IO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



4. We suppose the layers of air to differ gradually from one to 

 another in transparency. This, according to Mr. Very, may be true 

 for some atmospheric elements, but there are others which are sharply 

 restricted to definite layers or other definitely formed volumes, so 

 that the ordinary air-mass formula fails for this cause. 



5. A considerable amount of solar radiation is said by Mr. Very 

 to be definitely lost to measurement in the atmosphere. The Smith- 

 sonian observations, he says, give merely the quantity A — B, where 

 B represents the absorption occurring in fine lines of atmospheric 

 origin, or radiation cut off by particles too gross to diffract the rays, 

 or that which is arrested by bands of absorption not composed of fine 

 lines, but large and diffused, and incapable of being distinguished 

 certainly amidst the crowd of lines and bands which occur in the 

 spectrum. 



6. The authors underestimate, according to Very, the solar inten- 

 sity in the infra-red part of the spectrum where terrestrial rays are 

 sent out. For they suppose the energy there is comparable to that 

 of a "black body" at 6,ooo°, whereas the sun's radiation is much 

 richer in long waves than that of a body at 6,ooo°. The solar 

 radiation does not correspond to that of a body of uniform tempera- 

 ture, but its infra-red part corresponds to a body at a higher 

 temperature than does its visible part. 



7. Mr. Kron is of the opinion that the authors underestimate the 

 solar radiation in the ultra-violet spectrum, owing to the powerful 

 atmospheric absorption there. 



8. Mr. Bigelow finds from thermodynamic considerations that our 

 solar constant values represent the intensity at about 40 kilometers 

 altitude, where the atmospheric pressure is less than yoVo of that at 

 the sea level, but that between this and the limit of the atmosphere 

 the radiation increases from 1.93 to 4.0 calories ! 



REPLY TO THESE CRITICISMS 



First objection. — In regard to (1) we may remark, in addition to 

 what we have said above, that nearly all the pyrheliometry now being 

 done in the world is done with Angstrom, Marvin, Michelson, or 

 Smithsonian pyrheliometers. These represent five independent 

 attempts to fix the standard scale of radiation. They have been 

 many times compared with each other, and are found in accord to 

 within less than 4 per cent, and now, in view of A. K. Angstrom's 

 researches, perhaps to less than 2 per cent. Of these scales of 

 pyrheliometry, ours gives the highest readings. We have devoted 



