400 LECTURE XLII. 



is almost constantly visible ; and in these climates it may often be distin- 

 guished in the beginning of March, after the termination of twilight, 

 exhibiting the appearance of a narrow triangle, somewhat rounded off, of a 

 whiteness resembling the milky way, ascending from the sun as a base, like 

 the projection or section of a very flat spheroid, and extending to a disWi^e 

 of more than 50 from the sun. The whole orbit of Venus never subtends 

 so great an angle from the earth as 96, consequently this substance must 

 occasionally involve both Mercury and Venus ; and if it were not extremely 

 rare, it would produce some disturbance in their motions ; while in fact it 

 does not appear to impede the progress even of the tails of the comets, which 

 are probably themselves of very inconsiderable density. It cannot a 

 continuous fluid atmosphere, revolving with the same velocity as the; r an ; 

 for the gravitation of such an atmosphere would cause it to assume a form 

 more nearly spherical ; and the only probable manner in which it can be 

 supposed to retain its figure, is by means of a revolution much more rapid 

 than the sun's rotation. Some persons have attributed the appearance to 

 the refraction of the earth's atmosphere only ; but if it arose from any such 

 cause as this, its direction could scarcely be oblique with respect to the 

 horizon, and it is highly improbable that it should always happen to coin- 

 cide with the plane of the sun's rotation. (Plate XXXI. Fig. 470.) 



The sun is accompanied in his progressive motion among the fixed stars 

 by ten [eleven] planetary bodies, of different magnitudes, revolving round 

 him, from west to east, in orbits approaching to circles, and visible to us by 

 means of the light which they receive from him. These are Mercury, Venus, 

 the Earth, Mars, Juno, Pallas, Ceres [Vesta], Jupiter, Saturn, and the 

 Georgian planet. It is unnecessary to adduce at present any arguments to 

 prove the actual existence or direction of any of these motions ; their com- 

 plete agreement with the visible phenomena of the heavens, and with the 

 laws of gravitation, will hereafter appear to afford sufficient evidence of the 

 accuracy of the received theory of the arrangement of the solar system. The 

 motion of the earth is the most unanswerably proved by the apparent aber- 

 ration of the fixed stars, derived from the different directions of this motion 

 at different times, and corresponding precisely with the known velocity of 

 light, deduced from observations of a very different kind. That the planets 

 receive their light from the sun, is undeniably shown by the appearance of 

 the discs of many of them, when viewed through a telescope, those parts of 

 their surfaces only being luminous, on which the sun shines at the time of 

 observation. 



These planets are neither all in one plane, nor does any one of them 

 remain precisely in the same plane at all times ; but their deviations from 

 their respective planes are inconsiderable, and they are commonly repre- 

 sented by supposing each planet to revolve in a plane passing through the 

 sun, and the situation of this plane to be liable to slight variations. There 

 is, however, a certain imaginary plane, determinable from the situations, 

 the velocities, and the masses of the planets, which, like the centre of inertia, 

 never changes its position on account of any mutual actions of the bodies of 

 the system, and this plane of inertia is called the fixed ecliptic. Its 

 situation is nearly half way between the orbits of Jupiter and of Saturn ; 



