438 



S. P. Langley — Temperature of the Moon. 



The memoir, of which an abstract is here given, contains num- 

 erous subsidiary researches and observations for which we must 

 refer the reader to the original, only here mentioning two : 



First. — Temperature correction of a rock salt prism. This 

 investigation of the change of refrangibility due to tempera- 

 ture was carried only far enough to give such approximate 

 accuracy as served our immediate purpose, giving a value of 

 — 13" of arc for each degree Centigrade, but subsequent meas- 

 urements have materially modified this, and I give the value 

 now adopted, which is — 9 7/, 5 the formula being 



d t o=d^-(t— 20)10" 

 where d— deviation, and t— temperature in Centigrade degrees. 



Second. — Comparison of the intrinsic intensity of solar radia- 

 tion with that of the electric arc in different parts of the 

 spectrum. This observation of the comparative intensity of 

 the sun and the electric light is given in the original memoir 

 only so far as to show that it brings independent evidence of a 

 large atmospheric absorption of the extreme infra-red rays and 

 enables us to estimate approximately the amount of this absorp- 

 tion at each point in the spectrum. The observations were not 

 repeated to obtain such a thorough comparison as would be 

 desirable. As they have never been printed, however, and 

 since, as far as we know,. none other such exist, we will give 

 them here under the caution that they are to be considered 

 only first approximations. The light was that from the pit of 

 the positive carbon of gas coke, one inch in diameter, with the 

 current derived from a dynamo, actuated by an engine of 

 ten horse power, and therefore certainly at least as intrinsically 

 hot and bright as any smaller arc-light in more common use, 

 and presumably much more so. The apparatus was that 

 already described in the memoir " On Hitherto Unrecognized 

 Wave-lengths."* 



The fol'owing table gives the observed galvanometer d< flec- 

 tions after applying a multiplying factor for the shunt, which 

 had to be used for the larger readings : 



1 



Deviation 

 Rock Salt 

 60° Prism. 



Wave- 

 Length. 



3 



Observed 



deflection 



sun after 



absorption. 



4 



Observed 



deflection 



arc. 



5 



Calculated 



deflection 



sun "without 



absorption. 



6 



Observed 

 ratio sun 

 and arc. 



< 7 

 Calculated 



ratio of 

 unabsorbed 

 sun and arc. 





/ 



/" 



div. 



div. 



div. 







43 



53 



373 



7 5 



1-3 



34- 



5-77 



26-0 



43 



17 



0.398 



11-5 



1-5 



38- 



7-67 



21-8 



41 



54 



0-489 



33 5 



5.3 



60- 



6 32 



14-4 



41 



05 



0-587 



1040 



10-5 



115- 



9 90 



111 



40 



45 



0-663 



201-0 



220 



204- 



9 27 



10-0 



40 



27 



749 



432-0 



43-5 



450- 



9 93 



91 



40 



05 



96 



1 073-0 



215-0 



1 763- 



4-99 



8-2 



39 



54 



1-13 



1 783-0 



459-0 



3 534- 



3-88 



7-7 



39 



20 



2-87 



1 905-0 



882-0 



5 645- 



2-16 



6-4 



39 



00 



4-3 



5210 



235-0 



1 363- 



2-22 



5-8 



38 



45 



56 



75.0 



157-0 



832- 



048 



5 3 



38 



00 



104 



195 



39-0 



156- 



0-50 



40 



See this Journal, xxxii, August, 1886. 



