NO. 8 SAMUEL PIERPONT LANGLEY ABBOT 33 



was directed — a statement which will not surprise any professional 

 astronomer. It is made here to emphasize the like statement that 

 there is, then, no limit to our power of perception of tremors. These 

 are, it will be remembered, instances which may be paralleled in 

 illustrations drawn from the use of other senses, and not peculiar to 

 the present observation. 



" Clearly, we may never distinguish the entire number of solar 

 lines which exist here more than we could in visible spectra by the use 

 of the eye or by photography. In every case there must finally come a 

 time when we must stop our investigations because we have reached 

 a degree of minuteness in the solar lines corresponding to the inter- 

 vening disturbances due to terrestrial causes, which we can never 

 eliminate." 



"ON A POSSIBLE VARIATION OF THE SOLAR RADIATION AND 



ITS PROBABLE EFFECT ON TERRESTRIAL 



TEMPERATURES " 



This was Langley's last important paper. It was based on observa- 

 tions by Mr. Fowle and the present writer made at Washington. After 

 long experience in far better observing locations we cannot suppose 

 that the solar variations indicated in 1903 were real. Nevertheless 

 they embarked us on a long endeavor to determine accurately the limits 

 of the solar variation and its effects on weather. This investigation 

 now [1934] seems certain to be of quite as great importance as 

 Langley ever dreamed, for it gives promise of long-range weather 

 forecasting, not only for seasons but for years in advance. But let 

 us quote from the paper. 



" The purpose of the present communication is primarily to discuss 

 the validity of a surmise we may entertain, founded on observation 

 here, as to certain possible changes in the solar constant. There is 

 especially discussed a possible falling off of solar radiation about the 

 close of March 1903, as indicated by certain recent values of solar 

 radiation computed from observations here, and compared with ac- 

 tually observed temperatures for eighty-nine stations of the North 

 Temperate Zone. 



" The homogeneous rays are observed here by the bolometer, and 

 the holographic curves from which the atmospheric extinction of 

 radiation is inferred, traced by the movement of the spot of light upon 

 the galvanometer scale, are now very much more satisfactory than 

 formerly. They represent an immense gain over the conditions operat- 



