,1888.] Presidents Address. xxix 



heat we wish to determine are allowed to fall on the wire in one of 

 the arms and any increase of temperature acting on the platinum wire 

 retards the flow of electricity ; the balance is disturbed and the effect 

 is immediately shewn on a galvanometer inserted in the circuit. With 

 this instrument he found in 1885 that besides the reflected heat from 

 the Moon's surface there were distinct traces of radiated heat, and 

 he came to the conclusion that the temperature of the surface was 

 about the freezing point. But even this amount of heat implies 

 some atmosphere on the Moon, and the existence of which has always 

 . been negatived by other observations. 



THE SUN. 



The great question of the Sun's distance from the earth is still 

 unsettled, though the uncertainty has been reduced to within narrow 

 limits. First, in the magnitude of the undertaking, came the expedition 

 for observing the Transits of Venus over the Sun's disc in 1874 and 

 1882. For the former one especially ahnost every civilized nation 

 in the world equipped parties of observers, who were despatched- to 

 all quarters of the globe where the phenomenon could be favourably 

 observed. It was fondly hoped by many that these two transits 

 would settle the question of the Sun's distance beyond doubt or cavil, 

 but this opinion was not shared by those who had studied the reports 

 of the appearances observed in the transits of 1761 and 1769. But 

 : still it was expected that with greatly improved instruments and more 

 .exp rience good results would be obtained ; besides it would have 

 been au unpardonable piece of neglect not to observe these transits, 

 whicii are only visible to one generation out of three. And good 

 .results wei^e obtained, better I think in the transit of 1882 than in 

 that of 1874 ; but not by any means good enough to settle the 

 question. In fact the method is theoretical iy a very powerful one, 

 but the optical appearances presented change so gradually and are 

 so perplexing under various conditions of definition that it is almost 

 impossible for observers, widely separated and under different atmos- 

 pheric conditions, to hit upon exactly the same phase for observation. 

 And here I should like to record my protest against a statement one 

 .often meets with in popular astronomical literature ; it is said that the 

 metiiod is worthless because different computers have from the same 

 datn deduced such different values for the Sun's parallax as 8"*57 and 

 ,8"'i^M. It is quite true that from the transit of 1769 Encke did deduce 

 .the alue 8"'57, and on the authority of his discussion the value of 



