48 



ASTRONOMICAL PHENOMENA AND PROGRESS. 



or a statement of its conditions, and what he 

 proposes to do during the coming year. It is 

 proposed to distribute this programme among 

 other observatories, in order to avoid the du- 

 plication of work. A recommendation of the 

 preceding year is also renewed that a board of 

 visitors be appointed annually to examine into 

 the conduct of the observatory and make such 

 suggestions as may add to its general useful- 

 ness. With the great equatorial Prof. Hall has 

 continued his observations of the satellites 

 of the planets Mars, Saturn, Uranus, and Nep- 

 tune, and has also made some determinations of 

 stellar parallax. Good progress has been made 

 in reducing the observations of these satellites 

 made in preceding years, and it is proposed to 

 continue it steadily until all the observations 

 have been completely reduced and the final re- 

 sults obtained. With the transit circle Prof. 

 Eastman has continued the observations upon 

 the sun, moon, and planets, as made in previous 

 years. The old equatorial, in charge of Com- 

 mander Sampson, has been employed princi- 

 pally in observing comets, small planets, and 

 occultations. Time-signals are sent to various 

 points in the United States as usual, and since 

 Nov. 18, 1883, the noon-signal has been given 

 corresponding to the seventy-fifth meridian 

 west of Greenwich, or eight minutes twelve 

 seconds before Washington noon. An appro- 

 priation for beginning work upon the proposed 

 new observatory, for which a site was purchased 

 three years ago, is again requested. 



Physical Constitution of the Sun. In previous 

 volumes of this " Cyclopedia " mention has 

 been made of Prof. Langley's researches upon 

 the sun's heat. These researches mark an epoch 

 in the history of the subject, from the facts that 

 those made on Mount Whitney were made un- 

 der conditions more favorable than any which 

 have hitherto been obtained, that they were 

 made with a newly invented instrument of ex- 

 traordinary sensitiveness (the bolometer), and 

 that the law of absorption in the atmosphere 

 was taken into account more accurately than by 

 previous investigators. Beginning with ques- 

 tions of atmospheric absorption, Prof. Lang- 

 ley remarks that in all previous investigations 

 of the subject it has been assumed that every 

 successive layer of atmosphere through which 

 the solar rays passed absorbed a quantity of 

 solar heat proportional to its density and the 

 length of the path of light through it. He 

 showed that, although this law was true of 

 light of any one color, it was not true when 

 the whole light and heat of every color or wave- 

 length was measured. To show the principle 

 involved let us suppose light of two colors, red 

 and blue, to come from the sun to our atmos- 

 phere. Let us also imagine that the atmos- 

 phere exerts a very powerful absorption on the 

 blue light, but suffers the red light to pass free- 

 ly. Then a largo quantity of bine light will be 

 absorbed in the upper regions of the air, thus 

 leaving very little of that color to reach the 

 lower levels. The result will be that a small- 



er proportion of the total light will be absorbed 

 at the lower levels, because nearly all the blue 

 light having already been absorbed, there will 

 be none of that color left to be absorbed, while 

 the red light passes freely in any case. Now, 

 since observations can only be made at the 

 lower strata near the earth, it follows that the 

 rate of absorption in these strata will be less 

 than in the higher strata. Consequently, the 

 allowance for absorption will be too small, and 

 the quantity of heat emitted by the sun, as cal- 

 culated from observation at the earth's surface, 

 will likewise be too small. If the difference of 

 the rates of absorption depended only upon the 

 red and blue, as we have supposed, the prob- 

 lem would be easy of solution ; in fact, however, 

 the light which is most absorbed is scattered 

 all through the spectrum, as is shown by the 

 dark lines, and it thus becomes impossible to 

 make an accurate calculation. Prof. Langley, 

 however, found that from the best estimate 

 that he could make it was probable that 40 per 

 cent, of the total amount of heat radiated by 

 the sun toward the earth was absorbed in the 

 clearest atmosphere. By measuring the quan- 

 tity of "heat which actually reached his in- 

 strument, and allowing for the absorption, he 

 reached the following conclusion : 



Let a plane surface measuring one square 

 centimetre be exposed perpendicularly to the 

 sun's heat at the mean distance of the earth 

 from the sun ; and let the absorbing atmos- 

 phere be entirely removed. Then the quan- 

 tity of heat which will fall on that surface will 

 be such as will raise the temperature of one 

 gramme of water at the rate of nearly 3 cen- 

 tigrade per minute. This result is considera- 

 bly greater than that obtained by previous ex- 

 perimenters who observed under less favorable 

 conditions and did not properly allow for ab- 

 sorption. 



In connection with these researches Prof. 

 Langley has also investigated the heat spectrum 

 of the sun far below the ordinary visible spec- 

 trum, and found in it a great number of lines 

 produced by the absorption of the sun's atmos- 

 phere or that of the earth. Respecting the 

 apparatus with which these determinations 

 were made, Prof. Langley remarks that although 

 its results are better than those generally ob- 

 tained in heat measures, it is necessarily infe- 

 rior to the eye, and that its use may possibly 

 at some future time be superseded by photog- 

 raphy. The general result maybe summed up 

 as follows : Besides the light-waves that can be 

 perceived by the eye, the sun sends out heat- 

 waves, which differ from the light-waves in 

 nothing except being of greater length and 

 therefore imperceptible to the eye. This has 

 been long known, but Prof. Langley, by in- 

 venting a species of artificial eye, as embodied 

 in the bolometer, has been enabled to investi- 

 gate these obscure rays and measure their wave- 

 length to a higher degree of accuracy than had 

 before been attained. He concludes that this 

 dark spectrum is far longer than was supposed, 



