

1893.] Professor Reynolds on Fluid Motion. 129 



WEEKLY EVENING MEETING, 



Friday, June 2, 1893. 



Sir Douglas Galton, K.C.B. D.C.L. LL.D. F.R.S. Vice-President, 



in the Chair. 



Professor Osborne Reynolds, M.A. LL.D. F.R.S. 



Study of Fluid Motion by means of Coloured Bands. 



In his charming story of I ' The Purloined Letter,' Edgar Allan Poe 

 tells how all the efforts and artifices of the Paris police to obtain 

 possession of a certain letter, known to be in a particular room, were 

 completely baffled for months by the simple plan of leaving the 

 letter in an unsealed envelope in a letter-rack, and so destroying all 

 curiosity as to its contents ; and how the letter was at last found there 

 by a young man who was not a professional member of the force. 

 Closely analogous to this is the story I have to set before you to- 

 night — how certain mysteries of fluid motion, which have resisted all 

 attempts to penetrate them are at last explained by the simplest 

 means and in the most obvious manner. 



This indeed is no new story in science. The method adopted by 

 the minister, D., to secrete his letter appears to be the favourite of 

 Nature in keeping her secrets, and the history of science teems with 

 instances in which keys, after being long sought amongst the grander 

 phenomena, have been found at last not hidden with care, but 

 scattered about, almost openly, in the most commonplace incidents of 

 every-day life which have excited no curiosity. 



This was the case in physical astronomy — to which I shall return 

 after having reminded you that the motion of matter in the universe 

 naturally divides itself into three classes. 



1. The motion of bodies as a whole — as a grand illustration of 

 which we have the heavenly bodies, or more humble, but not less 

 effective, the motion of a pendulum, or a falling body. 



2. The relative motion of the different parts of the same fluid or 

 elastic body — for the illustration of which we may go to the grand 

 phenomena presented by the tide, the whirlwind, or the transmission 

 of sound, but which is equally well illustrated by the oscillatory 

 motion of the wave, as shown by the motion of its surface, and 

 by the motion of this jelly, which, although the most homely illus- 

 tration, affords by far the best illustration of the properties of an 

 elastic solid. 



3. The inter-motions of a number of bodies amongst each other 

 — to which class belong the motions of the molecules of matter 



Vol. XIV. (No. 87.) k 



