July 12, 1895.] 



SCIENCE. 



31 



constants of precession and nutation, the 

 parallactic inequality of the Moon, the lunar 

 inequality of the Earth, the masses of the 

 Earth and Moon, the ratio of the solar and 

 lunar tides, the constant of aberration, the 

 velocity of light and the light-equation.'' 

 Collating the great mass of astronomical, 

 geodetic, gravitational and tidal results 

 which have been accumulating for the past 

 two centuries and applying the mathemat- 

 ical process known as a ' least square adjust- 

 ment,' he derives the value 8". 809 ± 0".006, 

 giving for the mean distance between the 

 centres of Sun and Earth, 92,797,000 miles. 

 A valuable bibliography of the entire sub- 

 ject concludes Professor Harkness's paper. 



Professor ISTewcomb, in his Elements of the 

 Four Inner Planets and the Fundamental Con- 

 stants of Astronomy (Washington, 1895), on 

 which his new tables of the principal planets 

 of the solar system are founded, derives 

 from his discussion of all existing data a 

 definitive value for the Sun's parallax eqiial 

 to 8". 790. This important paper is a sup- 

 plement to the American Ephemeris for 1897. 



Possible changes of the Sun's diameter 

 from time to time have been critically in- 

 vestigated by Dr. Auwers, of Berlin, and 

 Professor Newcomb, with negative results; 

 nor are the observations yet made sufficient 

 to disclose any difference between equatorial 

 and polar diameters. The heliometer af- 

 fords the best means of measuring the Sun's 

 apparent diameter or the angle subtended 

 by its disk. The orbit of the Earth being- 

 elliptical, this diameter changes in the in- 

 verse proportion of the Earth's varying dis- 

 tance from the Sun; at the beginning of the 

 year it is 32' 32" and 31' 28" early in July, 

 the mean value being 32' 0". Supposing 

 the form of the Earth's orbit unknown, 

 daily measures of the Sun's varying diam- 

 eter would alone, in the course of a year, 

 enable the precise determination of the 

 figure of the orbit, so accurately can these 

 measures now be made. When at its mean 



distance from the Earth, the linear equiva- 

 lent of one second of arc at the Sun is 450 

 miles. The present uncertainty in the solar 

 diameter does not much exceed 2" ; that is 

 to say about 900 miles, or quite approxi- 

 mately TuVij of the entire diameter. Dr. 

 Auwers's recent value of the semi-diameter 

 is 15' 59". 63 ; and if we take the mean dis- 

 tance of the Sun at 93,000,000 miles, this 

 numerical relation gives the Sun's diameter 

 865,350 miles. 



A simple relation between the Sun's mass 

 and its dimensions relatively to the Earth 

 enables us to determine that the force of 

 gravity at the Sun's surface is 27f times 

 greater than it is here ; so that while a 

 body on the Earth falls only 16.1 feet in the 

 first second of time, at the Sun its fall in a 

 corresponding interval would be no less than 

 444 feet. If a hall clock were transported 

 to the Sun, its leisurely pendulum would 

 vibrate more than five times as rapidly. 

 So great is the Sun's mass that a body fall- 

 ing freely toward it from a distance indefln- 

 iteljr great would, on reaching the Sun, have 

 acquired a velocity of 383 miles per second. 

 The great Krupp gun exhibited at the 

 World's Fair in 1893, if fired from Chamou- 

 nix in the direction of Mont Blanc, at an 

 elevation of 44°, would propel its projec- 

 tile of 475 pounds in a curve meeting the 

 earth at Pre-Saint-Didier, 12.V miles from 

 Chamounix, and whose highest point would 

 be more than a mile above the summit of 

 Mont Blanc. If we could suppose the same 

 gun to be fired similarly on the Sun, so 

 great is the force of gravity there that the 

 projectile would be brought down to rest 

 about half a mile from the muzzle. 



From groups of the faculse. Dr. Wilsing 

 has found that the Sun's equator revolves 

 in 25''.23; but these observations are ex- 

 ceedingly difficult, and a repetition of the 

 work is desirable. Professor Young and 

 Dr. Crew have determined the period of ro- 

 tation of the Sun's equator by means of the 



