BOOK II.] MOTION OP LIBEr.TT. 639 



and ill spvin^^s Ttf watclics. An cxnct instance of tlie motion of 

 compressed air is seen in children's popguns, which thej malce 

 by scoopinpf out elder-branches or some such matter, and forcing 

 in a piece of some pulpy root or the like, at each end ; then they 

 force the root or other pellet with a ramrod to the opposite end, 

 from which the lower pellet is emitted and projected with a 

 report, and that before it is touched by the other piece of root or 

 pellet, or by the ramrod. We have examples of their escape 

 from tension, in the motion of the air that remains in glass eggs 

 after suction, in strings, leather, and cloth, which recoil after 

 tension, unless it be long continued. The schools define tliis by 

 the term of motion from the form of the element; injudiciously 

 enough, since this motion is to be found not only in air, water, 

 or fire, but in every species of solid, as wood, iron, lead, cloth, 

 parchment, &c., each of which has it3 own proper size, and i? 

 witli difficulty stretched to any othe-r. Since, however, this 

 motion of liberty is the most obvious of all, and to bo seen in an 

 infinite number of cases, it will be as well to distinguish it cor- 

 rectly and clearly; for some most carJessly confound this with 

 the two others of resistance and connection ; namely, the freedom 

 from pressure with the former, and that from tension with the 

 latter, as if bodies when compressed yielded or expanded to pre- 

 vent a penetration of dimensions, and when stretched rebounded 

 and contracted themselves to prevent a vacuum. But if the air, 

 when compressed, could be brought to the density of water, or 

 wood to that of stone, there would be no need of any penetration 

 of dimensions, and yet the compression would be much greater 

 than they actually admit of. So if water could be expanded till 

 it became as rare as air, or stone as rare as wood, there would 

 be no need of a vacuum, and yet the expansion would be much 

 greater than they actually admit of. We do not, therefore, 

 arrive at a penetration of dimensions or a vacuum before the 

 extremes of condensation and rarefaction, whilst the motion we 

 speak of stops and exerts itself much witliin them, and ia 

 nothing more than a desire of bodies to preserve their specific 

 density (or, if it be preferred, their form), and not to desert them 

 suddenly, but only to change by degrees, and of their own 

 accord. It is, however, much more necessary to intimate to 

 mankind (because many other points depend upon this), that the 

 violent motion which we call mechanical, and Democritus (who, 

 in explaining his primary motions, is to be ranked even below 

 the middling class of philosophers) termed the motion of a blow, 

 is nothing else than this motion of liberty, namely, a tendency 

 to relaxation from compression. For in all simple impulsion or 

 flight through the air, the body is not displaced or moved in 

 space, until its parts are placed in an unnatural state, and com- 

 pressed by the impelling force. When that takes place, the 



