386 HISTORY OF PHYSICAL ASTROX03IY. 



matical hypotheses, which ought to be considered as mere steps o f 

 calculation. " Since, therefore," he continues, 1 " each science has 

 hitherto been a slight and ill-constructed thing, we must assuredly take 

 a firmer stand; our ground being, that these two subjects, which on 

 account of the narrowness of men's views and the traditions of pro- 

 fessors have been so long dissevered, are, in fact, one and the same 

 thing, and compose one body of science." It must be allowed that, 

 however erroneous might be the points of Bacon's positive astronomi- 

 cal creed, these general views of the nature and position of the science 

 are most sound and philosophical. 



(Kepler) In his attempts to suggest a right physical view of the 

 starry heavens and their relation to the earth, Bacon failed, along with 

 all the writers of his time. It has already been stated that the main 

 cause of this failure was the want of a knowledge of the true theory of 

 motion ; the non-existence of the science of Dynamics. At the time 

 of Bacon and Kepler, it was only just beginning to be possible to re- 

 duce the heavenly motions to the laws of earthly motion, because the 

 latter were only just then divulged. Accordingly, we have seen that 

 the whole of Kepler's physical speculations proceed upon an ignorance 

 of the first law of motion, and assume it to be the main problem of the 

 physical astronomer to assign the cause which keeps up the motions of 

 the planets. Kepler's doctrine is, that a certain Force or Virtue resides 

 in the sun, by which all bodies within his influence are carried round 

 him. He illustrates* the nature of this Virtue in various ways, com- 

 paring it to Light, and to the Magnetic Power, which it resembles in 

 the circumstances of operating at a distance, and also in exercising a 

 feebler influence as the distance becomes greater. But it was obvious 

 that these comparisons were very imperfect; for they do not explain 

 how the sun produces in a body at a distance a motion athwart the 

 line of emanation ; and though Kepler introduced 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, suggest- 

 ed a much more substantial 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 



Vol. ix. 221. 2 De Stella Mortis, P. 3. c. xxxiv. 



