144 CLASSICS OF MODERN SCIENCE 

 of rotation and revolution of the planets and satellites m the same 

 direction, and in orbits very little inclined to each other, likewise 

 explains why the motions of the comets deviate from this general law. 



The great eccentricity of the orbits of the comets, is also a result of 

 our hypothesis. If those orbits are elliptic, they are very elongated, 

 since their greater axes are at least equal to the radius of the sphere 

 of activity of the Sun. But these orbits may be hyperbolic ; and if the 

 axes of these hyperbolae are not very great with respect to the mean 

 distance of the Sun from the Earth, the motion of the comets which 

 describe them will appear to be sensibly hyperbolic. However, with 

 respect to the hundred comets, of which the elements are known, not 

 one appears to move in a hyperbola ; hence the chances which assign 

 a sensible hyperbola are extremely rare relatively to the contrary 

 chances. The comets are so small, that they only become sensible 

 when their perihelion distance is inconsiderable. Hitherto this dis- 

 tance has not surpassed twice the diameter of the Earth's orbit, and 

 most frequently, it has been less than the radius of this orbit. We 

 may conceive, that in order to approach so near to the Sun, their ve- 

 locity at the moment of their ingress within its sphere of activity, 

 must have an intensity and direction confined within very narrow 

 limits. If we determine by the analysis of probabilities, the ratio 

 of the chances which in these limits, assign a sensible hyperbola to 

 the chances which assign an orbit, which may without sensible error 

 be confounded with a parabola, it will be found that there is at least 

 six thousand to unity that a nebula which penetrates within the sphere 

 of the Sun's activity so as to be observed, will either describe a 

 very elongated ellipse, or an hyperbola, which, in consequence of the 

 magnitude of its axis will be as to sense confounded with a par- 

 abola in the part of its orbit which is observed. It is not therefore 

 surprising that hitherto no hyperbolic motions have been recognized. 



The attraction of the planets, and perhaps also the resistance of the 

 ethereal media, ought to change several cometary orbits into ellipses, 

 of which the greater axes are much less than the radius of the sphere 

 of the solar activity. It is probable that such a change was produced 

 in the orbit of the comet of 1759, the greater axis of which was not 

 more than thirty-five times the distance of the Sun from the Earth. 

 A still greater change was produced in the orbits of the comets of 

 1770 and of 1805. 



