BOOK H.] MOTION OF TREPIDATION. 551 



variation of the polos if they be moveable ; which, nowever, 

 only affects revolution when circular. The motion in question 

 is, according to common and long-received opinion, considered to 

 be that of the heavenly bodies. There exists, however, with 

 regard to this, a considerable dispute between some ot the 

 ancients as well as moderns, who have attributed a motion of 

 revolution to the earth. A much more reasonable controversy, 

 perhaps, exists (if it be not a matter beyond dispute), whether 

 the motion in question (on the hypothesis of the earth s being 

 fixed) is confined to the heavens, or rather descends and is com 

 municated to the air and water. The rotation of missiles, as in 

 darts, musket-balls, and the like, we refer entirely to the motion 

 of liberty. 



Let the eighteenth motion be that of trepidation,* to which (in 

 the sense assigned to it by astronomers) we do not give much 

 credit ; but in our serious and general search after the tendencies 

 of natural bodies, this motion occurs, and appears worthy of 

 forming a distinct species. It is the motion of an (as it were) 

 eternal captivity ; when bodies, for instance, being placed 

 not altogether according to their nature, and yet not exactly ill, 

 constantly tremble, and are restless, not contented with their 

 position, and yet not daring to advance. Such is the motion of 

 the heart and pulse of animals, and it must necessarily occur in 

 all bodies which are situated in a mean state, between con 

 veniences and inconveniences ; so that being removed from their 

 proper position, they strive to escape, are repulsed, and again 

 continue to make the attempt. 



Let the nineteenth and last motion be one which can scarcely 

 be termed a motion, and yet is one ; and which we may call the 

 motion of repose, or of abhorrence of motion. It is by this 

 motion that the earth stands by its own weight, whilst its ex 

 tremes move towards the middle, not to an imaginary centre, 

 but in order to unite. It is owing to the same tendency, that all 

 bodies of considerable density abhor motion, and their only 

 tendency is not to move, whicn nature they preserve, although 

 excited and urged in a variety of ways to motion. But if they 

 be compelled to move, yet do they always appear anxious to 

 recover their former state, and to cease from motion, in which 

 respect they certainly appear active, and attempt it with sufficient 

 swiftness and rapidity, as if fatigued, and impatient of delay. 

 We can only have a partial representation of this tendency, 

 because with us every tangible substance is not only not con- 



* The motion of trepidation, as Bacon calls it, was attributed by the 

 ancient astronomers to the eight spheres, relative to the precession of tha 

 equinoxes. Galileo was the first to observe this kind of lunar motion, 

 Ed. 



