ACADEMY OF SCONCES.] BIOGRAPHY. H 



observations of the planets and the moon to a homogeneous system, and Newcomb's Catalogue 

 of (1596) Fundamental Stars, reduced to an absolute system, to meet the needs of tbe almanacs 

 of the different nations. 



The need of a more accurate determination of the velocity of ligbt was pointed out by New- 

 comb in 1867. He brought the subject before the National Academy in 1878, and in response to 

 the academy's favorable report the Congress appropriated $5,000 to defray the expense of the 

 determination. The problem was assigned to Newcomb. He employed Foucault's method, 

 with improvements and refinements. His principal observing stations were at the foot of the 

 Washington Monument and at Fort Myer across the Potomac River. The distance between 

 the mirrors at the two stations was 3,721.21 meters. The observations were conducted in the 

 years 18S0-1SS2, and they led to a concluded velocity of light in vacuo of 299,860 ± 30 kilometers 

 per second. Michelson's value, determined at Cleveland in 1882, was 299,853 kilometers per 

 second. The percentage of error in these values is thought to be very small. Newcomb's 

 value of the velocity, combined with Nyren's value of the constant of aberration, 0."492, led 

 to a corresponding value of 8. "794 for the solar parallax. Unfortunately the many values 

 assigned to the aberration constant by the various investigators differ, for reasons as yet unex- 

 plained for the most part, and the related values for the solar parallax are correspondingly 

 uncertain. 



A condensed resume of the investigations of the motions of Mercury, Venus, Earth and Mars, 

 of the masses of these planets, of the constants of precession and nutation, of the solar parallax, 

 of the mass of the moon, etc., was published in 1895 as a supplement to the American Ephemeris, 

 under the title of The Elements of the Four Inner Planets and the Fundamental Constants of 

 Astronomy. This useful volume contains also Newcomb's attempt to account for the outstand- 

 ing discrepancies a of the motions of Mercury, Venus, the Earth andMars, of which by far the most 

 noteworthy is that concerning the perihelion of Mercury's orbit. He discusses the principal 

 variations that would be produced in the motions of the planets by modifications in, or additions 

 to, the forces normally considered, from the following sources: 



1. An assumed nonsphericity of the sun. 



2. An assumed intramercurial ring or group of planetoids. 



3. The mass of the diffused matter which reflects the zodiacal light. 



4. An assumed ring of planetoids between the orbits of Mercury and Venus. 



5. A minute deviation of the law of gravitation from the exact inverse squares of the 

 distances. 



He concludes, in effect, that all of these assumptions are untenable. 



What we may call Newcomb's minor contributions to astronomical knowledge were numer- 

 ous. We take space to describe a few of them. 



Euler's investigation of the motions of the earth had led to the result that if the axis of 

 rotation does not coincide with the axis of the earth's spheroidal figure there must be a minute 

 variation of terrestrial latitudes in a period of 10 months. Several able astronomers searched 

 unsuccessfully for evidence of such a variation in existing observations. Kiistner's observations 

 at the Berlin Observatory in the eighties pointed unmistakably to a latitude variation. This 

 led Chandler to institute an exhaustive study of the Greenwich and other observations, which 

 established the existence of minute latitude variations with a principal period of about 14 

 months. Newcomb retraced Euler's steps, and confirmed his conclusion that an absolutely rigid 

 and nondeformable earth would call for a period of 10 months, but deduced the new result 

 that if the body of the earth were, on the average, only as rigid as steel, then Euler's period 

 would be lengthened to 14 months, and thus be brought into close agreement with observation. 

 Newcomb's investigation supplied, in fact, our first reliable determination of the earth's rigid- 

 ity and his result has been substantially confirmed by the work of several observers on the 

 tidal deformations of the solid earth and by the extensive series of observations on the trans- 

 mission of earthquake waves through the earth's deep interior. 



o At the date of writing this biographical memoir (February, 1916) de Sitter's papers appl5*ing Einstein's theory of relativity to this 

 problem had not yet been published. W. \V. C 



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