ADVISORY COMMITTEJE; ON ASTRONOMY 105 



special provisions for breathing, for the indispensable condition in 

 future successful study of the heat radiation of the Sun is an alti- 

 tude where the greater portion of our atmosphere lies below us. 

 This is for the upper site only, which is in connection with the near 

 lower observatory in view from the upper, where the routine work 

 will be carried on. 



I wish, while repeating that this altitude is indispensable for some 

 of the objects of the proposed solar observatory, to observe that it 

 is not so for all. I wish also to recall that the whole plan involves 

 primarily the use of special apparatus for solar observations. This 

 primary use admits, nevertheless, that much of this apparatus can 

 be advantageously employed for the study of such other objects as 

 the photography of nebulae, the Moon, or visual or photographic 

 topography of Mars, though you will notice that these latter pur- 

 poses may demand special apparatus other than that here indicated, 

 which concerns the Sun primarily. 



A necessary preliminary for the choice of any such twin site will 

 be an examination by an expert of the conditions of " seeing " in 

 the daytime for solar purposes only, to find whether the seeing is 

 good for this purpose, quite irrespective of the condition of the vision 

 at night, and this should be done soon, if at all, since the time of 

 the next sun-spot maximum draws near. 



The researches of which I particularly treat here chiefly involve 

 measures of the heating effects of radiation, 



I. Principal Objects of Inquiry of a Distinctly Solar 



Observatory. 



(a) To determine the "solar constant," so called — that is, the 

 heat equivalent of the solar rays falling perpendicularly upon a 

 given area outside of the earth's atmosphere in a second of time. 



{U) Whether this quantity be fixed or variable, and if the latter, 

 how it varies through a term of years, and especially what connec- 

 tion exists between such variation and the sun-spot cycle. 



(r) To determine what absorption the solar beam experiences in 

 passing through the earth's atmosphere. The complete answer to 

 this question implies a knowledge of the transparency of all layers 

 of the air — high, medium, and low — and for all wave-lengths. It 

 should also imply a repetition at both stations of the detailed infra- 

 red line spectrum research already made. 



{d) What absorption does the solar beam experience in passing 



