260 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



who, in his determination of the solar constant (the amount of heat 

 received from the sun) , shows that this is 1 or 2 per cent less at sun 

 spot maximum than at minimum. The absolute amount of the sun's 

 heat at the surface of the earth is 1.9 calories, which may be more 

 simply stated as the amount of heat per square centimeter which will 

 raise 1 cubic centimeter of water 1.9° C. or 3.5° F. in temperature in 

 one minute, if the atmospheric absorption is neglected. It has also 

 been proved by Prof. Abbot that there are irregular variations in this 

 quantity, and it is hoped that a knowledge of these variations may be 

 of value in helping to predict temperature and meteorological changes 

 on the earth; a problem whose solution, even with all the advances 

 in science, seems as far off as ever. 



Another interesting problem, which at the meeting of the solar 

 union was advanced a stage, is the determination of the solar rota- 

 tion by the displacement of the spectral lines at opposite limbs of the 

 sun. Owing to the rotation of the sun on its axis in about 26 days, one 

 limb approaches and the other limb recedes from us, with a velocity 

 at the equator of about 2 kilometers per second. If the spectra of the 

 two limbs are brought side by side on the plate, the lines of the former 

 will be displaced to the violet, of the latter to the red; and with a high 

 dispersion spectrograph this displacement will be quite noticeable, of 

 the order of one-tenth of a millimeter. Some work has been done on 

 this problem by Duner at Upsala and by Halm at Edinburgh visually, 

 and more recently by Adams at Mount Wilson photographically. 

 Besides determining the rate, and the law of decrease of rotation with 

 different latitudes, there are other interesting problems, such as 

 variations of the rate for lines of different substances, which require 

 solution. A combined attack by six institutions, of which the 

 Dominion Observatory is one, on different well-distributed regions of 

 the spectrum has been arranged, and, in addition, each observer is to 

 measure a common region for comparison of results and removal of 

 s} 7 stematic error. 



Besides these definite advances, much other work in the distribu- 

 tion of the gases and metallic vapors over the photosphere, in com- 

 paring the spectra of the limb and center of the sun and along many 

 other lines, has been recently accomplished; and we may confidently 

 look for rapid development and increase of our knowledge of the con- 

 stitution of our luminary in the near future. 



Although the study of the sun is most intimately connected with 

 that of the stars, which was recognized at the solar union by the 

 appointment of a committee to discuss the question of the classifica- 

 tion of stellar spectra, yet we may perhaps turn for a moment to the 

 other members of our solar system and see if any new light has 

 recently been thrown upon the interesting question of conditions on 

 other planets. The perennial question of the objective existence of 



