86 REPORT OF THE SECRETARY. 



merits of the Observatory and its works. This report was referred in the Senate to 

 the Committee on the Library and ordered to be printed with all its inclosures. It 

 therefore became necessary to supply some new copies of illustrations for the 

 engravers, and opportunity was offered to correct a number of errors discovered in 

 Volume I of the Annals, so that this new edition, forming a part of Senate Document 

 No. 20, is considerably improved. The preparation of the first drafts for the report, 

 the preparation of new illustrations, and the reading of proofs naturally occupied con- 

 siderable time of the Observatory staff. Several hundred copies of this edition of 

 the Annals were secured for the Observatory, and thus it will be possible to supply 

 a limited number to those who may from time to time make request for them. 



Of the numerous inquiries addressed to the Institution by correspondents all over 

 this country and the world, many relating to astronomy, physics, and chemistry are 

 referred to this Observatory for answer, and thus an amount of time by no means 

 trifling is spent on work of no immediate concern to the Observatory researches. 

 However, the search for the information asked for so frequently leads to obtaining 

 knowledge of value in our own work, that it is on the whole, perhaps, time well 

 spent, independent of considerations of the convenience of the Institution's 

 correspondents. 



(2) Progress of ■invcstigatiuvs. 



The solar radiation; its total amount and its absorption.— Yon showed long since that 

 ordinary actinometry, or measures confined to the investigations of white light, can 

 never furnish trustworthy data in relation to the output of the sun, since the 

 absorption of our air is so different for different wave lengths that it is only by going 

 further and examining the spectrum in detail that satisfactory results are obtain- 

 able. This kind of energy spectrum work has been taken up very extensively here 

 in the past year. 



The solar energy spectrum.— The early work of this Observatory described in Volume 

 1 of its Annals was chiefly concerned with the minute absorption bands in the infra- 

 red solar spectrum. As all readers of the Annals are aware, automatic prismatic 

 energy curves were made by the aid of the holographic process, and such inflections 

 of these curves as were found not accidental in their origin corresponded to the 

 so-called Fraunhofer lines of the visible spectrum. Less attention was paid to the 

 whole height of these curves at their various parts than to these numerous small 

 variations in height. Still, a chapter of the Annals was devoted to the inferences 

 relating to the absorption of our atmosphere which could be drawn from the data 

 relating to secular variations in the general form of the curves then available. But 

 since the curves were taken at a slow speed to allow full time for recording the 

 small inflections, the total heights were less comparable, owing to possible changes 

 in the transparency of the air, the altitude of the sun, and the behavior of the appa- 

 ratus which miglit intervene l)ctweeu start and close of a curve. 



NEW wo UK. 



The absorption of Ike atmosphere and of the xnlar rn^v/oyje-— This present year care has 

 been taken to obtain energy curves whose heights shall be comparable. It was the 

 purpose of this work to study the general absorption of the sun's envelope, the gen- 

 eral absorption of the earth's atmosphere, and the changes in the selective absorp- 

 tion of water vapor in the latter. It is the further object toward which these studies 



Astrophysical Observatory under your superintendence. The bolometer, Avhich 

 science owes to your inventive resource, is one of the boldest and most original 

 instruments ever devised for scientific research. It permits the exploration of a field 

 which is entirely closed to any other means of investigation. The many results 

 which you have obtained with this instrument are deemed by all astronomers of the 

 very highest importance in the study of those parts of the spectrum concerning 

 which we should otherwise have remained in ignorance." — Sir Robrkt S. Ball. 



