BOOK II.] MEASURES OP MOTION. 531 



Yet all these powers, whetlier acting at a small or great dis- 

 tance, certainly act -witliin definite distances, wliicli are well 

 ascertained by nature ^ so that there is a limit depending either 

 on the mass or quantity of the bodies, the vigour or faintness of 

 the powers, or the favourable or impeding nature of the medium, 

 all of which should be taken into account and observed. We 

 must also note the boundaries of violent motions, such as mis- 

 siles, projectiles, wheels, and the like, since they are also mani- 

 festly confined to certain limits. 



Some motions and virtues are to he found of a directly con- 

 trary nature to these, which act in contact but not at a distance; 

 namely, such as operate at a distance and not in contact, and 

 aijain act with less force at a less distance, and the reverse. 

 Sight, for instance, is not easily effective in contact, but requires 

 a medium and distance; although I remember having heard 

 from a person deserving of credit, that in being cured of a cata- 

 ract (which was done by putting a small silver needle within the 

 first coat of the eye, to remove tlie thin pellicle of the cataract, 

 and force it into a corner of the eye), he had distinctly seen the 

 needle moving across the pupil. Still, though this may be true, 

 it is clear that large bodies cannot be seen well or distinctly, 

 imless at the vertex of a cone, where the rays from the object 

 meet at some distance from the eye. In old persons the eye sees 

 better if the object be moved a little farther, and not nearer. 

 Again, it is certain that in projectiles the impact is not so violent 

 at too short a distance as a little afterwards.^ Such are the 

 observations to be made on the measure of motions as regards 

 distance. 



There is another measure of motion in space which must not 

 be passed over, not relating to progressive but spherical motion, 

 —that is, the expansion of bodies into a greater, or their contrac- 

 tion into a lesser sphere. For in our measure of this motion we 

 must inquire what degree of compression or extension bodies 

 easily and readily admit of, according to their nature, and at 

 what point they begin to resist it, so as at last to bear it no fur- 

 ther, — as when an inflated bladder is compressed, it allows a 

 certain compression cf the air, but if this be increased, the air 

 does not suffer it, and the bladder is burst. 



' This is not true except when the projectile acquires greater velocity 

 ftt every successive instont of its course, which is never the case except 

 with falling bodies. Bacon appears to have been led into the opinion 

 from observing that gun-shots pierce many objects at a distance from 

 which they rebound when brought within a certain proximity of con- 

 tact. This apparent inconsistency, however, arises from the resistance 

 ox the parts of the object, which velocity combined with force is necea* 

 iwry to overcome, iia. 



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