68 REPOET OF THE SECRETARY. 



siJectrometer chamber in the place of lime, as heretofore. A lead tank about 6 feet 

 long and 15 inches wide is supplied with about a gallon at a time of crude sulphuric 

 acid. This acid will abstract more than an equal volume of water from the air 

 before requiring changing, and its use has been attended with very satisfactory 

 results in preserving the faces-of the rock-salt optical apparatus. 



GENERAL ALTEKATIONS. 



The Observatoiy has received necessary repairs, and has been repainted. Its color 

 is now a light drab, and the roof is white in place of the previous brown. It is 

 found that the diflterence of the temperature maxima of thermometers within and 

 without the Observatory has become nearly 4*^ C. greater since the repainting. This 

 greatly ameliorates the condition of the observers. 



It is proposed to introduce a system of cooling by ammonia gas, to be regulated 

 automatically, so that the apparatus can be maintained through the months of 

 April, May, June, July, August, September, and October, as in the winter, at a 

 standard, constant temperature. As much of the clear observing weather comes in 

 these months, much is hoped from this change. 



PEKSONNEL. 



Mr. L. E. Emerson closed his period of service at the Observatory July 1, 1897. 



SUMMARY. 



I may sum up the result of the year's work by saying that the positions of between 

 200 and 300 lines in the infra-red solar spectrum have been accurately established ; 

 that an account of the researches of the observatory has been prepared for publica- 

 tion ; and that the instrumental equipment has reached a state of excellence never 

 before equaled. 



Respectfully submitted. 



C. G. Abbot, 

 Jid Acting in Charge, Astrophysical Observatory. 

 Mr. S. P. Langley. 



Secretary of the Smithsonian Institution. 



