Mr. E. J. Stone on the Heating -powers ofArcturus and a. Lyra. 377 



of one of the negative eyepieces of the great equatorial. This com- 

 pletely shuts the pile up in the telescope-tube. A thick flannel bag 



( 



<x 



fi I 



is then wrapped over the eyepiece and terminals. The bag is pre- 

 vented from actually touching the case of the pile, and is useful in 

 preventing the irregular action of draughts upon the case of the pile 

 and terminals. The wires are led from the terminals of the pile to 

 the observatory library, where I have placed the reflecting galvano- 

 meter. This separation of the galvanometer from the telescope is 

 most inconvenient ; but it was absolutely necessary, on account of the 

 large moving masses of iron in the observing-room. 



The two faces a and (3 of the pile are so nearly alike, that the re- 

 sultant current generated by any equal heating of them is exceedingly 

 feeble. 



The telescope is first directed so that the star falls between the 

 faces a and /3, and allowed to remain thus until the needle is nearly 

 steady at the zero. 



The star is then placed alternately upon the faces a and /3, and 

 the corresponding readings of the galvanometer taken as soon as the 

 needle appears to have taken up its position, which usually takes 

 place in about ten minutes. In order to avoid changes of zero, I 

 have always reduced those readings by comparing a reading with star 

 on face a with the mean of two readings with star on /3, taken before 

 and after the reading with star on a, or vice versa. 



With this precaution I have never met with any anomalous results, 

 although in making the observations I have usually joined over the 

 terminals without knowing the direction for heat, and have left this 

 undetermined until the completion of the observations. I mention 

 this because the differences in the readings for star on a and star on 

 /3 in the state in which I use my galvanometer are small. 



On many nights, when very slight appearances of cloud prevailed, 

 I have not been able to make any satisfactory observations at all. 



The number of divisions over which the spot of light travels on 

 the galvanometer- scale for a given difference of temperature of the 

 faces a and (3 is of course dependent upon many circumstances, and 

 especially upon the position of the sensitiveness-regulation magnet of 

 the galvanometer. 



I have thought it useless, therefore, to publish any results unless 

 obtained upon nights when the state of the galvanometer was elimi- 

 nated by referring to an independent source of heat. The way in 

 which this has been attempted is as follows : — 



After obtaining the differences in the position of the spot of light 



