﻿Sir W. Thomson on Approach caused by Vibration. 425 



is an attraction on the whole towards the tuning-fork, into what- 

 ever position the tuning-fork be turned relatively to it. 



Your seventh experiment* has interested me even more than 

 any of the others. It illustrates the elementary law of pres- 

 sure in hydrokinetics, not by showing effects of fluid pressure 

 on portions of a solid bounding surface, as all other illustrative 

 experiments hitherto known to me have done, but by showing 

 an effect of diminished fluid pressure throughout more rapidly 

 moving portions of the finite mass of the fluid itself. This 

 effect consists of a slight degree of expansion, depending on the 

 air not being perfectly incompressible. The volume occupied 

 by the more rapidly moving portions becoming slightly aug- 

 mented, the remainder of tbe fluid would be condensed were the 

 whole contained within an altogether fixed boundary. A move- 

 able portion of this boundary (that is, the surface of the liquid 

 in your tube) yields and shows to the eye the effect of the dimi- 

 nished pressure through the rapidly moving portions. 



No branch of abstract dynamics has had a greater charm for 

 the mathematical worker than hydrokinetics; but it has not 

 hitherto been made generally attractive by experimental illustra- 

 tions. Such refined and beautiful experiments as those you 

 describe, and especially your seventh, tend notably to give to 

 this branch of dynamics quite a different place in popular esti- 

 mation from that which it has held ; but what is perhaps of 

 even more importance, they help greatly to clear the ideas of 

 those who have made it a subject of mathematical study. 



Yours truly, 



Professor Guthrie. William Thomson. 



Nov. 16, 1870. 

 Dear Sir, — On writing to you on the 14th I forgot to remark 

 that the dynamical theory demonstrates the truth of your pre- 

 diction in § 18f. It shows that the amounts of the expansions 

 of different gases in equal and similar arrangements exhibiting 

 your 7th experiment^ are simply as their densities, it being 

 assumed always that the whole amount of expansion is such as 

 to produce only an infinitesimal change of density in any part of 

 the gas. 



I remain, dear Sir, 



Yours truly, 

 Professor Guthrie. William Thomson. 



* Experiment 7 in Proceedings Roy Soc. vol. xix. p. 38, or experiment 

 10, Phil. Mag. Nov. 1870.— F. G. 



t Proceedings Roy. Soc. No. 123, § 18. Phil. Mag. Nov. 1870, § 27. 

 1 Proceedings Roy. Soc. No. 123, Ex. 7. Phil. Mag. Nov. 1870. Ex. 10. 



