636 



The Earl of Bosse 



[May 31 : 



As to future extension of these observations. Notwithstanding 

 all discouragements of weather, we shall still watch for eclipses and 

 even " near approaches " at Full-Moon ; but we much desire a more 

 sensitive apparatus, and we must turn our thoughts to the radiomicro- 

 meter with suitable siderostat, to the bolometer, or to what I should 

 much prefer, a more sensitive reflecting galvanometer to be used with 

 the thermocouple. I believe that skilful and delicate hands can 

 improve much upon the present instrument. 



Fig. 16. 



It may be asked — can heat experiments be extended to other 

 Heavenly Bodies ? The step is a long one. The relative proportions 

 of light coming to us have been estimated as below — 



Full Moon 

 Venus 



= 1325; 



and 



Full Moon 

 Sirius, 



Full Moon 

 Jupiter 



12,703, 



= 6426 



or, if we assume the Moon's heat-radiation as compared with her 

 light to be eight times as great as that of the Sun, or a star, the 

 proportion between the heat-radiations of the Moon and of Sirius is 

 100,000. Much greater sensitiveness is therefore needed. It will 

 be quite possible, I believe, even with the present thermopile and 

 galvanometer to measure the radiation from the various parts of tbe 



