The Rev. J. Challis on the Motion of Fluids. \'11 



the effect ? Might it not be as reasonably inferred, in the case 

 of light, that, although its transmission does not require the 

 interposition of a pane of glass, yet, that when such a pane is 

 interposed, since the light is not intercepted, there is reason 

 to suppose an active cooperation of the vitreous particles in 

 aid of the radiation ? It may be expedient here to advert to 

 the fact, that Prof Henry has found a metallic plate to inter- 

 fere with the dynamic induction of one flat helix upon another. 

 I have myself been witness of this result. 



74'. Does not magnetic or electro-dynamic induction take 

 place as well in vacuo as in plena ^ Has the presence of any 

 gas been found to promote or retard that species of reaction ? 

 It appears that, agreeably to your experiments, ponderable 

 bodies, when made to intervene, did not enhance the influence 

 in question, while in some of those performed by Henry it 

 was intercepted by them. Does it not follow that ponderable 

 particles may impede, but cannot assist in this process ? 



75. I am happy to find that, among the opinions which I 

 expressed in my letter to you, although there are several in 

 which you do not concur, there are some which you esteem 

 of importance, though you do not consider yourself justified 

 in extending to them your sanction, being constrained, in the 

 present state of human knowletlge, to hold your judgment in 

 suspense. For the present I shall here take leave of this sub- 

 ject, having already so extended my letter as to occupy too 

 much of your valuable time. I am aware that as yet I have 

 not sufficiently studied many of the intricate results of your 

 sagacity, ingenuity and consummate skill in experimental in- 

 vestigations. When I shall have time to make them the sub- 

 ject of the careful consideration which they merit, I may ven- 

 ture to subject your patience to some further trials. 



Philadelphia, Jan. 1, 1841. 



LXX V. On the Pmiciples of the Application of Analj/sis to the 

 Motion of Fluids. By the Rev. J. Challis, M./I., l^lumian 

 Professor of Astronomy and Experimental Philosophy in the 

 University of Cambridge*. 



T^OR the sake of simplicity, the following remarks will be 

 *■ restricted to the motion of an incompressible fluid : 

 they may without difficulty be extended to fluid motion in 

 general. 



It is well known that one of the fundamental equations of 

 fluid motion is obtained on the principle that the mass of each 

 small element remains the same while its position and form 

 • Coiiiiiiuiiicatcii by the Author. 



