1890.] 



On the Heat of the Moon and Stars. 



497 



from moment to moment and not only at ten seconds intervals, I 

 should record the movement of the index photographically. It would 

 also be an advantage to have the telescope under cover, owing to 

 the annoyance caused by wind. I did not adopt this plan, as I did 

 not wish the apparatus to be influenced by any unnecessary source of 

 heat. Now that I have found how insensitive it is to everything in 

 the nature of radiation, except that which is focussed by the tele- 

 scope upon the disc, I know that this precaution was' unnecessary as 

 well as inconvenient. The apparatus, as designed, is not suitable for 

 spectroscopic examination of the moon's heat, and so I have not 

 attempted to repeat any of Langley's observations. I may state that, 

 though neither the properties of the circuit employed nor the ten 

 seconds readings were suitable for detecting local variations of the 

 moon's heat, due to bright or dark spots, yet I did expect to find 

 indications of local differences. None, however, were apparent, but I 

 can hardly imagine that with a much smaller disc, a more rapid period, 

 and a photographic record local variations could not be deter- 

 mined. 



As my observations on the moon's heat are so small in number and 

 were made at such irregular intervals, I have not applied Lord 

 Rosse's correction* for its absorption by the air. 



The curves of the moon's heat require a slight correction for the 

 lag of the apparatus, which I have not attempted to make and which 

 it would be useless to make unless the curves were drawn by a more 

 perfect method. The correction could be reduced to a great extent 

 by causing the telescope or a siderostat to so move as not quite to 

 keep pace with the earth's rotation, so that a greater time would be 

 occupied by a transit ; but it would be unsafe to prolong this time to 

 a great extent, for, besides the greater uncertainty in the position of 

 the zero, a slight change would be produced by the varying altitude 

 of the part of the sky under observation, and this would change the 

 atmospheric absorption. As the telescope is moved from the horizon 

 to the zenith the index moves over the scale to an extent which is 

 very variable, but in the direction of cold. 



I should have stated that very thin clear mica absorbs a large 

 proportion of the moon's heat. On the night that I tried this I was 

 unable to measure the proportion, as while with the mica I obtained a 

 deflection (not recorded in my notes) of roughly 200 mm., without 

 the mica the index went off the scale and I had no stop to limit the 

 radiation. 



Anticipating trouble owing to the varying state of the sky, such as 

 was found by Mr. Stone, I devised a form of radio-micrometer circuit 

 which should be differential. Thus, calling the alloys used A and B, 



* « Phil. Trans.,' 1873, p. 598. 



