SCIENCE. 



1 1 



THE CAMBRIDGE OBSERVATORY. 



The Annual Report of Prof. Pickering, Director of 

 Harvard College Observatory, shows that the Observa- 

 tory has been in a most prosperous condition during the 

 past year, and if the same financial support is extended 

 to it in the future that has been so generously offered in 

 the past few years, it will be enabled to retain its place, 

 inferior to no other Observatory in the country. The 

 work carried on at the Cambridge Observatory consists 

 of observations with the 15m. Equatorial, with the Meri- 

 dian Circle and Meridian Photomster, together with the 

 attendant reductions ; and in the distribution of time- 

 signals over the greater part of New England. 



With the large equatorial, many important observa- 

 tions upon the satellites of Mars were made during the 

 opposition of that planet. Employing the method of 

 reducing the light of the planet, by colored glass (a 

 method first used at this Observatory), the number of 

 observed position angles of Deimos was 825 ; of Phobos, 

 278 ; and that of observed distances, 248. The probable 

 errors of one setting were respectively o.6\ 0.9' and 0.6". 

 Besides the micrometric work, many photometric obser- 

 vations were made, the results of which indicate that if 

 we assume the satellites to have a capacity for reflecting 

 sunlight equal to that of Mars itself, Deimos has a diam- 

 eter of about six, and Pnobos of about seven miles. The 

 photometric observations upon the eclipses of Jupiter's 

 satellites give reason to believe that by this method the 

 determination of longitudes may be made as accurately 

 as by occupations or lunar culminations. Measurements 

 of the light of planetary nebulae have been continued. 

 The spectra of nebulas are also observed through a direct 

 vision prism placed between the object glass and eye- 

 piece of the telescope. The planetary nebulae retain 

 their shape under these circumstances, obviously indi- 

 cating that their light is monochromatic. The difference 

 between monochromatic objects and ordinary stars is so 

 marked when thus examined, that a method of detecting 

 small nebulas was at once suggested, and a compara- 

 tively short search revealed three such objects. The 

 most remarkable discovery, however, was in the spec- 

 trum of the star Oeltzen 17681, R.A. i8h. im. 17s., 

 Dec. —2i° 1', which shows that the light is concentrated 

 in two poin f s of the spectrum, one in the blue, the other 

 in the yellow. A faint, continuous spsctrum is also seen. 



Between Sept. 24, 1879 an ^ Nov. 1, 1880, observations 

 were made with the Meridian Circle on 277 days, the 

 work being confined to the determination of the absolute 

 co-ordinates of 109 fundamental stars, in connection 

 with which observations of the sun and of Polaris were 

 made as often as possible. Up to Nov. t, 1880, 183 ob- 

 servations of Polaris had been obta.ned, 131 of the Sun 

 and 1760 of Fundamental Stars. To furnish the means 

 of measuring the variation of the instrumental changes 

 between one culmination of Polaris and the next, a col- 

 limator with focal length of 206 feet was constructed 

 and ha» given excellent results. 



A Meridian Photometer devised by Prof, Pickering has 

 been used in continuing the measurement of the light of 

 all stars visible to the naked eye between the north pole 

 and the parallel of 30 south declination. Over 40,000 

 separate settings have already been made, v and it is prob- 

 able that the work will be completed in October next. 

 The instrument, as its name implies, is mounted in the 

 meridian and forms polarized images of the pole star 

 and the star to be observed, which are brought to equal- 

 ity by turning a Nicol prism. 



The time signals from the Observatory are distributed 

 to the railroads and several prominent jewelers in Boston, 

 and through the railroad companies to many of the 

 neighboring towns. By the co-operation of the United 

 States Signal Service Officer a time-ball is dropped in 

 Boston at noon. The signals are also used in connec- 

 tion with those from the United States Naval Observa- 



tory, and the Allegheny City Observatory for the regu- 

 lation of the New York time service. 



During the past year, the second part of Volume XI of 

 the Annals of the Observatory, containing a discussion 

 of 25,000 photometric observations made with the great 

 equatorial, and Volume XII containing the results of ob- 

 servations made by Prof. W. A. Rogers in 1874 and 

 1875 w ' tn the Meridian Circle have been completed and 

 distributed, and six more volumes are in a more or less 

 advanced state of preparation. W. C. W. 



Washington, D. C. 



ON THE THERMAL BALANCE* 

 Bv Prof. S. P. Langley. 



When the thermometer is not sufficiently sensitive for 

 delicate investigation of radiant heat, scientific men have 

 been accustomed, since the time of Melloni, to the use 

 of the thermopile, an instrument which, employed in 

 connection with the galvanometer, permits the making 

 of numerous important measures. It has not been im- 

 proved materially in the last fifty) ears. Meanwhile, many 

 problems of both high theoretical and practical interest 

 have arisen, which cannot be solved without a more 

 sensitive and accura'e instrument. One of these prob- 

 lems is the measurement of the distribution of radiant 

 energy in a pure spectrum, when the rays have not passed 

 through any prism. I could obtain no accurate results 

 with the thermopile. I was forced to invent a more sen- 

 sitive instrument for this special investigation, and, having 

 done so, I believe it will be of general utility. The prin- 

 ciple of the new apparatus has been applied by Dr. Sie- 

 mens and others to other purposes. I spent several 

 months in making it, as I hope, a useful working tool for 

 the physicist and the physical astronomer. It is founded 

 on the principle that, if a wire conveying an electric cur- 

 rent be heated, less electricity flows through it than be- 

 fore. If two such wires, carrying equal currents from a 

 powerful battery, meet in a recording apparatus (the 

 galvanometer) the index of the instrument — pushed in 

 two opposite ways by exactly equal forces— will remain 

 at rest. If one current be diminished by warming ever 

 so little the wire that conveys it, the other current causes 

 the index to swing with a force due, not directly to the 

 feeble heat which wanned the wire, but to the power of 

 the battery which this feeble heat controls. 



The application of this principle is thus made: Iron or 

 steel is rolled into sheets of extreme thinness. I have 

 succeeded in rolling sheets of steel made at the works of 

 Miller & Parkin, Pittsburg, Penn., until it took 8000 of 

 them to make the thickness of an inch. Of the platina 

 sheets rolled at the United States Mint in Philadelphia, 

 fifty laid one on another do not together equal the thick- 

 ness of light tissue paper. Minute strips of these, 1-32 of 

 an inch wide and % of an inch long, were united so as 

 to form a prominent part of the circuit, through which a 

 part of a powerful battery passed to the galvanometer. 

 Experiment proves that an almost inconceivably minute 

 warming of a set of these strips reduced the passage of 

 the electricity so as to produce very large indications on 

 the registering instrument. I have in the course of my 

 experiments thus far, found iron the most advantageous, 

 though other metals are still under trial. The instru- 

 ment thus formed is from ten to thirty times more sensi- 

 tive than the most delicate thermopile ; but this is almost 

 a secondary advantage compared with its great precision 

 and the readiness with which it is used. The thermo- 

 pile is very slow in its action. This new instrument, the 

 thermal-balance, takes up the heat and parts with it 

 again in a single second. It is almost as prompt as the 

 human eye itself. 



With reference to its accuracy, experiments prove 

 that the probable error of a single measurement made 



* Read before the National Academy of Sciences, N. Y., x88o 



