1869.] 



Mr. Huggins on the Heat of the Stars. 



311 



placed at some distance to preserve it from the influence of the ironwork of 

 the telescope, were covered with gutta percha, over which cotton-wool 

 was placed, and the whole wrapped round with strips of brown paper. 

 The binding-screws of the galvanometer were enclosed in a small cylinder 

 of sheet gutta percha, and filled with cotton-wool. These precautions 

 were necessary, as the approach of the hand to one of the binding-screws, 

 or even the impact upon it of the cooler air entering the observatory, was 

 sufficient to produce a deviation of the needle greater than was to be 

 expected from the stars. 



The apparatus was fixed to the telescope so that the surface of the ther- 

 mopile would be at the focal point of the object-glass. The apparatus 

 was allowed to remain attached to the telescope for hours, or sometimes for 

 days, the wires being in connexion with the galvanometer, until the heat had 

 become uniformly distributed within the apparatus containing the pile, and 

 the needle remained at zero, or was steadily deflected to the extent of a 

 degree or two from zero. 



When observations were to be made, the shutter of the dome was opened, 

 and the telescope, by means of the finder, was directed to a part of the sky 

 near the star to be examined where there were no bright stars. In this 

 state of things the needle was watched, and if in four or five minutes no 

 deviation of the needle had taken place, then by means of the finder the 

 telescope was moved the small distance necessary to bring the image of the 

 star exactly upon the face of the pile, which could be ascertained by the 

 position of the star as seen in the finder. The image of the star was kept 

 upon the small pile by means of the clock- motion attached to the tele- 

 scope. The needle was then watched during five minutes or longer ; almost 

 always the needle began to move as soon as the image of the star fell upon 

 it. The telescope was then moved, so as to direct it again to the sky near 

 the star. Generally in one or two minutes the needle began to return 

 towards its original position. 



In a similar manner twelve to twenty observations of the same star were 

 made. These observations were repeated on other nights. 



The mean of a number of observations of Sirius, which did not differ 

 greatly from each other, gives a deflection of the needle of 2°. 



The observations of Pollux 



No effect was produced on the needle by Castor. 



Regulus gave a deflection of 3°. 



In one observation Arcturus deflected the needle 3° in 15 minutes. 



The observations of the full moon were not accordant. On one night a 

 sensible effect was shown by the needle ; but at another time the indications 

 of heat were excessively small, and not sufficiently uniform to be trust- 

 worthy. 



It should be stated that several times anomalous indications were ob- 

 served, which were not traced to the disturbing cause. 



The results are not strictly comparable, as it is not certain that the 



