[ 481 ] 



IX. 



LUNAR EADIANT HEAT, MEASURED AT BIRR CASTLE OBSERYATORT, 

 DURING THE TOTAL ECLIPSE OF JANUARY 28, 1888. By OTTO BOBD- 

 DICKER, Ph. D. With an Introduction by THE EARL OF ROSSE, K.P., LL.D., 

 F.R.S., &c., President of the Royal Dublin Society. Plates LIII., LIV., LV. 



[Read Febeuaey 18, 1891.] 



Introduction. 



Some years ago it was suggested that it would be desirable to turn tbe large 

 Reflectors at Parsonstown, armed with the thermopile, upon the moon, with the 

 view of detecting, and, if possible, measuring its radiant heat. Several attem^Dts 

 to do so had been made elsewhere, but without encouraging results. Melloni had 

 employed a lens of three feet aperture of imperfect quality, but sufficiently good 

 for the purpose. Owing, however, to the large absorption of heat by glass he 

 obtained no very certain indications, his pile could not have received any of the 

 less refrangible heat rays through the glass, but it appeared quite possible that 

 with a speculum of equal aperture far more decided and really interesting and 

 instructive results might be arrived at. Accordingly, I procured a thermopile of 

 four elements, with surface of face half an inch square, from Messrs. Elliott, fixed 

 it in the principal focus of a concave reflector of three and a-half inches aperture 

 and three inches focus, and placed the whole in my three-foot telescope, the con- 

 cave mirror being situated at the focus, and directed towards the speculum. 

 Thus the whole of the Moon's radiant heat (and light) which entered the three- 

 foot tube was concentrated upon a spot of one-third of an inch in diameter, on 

 the centre of the face of the pile. At first the indications were very uncertain. 

 They were so masked by accidental disturbances that thej^ could be detected with 

 certainty only by taking the mean of a series of readings with the telescope 

 directed towards the Moon, and comparing it with that of a series with the 

 telescope turned away from the Moon. I then procured a second pile from Messrs. 

 Elliott, fitted it and a similar concave reflector close beside the first, and placed 

 both piles, but with poles in reversed position, in the same circuit with the 

 galvanometer. Thus, by turning the telescope, so that the Moon's image fell 



TKiNS. EOT. DUB. SOC, N.S, VOL. IV., PAET IS. 3 T 



