TEMPERATURE OF THE SURFACE OF THE MOON. 



MfasnremenU. 



21 



From these ineasnreineTits it was coiicliuled that the Hilger jrlass transmitted 2ft percent., the 

 glass A 28 per cent., ami the glass B 30 ])er cent, of the lunar radiation under the conditions iu 

 question. 



A similar series of excellently accordant observationson the same evening by another observer 

 placed the trr.nsmissibility of the Ililger glass at 27 per cent., of glass A at 27 per cent., and of 

 glass B at 26 per cent. 



The most striking feature about these results i.s their very fair agreement among themselves 

 and yet their discordance with the previons measures of June 21, which also exhibit no striking 

 discrepancies of au order equal to that existing between the two sets of measurements. Ifc 

 was therefore concluded that this discrepancy was in all probability chiefly due to purely local 

 causes affecting the couditiou of the a])paratns at the time of experiment, which we "have reason to 

 believe is in a large measure accounted for by the differing methods of exjiosure, and at the follow- 

 ing lunation the measures were repeated, varying these conditions with especial regard to the fol- 

 lowing points: (1) Temperature of different portions of the apparatus, particularly of the large 

 concave mirror; (2) temperature of the glass; and (3) place at which the glass was interposed. 

 For the piP'pose of varying tiie latter condition, the large pane of window glass, already referred 

 to, was fixed in a frame so fastened to the large flat of the siderostat that the lunar rays incident 

 on the flat were first obliged to pass normally throngh the glass, without, however, being inter- 

 cepted by it on their way to the aperture in the wall of the building.* Tlie glass could be instantly 

 withdrawn from the frame when desired and interposed immediately in front of the bolometer as 

 in previous experiments, or elsewhere iu the i)ath of the rays. These later experiments were car- 

 ried out with all due pi-ecautions, exposures bciing made only by inclining the siderostat mirror by 

 means of its azimuth motion iu the manner x>revionsly desciibed. The transmission of the large 

 pane for .solar rays was determined with special care by a long series of observations, and was 

 substantially the same as that given by former measures. The same apparent transmissibility was 

 found for all positions of the glass, whether in the open air above the siderostat mirror or imme- 

 diately in front of the bolometer case inside the building. 



Experiments on the eflect of varying the temperature of the glass and concave mirror by 

 warming were only partially successful, since the immediate eflect iu either case was naturally 

 that the progressive cooling of the heated object produced a violent "drift" of the galvanometer 

 needle, so that measurements could only be resumed when the temperature had fallen nearly to 

 that of the sujrounding objects. Within this limited range of temperature, however, the trans- 

 mission of the glass did not ai)i)ear to vary as it presumably would have done had there been any 

 change iu the bygrometric condition of the surface of the plates or other disturbing cause. 



"Since glass is atUermanous to radiations from sources of such temperature as the b(dometer strips or the walls of 

 the room, its position with reference to these might (conceivably) aft'oct the result. Although tlie mode of observa- 

 tion was calculated to eliminate any such effect, the experiment of placing the glass outside the building was there- 

 fore tried. 



