REPORT OF THE SECRETARY. 



89 



meter. All three instruments are now in use and, so far as known, 

 with satisfaction. 



Two silver-disk pyrheliomcters were standardized for the proposed 

 South American expedition. 



Considerable work was done on the apparatus mentioned last year, 

 designed to measure the constant of the fourth power radiation 

 formula. Owing to trouble found in maintaining a vacuum in the 

 apparatus no actual determinations were made. 



Much attention was devoted to the preparation of the equipment 

 of a solar constant expedition for South America. The purpose of 

 the expedition, as stated last year, is by cooperation with Mount 

 Wilson to secure daily values as far as possible throughout the year 

 for several years, and thus to investigate the influence of solar vari- 

 ation on terrestrial temperature. 

 Many improved devices were in- 

 vented and constructed for the ex- 

 pedition. Among them is a new 

 vacuum bolometer of very high sen- 

 sitiveness and in every way exem- 

 plary behavior. This instrument is 

 constructed in such a way as to be 

 sealed off when highly exhausted, 

 like an X-ray tube. Having no 

 cocks or windows it requires no 

 further attention to maintain a 

 vacuum indefinitely. The construc- 

 tion of the sensitive strips follows 

 the indications of mathematical 

 analysis covering the whole theory 

 of the bolometer, so that a maxi- 

 mum sensitiveness is obtained. A 

 similar instrument was prepared 



also for Mount Wilson work. The high sensitiveness of the new 

 bolometer is indicated by the statement that when used with the same 

 spectroscope and galvanometer employed in our Algerian expedition 

 of 1912 more than tenfold deflections on the solar spectrum were 

 observed with similar conditions. 



Another new instrument is a special machine designed to aid in 

 reducing spectro-bolometry, in solar constant work. Heretofore we 

 have plotted, on large cross-section paper, logarithms of observed 

 radiation against the air-masses traversed by the solar beam. Nearly 

 forty such plots, each of six points, are required to represent a 

 morning's spectro-bolometry. The plotted points fall in approxi- 

 mately straight lines, whose projection to the zero of air mass yields 



Fig. 



1. — Computing machine in use In 

 North Carolina. 



