68 



ANNUAL, KEPORT SMITHSONIAN INSTITUTION, 1911. 



much less for violet and ultra-violet rays than for red and infra-red ones, we 

 probably get infra-red rays from deeper-lying and hence hotter layers in the 

 sun than we do ultra-violet ones. 



We conclude that the solar radiation comes from sources ranging in tem- 

 perature perhaps between the limits 0,000° and 7,000° absolute centigrade, but 

 mostly from sources between 6,000° and 7,000°. 



Washington observations. — Further experiments have been made, 

 under Mr. Fowle's direction, on the transmission of radiation of great 

 wave lengths through long columns of air containing known qualities 

 of water vapor. Many of these observations are not yet reduced, 

 so that it is not yet proper to give a numerical summary of 

 results. The length of the column experimented upon has been 

 increased to 800 feet. The measurements cover the infra-red 

 spectrum, from the A line to a wave length of about 17/*. 

 The observations of the water contents of the air column 

 are made by means of pairs of wet and dry thermome- 

 ters located at a number of points along the path. 

 The air is thoroughly stirred before readings. Check 

 experiments by Mr. Aldrich, in which he drew the 

 air through phosphorus pentoxide tubes and 

 weighed the water absorbed, have confirmed 

 the accuracy of the water-vapor determina- 

 tions. Mr. Fowle has made a preliminary 

 comparison of the upper infra-red spec- 

 trum bands p, <r, r, <3?, SI>, and P., as 

 observed through the tube with 



ihe same bands as observed 

 through the whole atmosphere 

 at Washington, Mount Wilson, 

 and Mount Whitney. The re- 

 sults are most interesting, though 

 not yet ripe for publication, and 

 will probably lead to more exact 

 knowledge of the total quantity 

 of water vapor in the atmos- 

 phere, and its variation with 

 the altitude of the observer and 

 the season of the year. 



Reduction of observations. — 

 Upward of 100 days of solar- 

 constant measurements have 

 been made on Mount 

 Wilson on each of the 

 last several years. Each 

 day requires the equiva- 

 lent of three full days of 

 computation. This work 

 is being done at Washington by Messrs. Fowle and Aldrich and Miss Graves 

 and certain graphical parts of it by minor clerk Segal. The solar-constant 

 reductions are computed as far as the middle of the observing season of 1910. 

 Pyrheliometry. — Additional comparisons of the Mount Wilson secondary pyr- 

 heliometers have been made with primary standard pyrheliometer No. 3. These 

 are not yet all reduced, but such as have been finished confirm the results of 

 the previous fiscal year, so that we may regard the scale of absolute pyrheli- 



Fig. 3. — Abbot silver disk pyrheliometer. 



