1845] WRITINGS OF JOSEPH HENRY. 225 



M. Gautier has attempted to investigate the influence of 

 solar spots on terrestrial temperature, by comparing the tem- 

 perature of several places on the earth's surface during the 

 3'ears in which the spots were most abundant with those in 

 which the smallest number were perceptible. From all the 

 observations collected it seems to be indicated that during 

 the years in which the spots were the greatest in number the 

 heat had been a trifle less; but the results are far from being 

 sufficiently definite to settle the question ; and M. Gautier 

 remarks that a greater number of years of observation at a 

 greater number of stations will be necessary to establish a 

 permanent connection between these phenomena. 



The idea occurred to Professor Henry that much interest- 

 ing information relative to the sun might be derived from 

 the application of a thermo-electric apparatus to a picture of 

 the solar disc produced by a telescope on a screen in a dark 

 room. This idea was communicated to Professor Alexander, 

 who readily joined in the plan for reducing it to practice. 

 It was agreed that they should first attempt to settle the 

 question of the relative heat of the spots as compared with 

 the surrounding luminous portions of the sun's disc. The 

 first experiments were made on the 4th of January, 1845. 

 Mr. Alexander had observed a few days previous a very 

 larf^e spot, more than 10,000 miles in diameter, near the 

 middle of the disc. To produce the image of this spot a 

 telescope of four inches aperture and four and a half feet 

 focus was placed in the window of a dark room with a screen 

 behind it, on which the image of the spot was received. 

 The instrument was placed behind the screen with the end 

 slightly projecting through a hole made for the purpose, and 

 a small motion of the telescope was sufficient to throw the 

 image of the spot ofi^ or on the end of the pile. The spot 

 was very clearly defined, and might have been readily 

 daguerreotyped had the telescope been furnished with an 

 equatorial movement. The form of the penumbra of the 

 spot as it appeared on the screen was that of an irregular 

 oblong about two inches in one direction and an inch and a 

 half in the other. The dark central spot within the penumbra 

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