REPORT OP THE SECRETARY. 75 



unpromising weather, succeeded on September 3, under the most perfect sky and 

 in exceptionally dry air, in making a complete and satisfactory series of solar 

 constant measurements. A prism of quartz and two mirrors of magualium were 

 the only optical parts to affect the rays, so that it was possible to observe from 

 wave length 0.29 (a to wave length 3.0 /*. This extended region includes not only 

 all the visible but the ultra-violet and infra-red spectra, with sufficient com- 

 pleteness to include in the discussion apparently within 1 per cent of all the 

 rays which the sun sends the earth and to make the allowance for rays not 

 observed practically sure. During the same day Mr. Ingersoll observed with 

 the usual complete spectro-bolometric outfit on Mount Wilson, and his results 

 were in accord with what would be expected from his preceding and following 

 day's work there and agreed within 1 per cent with those obtained simultaneously 

 on Mount Whitney. 



In view of the agreement of results on the solar constant of radiation obtained 

 at sea level, 1 mile, and 2J miles elevation, it now seems highly probable that we 

 can really by Langley's method of homogeneous rays allow for losses in the air 

 and get the same values that we would observe directly if we could take our 

 instruments above the air altogether. 



The reduction of spectro-bolographic work to the absolute scale of pyrhelio- 

 metry enables us to give as the average value of the solar constant of radiation 

 for the epoch 1905 to 1909, 1.924 calories per square centimeter per minute. It 

 is probable that observations at sun-spot minimum will tend to raise this value 

 by rather more than 1 per cent, so that we may suppose the mean value of the 

 solar constant for a complete sun-spot cycle will be about 1.95 calories. 



Experiments made in 1909 at Mount Wilson with various optical systems 

 agree within their probable error with one another, and with the results obtained 

 on Mount Whitney in fixing the distribution of energy in the spectrum of the 

 sun outside the atmosphere. In the Mount Whitney work the curve of energy 

 distribution was followed to a wave-length estimated (not very accurately) as 

 0.29[i and it there practically reached zero intensity, although the quartz and 

 magnalium apparatus would have been capable of transmitting the rays, had 

 they existed, of much shorter wave-lengths. In the spectrum of the " perfect 

 radiator," corresponding to the apparent temperature of the sun, the intensity 

 of the ultra-violet rays would be of some importance for a considerably farther 

 stretch of wave-lengths beyond this. It therefore appears that either the earth's 

 atmosphere, even above Mount Whitney, or else the sun's envelope, effectually 

 hinders the solar rays. If it is the former, then it may be that the above-men- 

 tioned value of the solar constant should still be raised a few per cent. But the 

 known powerful selective absorption of vapors in the sun's envelope seems quite 

 reasonably competent alone to produce the observed weakness of the solar 

 spectrum in the ultra-violet. This view is confirmed by experiments of Miethe 

 and Lehmann, who found no extension of the solar spectrum with increasing 

 elevation, although they shifted their observing station from Berlin (50 meters) 

 to Monte Rosa (3,500 meters), thus greatly diminishing the layer of air 

 traversed. Their shortest wave-length was 0.2911^, closely agreeing with ours. 



From our experiments of 1909 the apparent average solar temperature is 

 6430°, 5840°, or 6200°of the absolute, according as we follow Wien's displace- 

 ment law, Stefan's law, or Planck's law as the method of computation. But the 

 temperature of the sun, apart from the uncertainty of terms when dealing with 

 such high values, is probably a quantity which has very various values, from 

 the center to the limb of the sun's disk, depending on the depth within the sun 

 at which the radiation originates. 



At Washington Messrs. Fowle and Aldrich have continued experiments on 

 the transmission by moist columns of air for long-wave radiation, though with 



