REPORT OF THE SiKCRKTAUY. 25 



Lord Rayleigh, Sir George Stokes, and other (Miiineiit foreign men of 

 science, whose ap})reeiation could not ))e charged Avitli being inHucnced 

 by national prejudice, as Avell as by ei^ually disinterested AniiM'ican 

 ones, all showing a very gratifying approbation of the work of the 

 Observatory ])y those best competent to judge its merits. 



The principal work of the Astrophysical Observatory during the 

 past 3'ear has continued to l)e the study of the sun and its radiation. 

 While fully acknowledging the interesting nsiture of astr()[)hysical 

 investigation of the stars and nebuhe, the study of the sun h'.is a far 

 superior practical importance, for were the former ])odies to bcAvholly 

 blotted out they would be missed chiefly as ()])jects of scientitic inter- 

 est, while with the sun would l)e abolished life itself. The solar 

 researches of the past year have mainly ])een concerned with deter- 

 mining the amount and nature of the absorption of solar I'adiation in 

 the earth's atmosphere and in the solar envelope. These researches 

 are preliminaiy to and form an essential part of the measurement of 

 the total radiation of the sun. A presumption exists, almost amount- 

 ing to certainty, that the total radiation of the sun is vai'iable in some 

 relation to the appearance of sun spots, l)ut nothing is yet known to 

 definitely flx the amount of this supposed varial)ility or to measure 

 its effect upon the earth, though that effect, if so iixed. can not but be 

 of interest to every inha])itant of the earth's surface. 



The instrumental means, which thus have been the sul)ject of inces- 

 sant study and improvement here during the past ten years, for inves- 

 tigating such questions, are more efficient than at an}' previous time. 

 It will l^e seen from the detailed report of the aid acting in charge that 

 automatic bolometric curves accurately representative of the amount 

 and distribution of the solar energy at the observer's station may now 

 be obtained in a few minutes, covering nearly the whole spectral region 

 which reaches sea level, and wdiere occurs nuich of the great and 

 varying absorption ])y water vapor which influences oui' terrestrial 

 temperatures so greatly. 



Some twenty 3^ears ago the writer invented a then new instrument 

 for measuring minute quantities of heat, for, owing to circumstances 

 which this is not the place to detail, an accurate determination of the 

 possil)le variation in the enormous quantities of heat which the sun 

 sends the earth depends (paradoxically) upon the ability to measure 

 smaller (|uantities of heat than the most delicate thermometer can 

 possibly do. The " bol<)met(u-," tin* instrument of tlu^ writiM-'s inven- 

 tion, which is in rjuestion, was abh' to measure the tiiiMi unheard of 

 ([uantit}^ of somewhat moi-e than one one-hundred-thousandth of a 

 degree. Since thcMi, during fifteen years of constant advance, latterly 

 associated with a great improvement of the adjuncts, particularly of 

 the galvonomet(>r, at the hands of Mr. Abbot, this has been brought 

 to measure somewhat less than one one-hundred-millionth of a degree. 



