with Prof. S. Alex- 

 ander, and consisted in throwing an image of the sun on a screen in a 

 dark room by drawing out the eye-piece of a telescope. Through a hole 

 in the screen the end of a sensitive thermo-i)ile was projected, the wires 

 of which were connected with a galvanometer. By slightly moving the 

 smaller end of the telescope diiferent parts of the image of the sun could 

 be thrown on the end of the thermo-pile, and by the deviation of the 

 needle of the galvanometer the variation of the heat was indicated. In 

 this way it was proved that the spots radiated less heat than the adja- 

 cent parts, and that all parts of the sun's surface did not give otf an 

 equal amount of heat. 



XI. Another series of exj)eriments was made with what was called a 

 thermal telescope. This iHstrument consisted of a long hollow cone of 

 pasteboard, lined with silver leaf and painted outside with lampblack. 

 The angle at the apex of this cone was such as to cause all the parallel 

 rays from a distant object entering the larger end of the cone to be re- 

 flected on to the end of a thermo-pile, the poles of which were connected 

 with a delicate galvanometer. When»the axis of this conical reflector 

 was directed toward a distant object of greater or less temperature than 

 the surrounding bodies, the difference was immediately indicated by the 

 deviation of the needle of the galvanometer. For example, vv'hen the 

 object was a horse in a distant field the radiant heat from the animal 

 was distinctly percei^tible at a distance of at least several hundred yards. 

 When this instrument was turned toward the celestial vault the radiant 

 heat was observed to increase from the zenith downward; when directed, 

 however, to different clouds, it Avas found to indicate in some cases a 

 greater and in others a less degree of radiation than the surrounding- 

 space. When the same instrument was directed to the moon a slight 

 increase of temperature was observed over that of the adjacent sky, but 

 this increase of heat was attributed to the reflection of the heat of the 

 sun from the surface of the moon, and not to the heat of the moon itself. 

 To show that this hyi)othesis is not inconsistent with the theory that the 

 moon has cooled down to the temx)erature of celestial space, a concave 

 mirror was made of ice, and a thermo-pile placed in the more distant 

 focus. When a flame of hydrogen, rendered luminous by a spiral plati- 

 num wire, was placed in the other focus, the needle of the galvanometer 

 attached to the pile indicated a reflection of heat, care being taken to 

 shade the pile by a screen with a small opening introduced between it 

 and the flame. 



XII. Another series of experiments connected with the preceding may 

 be mentioned here. It is well known that the light from a flame of 

 hydrogen is of very feeble intensity. The same is the case with that of 

 the compound blowpipe, while the temperature of the latter is exceed- 

 ingly high, sufficiently so to melt fine platinum wire. It is also well known 

 that by introducing lime or other solid substance into this flame its radi- 

 ant light is very much increased. I found that the radiant heat was 

 increased in a similar ratio, or, in other words, that in such cases the 

 radiant heat was commensurate with the radiant light, and that the 

 flame of the compound blowpipe, though of exceedingly high tempera- 

 ture, is a comparatively cool substance in regard to .radiant heat. To 

 study the relation of the temjierature of a flame to the amount of heat 



