ON THE MAGNITUDE OF THE SOLAR SYSTEM. 107 



even be concerned in the prodnction of gravitation itself. What could 

 be apparently more remote than these electric quantities and the solar 

 parallax? And yet we have here a relation between them, but we 

 make no use of it because as yet the same relation can be far more 

 accurately determined from experiments upou the velocity of light. 



]^ow, let us recall the quantities and methods of observation which 

 we have found to be involved, either directly or indirectly, with the 

 solar parallax. They are, the solar i>arallax, obtained from transits of 

 Venus, oppositions of Mars, and oppositions of certain asteroids; the 

 lunar parallax, found both directly and from measurements of the force 

 of gravity at the earth's surface; the constants of preceSvSion, nutation, 

 and aberration, obtained from observations of the stars; the parallac- 

 tic inequality of the moon; the lunar inequality of the earth, usually 

 obtained from observations of tlie sun, but recently found from heli- 

 ometer observations of certain asteroids; the mass of the earth, found 

 from the solar iiarallax and also from the periodic and secular pertur- 

 bations of Venus and Mars; the mass of the moon, found from the 

 lunar inequality of the earth and also from the ratio of the solar and 

 lunar components of tbe ocean tides; the masses of all the planets, 

 obtained from observations of their satellites whenever possible, and 

 when no satellites exist, then from observations of their mutual pertur- 

 bations, both i^eriodic and secular; the velocity of light, obtained from 

 experiments with revolving mirrors and toothed wiieels, together with 

 laboratory determinations of the index of refraction of atmospheric 

 air; the light equation, obtained from observations of the eclipses of 

 Jupiter's satellites; the figure of the earth, obtained from geodetic 

 tri angulations, measurements of the length of the seconds pendulum 

 in various latitudes, and observations of certain perturbations of the 

 moon; the mean density of the earth, obtained from measurements of 

 the attractions of mountains, from pendulum experiments in mines, 

 and from experiments on the attraction of known masses of matter 

 made either with torsion balances or with the most delicate chemical 

 balances; the surface density of the earth, obtained from geological 

 examinations of the surface strata; and, lastly, the law of distribution 

 of density in the interior of the earth, which in the present state of 

 geological knowledge we can do little more tlian guess at. 



Here, then, we have a large group of astronomical, geodetic, geological 

 and physical quantities which must all be considered in finding- the 

 solar parallax, and which are all so entangled with each other that no 

 one of them can be varied without aft'ecting all the rest. It is there- 

 fore impossible to make an accurate determination of any one of tbem 

 apart from the remainder of the group, and thus we are driven to the 

 conclusion that they must all be determined simultaneously. Such has 

 not been the practice of astronomers in the past, but it is the method 

 to which they must inevitably resort in the future. A cursory glance 

 at an analogous problem occurring in geodesy may be instructive. 



