238 TRANSACTIONS OF THE [mAY 17, 



really scientific measurements are only about fifteen years old, 

 due to Lord Eosse, at Parsonstown, and to Marie Davy, at Paris; 

 and they seemed to show that at the time of full moon we receive 

 from our satellite, not merely reflected heat, but warmth radiated 

 from the moon's surface; as if this surface were raised to a con- 

 siderable temperature by the long insolation to which it has been 

 exposed during the preceding fortnight. Lord Rosse estimated 

 the probable temperature of this heated rock to be as high as 

 from 300° to 500° F. 



But within the past four or five years this conclusion has been 

 called in question. Observations at Parsonstown, of the rapid 

 diminution of radiation during a lunar eclipse, seem to favor the 

 newer view, that the moon's surface, like that of a lofty moun- 

 tain-top on the earth, never gets very hot, since the absence of 

 air enables the solar heat to escape nearly as fast as it is received. 



Professor Langley's recent and still progressing work ui)on this 

 subject far excels in delicacy and elaborateness anytliing done 

 before. At first it seemed to show that the temperature of freez- 

 ing water was never reached even at the hottest parts of the lu- 

 nar surface ; but the later observations throw some doubt on 

 the legitimacy of this inference. It is found that the radiation 

 from the moon unquestionably contains a considerable percentage 

 of rays which have a wave-length longer than any of the heat 

 rays from melting ice ; and this fact has been supposed to make 

 it probable that the moon's surface was colder than tiie ice. But 

 then, within a few weeks, Professor Langley has found the long- 

 waved rays in the radiation from an electric arc! So the ques- 

 tion still hangs debatable. 



The Sun's Parallax. 



I think we may say that, during the past ten years, substan- 

 tial progress has been made with the problem of the solar paral- 

 lax. The transit of Venus, in 188'-i, adds whatever value its re- 

 sults may have, to those obtained eight years before ; but, on the 

 whole, so far as can be judged from the reductions thus far com- 

 pleted and published, it would seem likely that the outcome of 

 the transit observations will be simply to confirm the results ob- 

 tained by other methods. It may be that the data obtained 

 from the German heliometer measurements will prove more ac- 

 cordant and decisive than those derived from photographs and 

 from the contact observations; there are flying rumors tiiat they 

 will, but it will be necessary to await the official publication for 

 certain knowledge on this point. If they do not, we shall be 

 ol)liged, hereafter, to relegate transit observations to a second- 

 ary rank, as a means of determining the sun's distance. From 

 the various observations of the two transits, different computers 



