138 HISTORY OF PHYSICAL ASTRONOMY. 



ing at a distance, and also in exercising a feebler 

 influence as the distance becomes greater. But it 

 was obvious that these comparisons were very im- 

 perfect ; for they do not explain how the sun pro- 

 duces in a body at a distance a motion athwart 

 the line of emanation; and though Kepler intro- 

 duced an assumed rotation of the sun on his axis 

 as the cause of this effect, that such a cause could 

 produce the result could not be established by any 

 analogy of terrestrial motions. But another image 

 to which he referred, suggested a much more sub- 

 stantial and conceivable kind of mechanical action 

 by which the celestial motions might be produced, 

 namely, a current of fluid matter circulating round 

 the sun, and carrying the planet with it, like a 

 boat in a stream. In the Table of Contents of the 

 work on the planet Mars, the purport of the 

 chapter to which I have alluded is stated as fol- 

 lows: "A physical speculation, in which it is de- 

 monstrated that the vehicle of that Virtue which 

 urges the planets, circulates through the spaces of 

 the universe after the manner of a river or whirl- 

 pool (vortex,} moving quicker than the planets." 

 I think it will be found, by any one who reads 

 Kepler's phrases concerning the moving force, the 

 magnetic nature, the immaterial virtue of the 

 sun, that they convey no distinct conception, except 

 so far as they are interpreted by the expressions 

 just quoted. A vortex of fluid constantly whirling 

 round the sun, kept in this whirling motion by 



