194 Newton's Letters, Hypothesis and Experiments 



common water will scarce shrink by compression and swell 

 by relaxation, yet (so far as my observation reaches) spirit of 

 wine and oil will; and Mr. Boyle's experiment of a tadpole 

 shrinking very much by hard compressing the water in which 

 it swam, is an argument that animal juices do the same: and 

 as for their various pression by the ambient aether, it is plain 

 that that must be more or less, accordingly as there is more 

 or less aether within to sustain and counterpoise the pressure 

 of that without. If both {Ethers were equally dense, the 

 muscle would be at liberty as if pressed by neither: if there 

 were no aether within, the ambient would compress it with the 

 whole force of its spring. If the aether within were twice as 

 much dilated as that without, so as to have but half as much 

 springiness, the ambient would have half the force of its 

 springiness counterpoised thereby, and exercise but the other 

 half upon the muscle; and so in all other cases the ambient 

 compresses the muscle by the excess of the force of its spring- 

 iness above that of the springiness of the included. To vary 

 the compression of the muscle therefore, and so to swell and 

 shrink it, there needs nothing but to change the consistence 

 of the included aether; and a very little change may suffice, if 

 the spring of aether be supposed very strong, as I take it to 

 be many degrees stronger than that of air. 



Now for the changing the consistence of the aether, some 

 may be ready to grant that the soul may have an immediate 

 power over the whole aether in any part of the body, to swell 

 or shrink it at will; but then how depends the muscular mo- 

 tion on the nerves? Others therefore may be more apt to 

 think it done by some certain aetherial spirit included within 

 the dura mater, which the soul may have power to contract or 

 dilate at will in any muscle, and so cause it to flow thither 

 through the nerves ; but still there is a difficulty why this 

 force of the soul upon it does not take off the power of spring- 

 iness, whereby it should sustain more or less the force of 

 the outward aether. A third supposition may be, that the 

 soul has a power to inspire any muscle with this spirit, by im- 

 pelling it thither through the nerves ; but this too has its dif- 

 ficulties ; for it i*equires a forcible introducing the spring of 

 the aether in the muscles by pressure exerted from the parts 

 of the brain ; and it is hard to conceive how so great force 

 can be exercised amidst so tender matter as the brain is ; 

 and besides, why does not this aetherial spirit, being subtile 

 enough, and urged with so great force, go away through the 

 dura mater and skins of the muscle, or at least so much of the 

 other aether go out to make way for this which is crowded in ? 

 To take away these difficulties is a digression, but seeing the 



