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 
systematic 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 
