ADVISORY COMMITTEE ON ASTRONOMY 109 



In order to avoid the shifting of the beam with varying declination 

 of the Sun, each beam may encounter near the coelostat a second 

 mirror itself capable of traveling north and south on a track, and 

 to be moved every few days north or south as occasion requires. 

 In order to use the principal coelostat mirror at favorable angles 

 both forenoon and afternoon, its second mirror has two such tracks 

 close to the coelostat, which connect by a curve around the instru- 

 ment, so that the second mirror may be wheeled to the east or west 

 track, according to the hour angle of the Sun. The other beams 

 for other researches should have only one track position for their 

 second mirrors, choosing the position most favorable for morning 

 or for afternoon observation, as experience would dictate. 



While the beams from the second mirrors could be reflected in 

 any direction, a north-and-south one is to be chosen to insure entire 

 invariability of position. 



Accordingly the concave mirror of 30 inches or more aperture and 

 of 200 feet focus would be located north or south * of the coelostat 

 some 60 feet, and would send its beam horizontally through a tube 

 under the coelostat, where the solar image is received at the slit of 

 the spectro-bolometer apparatus. 



I have prepared a further list of apparatus for solar work, with 

 the prices of each piece, but in view of the brief time at my disposal 

 to prepare this preliminary statement, I think it better to give here 

 only the main points. 



All this is for the study of solar radiation, principally but not ex- 

 clusively for that of heat, and it is distinguished by the abundant use 

 of large mirrors, plane and concave, usually in connection with the 

 coelostat and catoptric telescopes. These are associated with special 

 buildings maintaining the bolometric and galvanometric trains at 

 constant temperature, and at the upper station there may be a special 

 construction to enable the observer to work in a special atmosphere. 

 There will also be a special teleferage system or other provision for 

 ready transport to the upper station. There will be two dioptric 

 telescopes of at least 12 inches aperture for photography, and ad- 

 juncts too numerous to be mentioned here, but which are indicated 

 fully in a separate note, to be communicated later. 



This list of apparatus can later, if desired, be furnished in detail, 

 with estimates for cost of each piece. 



The total estimated expenditure for this apparatus, with the build- 

 ings, is a little under $150,000; but this does not include the roads 



* According to which hemisphere is selected for the observatory. 



