1 64 



NATURE 



[June 14, 1894 



showing the horizontal axis of the spiral is half-way between the | 

 bottom and surface of the tank, in which the water is now 

 simply revolving round this axis. I 



This is the vortex in its simplest and rarest form (for a vorlex 

 cannot exist wilh its ends exposed'. Like an army it must have 

 its flanks protected ; hence a straight vorlex can only exist 

 where it has two surfaces to cover its flanks, and parallel ver- 

 tical surfaces are not common in nature. The vortex can bend, 

 and, wilh a horse shoe axis, can rest bolh its flanks on the same 

 surface, as this piece of clay, or unite ils ends with a ring axis, 

 which is its commonest form, as in the smoke ring. In both 

 these cases the vorlex will be in motion through the fluid, and 

 less easy to obser\'e. 



These vortices have no motion beyond the rotation because 

 they are half-way down the tank. If the vane were shorter 

 they would follow the vane ; if it were longer ihey would 

 leave it. 



In ih; same way, if instead of one vortex there were two 

 vortices, with their axis parallel, extending right across, the 

 one above another, they would move together along the lank. 

 I replace the float by another which has a vane suspended 

 from ii, so that the water can pass bolh above and below the 

 vane extending right across the middle portion of the tank. In 

 this case I in>litule two colour bands, one to pass over the top, 

 the other underneath the vane, which colour bands will render 

 visible a section of each vortex just as in the last case. I now 

 set the float in motion, and the two vortice; turn towards each 

 other in opposite directions. They are formed by the water 

 moving over the surface of the vane, downwards to get under it, 

 upwards to get over it, so that the rotation in the upper vortex 

 is opposite to that in the lower. All this is just the same as 

 before, but instead of these vortices standing still as before, they 

 follow at a definite distance from the vane, which continues ils 

 motion along the lank without resistance. 



Now this experiment shows, in the simplest form, (he modus 

 operandi by which internal waves can exist in fluid without any 

 motion in the external boundary. Not only is this plate moving 

 flatwise through the water, but it is followed by all the water, 

 coloured and uncoloured, enclosed in these cylindrical vortices. 

 Now, although there is no absolute surface visible, yet there is a 

 definite surface which encloses these moving vortices, and 

 separates them from the water which moves out of their way. 

 This surface will be rendered visible in another experiment I 

 shall show you. Thus, the water which has only wave motion 

 is bounded by a definite surface, the motion of which orresponds 

 to the wave ; but insid- this closed surface there is also water, 

 so that we cannot see the surface, and this water inside is moving 

 round and round, but so that ils motion at the bounding surface 

 is everywhere the same as that of Ihe outside water. 



The two masses of water do not mix. That outside moves 

 out of the way of and past the vortices over the bounding sur- 

 face, while the vortices move round and round inside the surface 

 in such a way that it is moving in exactly the same manner at 

 the surface as the wave surface outside. 



This is the key to the internal motion of water. You cannot 

 have a pure wave motion inside a mass of fluid with its boundaries 

 at rest, but you have a compound motion, a wave motion out- 

 side, and a vortex within, which fulfils the condition that there 

 shall be no sliding of the fluid over fluid at the boundary. 



A means which I hope may make the essential conditions of 

 thi< motion clearer occurred to me while preparing this lecture, 

 and to this I will now a<k your attention. 1 have here a num- 

 ber of layers of cotionwjol (wadding). Now I can force any 

 body along between these layers of wadding. They yield, as by 

 a wave, and let it go through ; but the w.idding must slide over 

 Ihe surface of the body so moving through it. And this it must 

 not do if it illustrate the conditions of fluid motion. Now, there 

 i» one way, and only one way, in which material can be got 

 through between the sheets of wa'iding without slipping. It 

 mmt roll through ; but this is not enough, bcoausc if it rolls on 

 Ihe under surface il will be slipping on the upper, liul if we 

 have two rollers, one on the lop of the other, between the sheets, 

 then Ihe lower roller rolls on the bottom sheet, Ihe upper roller 

 rolls again4t the upper sheet, so that there is no slipping between 

 Ihe rollers or the wadding, and, equally important, there is no 

 slipping between Ihe rollers as ihcy roll on each other. I have 

 only to pl.icc a sheet of canvas hclwcen the rollers and draw it 

 through : bolh the flannel rollers roll on llic canv.-is and on the 

 wadding, which Ihcy pass through without slipping, causing the 

 wadding to move in a wave outside them, and aflTording a com- 

 plete parable of Ihe vortex motion. 



NO. 1285, VOL. 50] 



I will now show by colour bands some of the more slrikinq 

 phenomena of internal motion, as presented by nature^ 

 favourite form of vortex, the vorlex ring, which may he de- 

 scribed as two horse-shoe vortices wilh their ends founded on 

 each other. 



To show the surface separating the water moving with the 

 vortex from that which gives «ay outside, I discharge lion; this 

 orifice a nuss of coloured water, which has a vortex ring in il 

 formed by the surface, as already described. You see the 

 beautifully-defined mass moving on slowly through the fluid, 

 with Ihe proper vortex ring motion, but very slow! It will not 

 go far before a change takes place, owing to the difl'usion of 

 Ihe vorlex motion across the bounding surface ; ihen the coloured 

 surface will be wound into the ring which will appear. The 

 mass approaches the disc in front. It cannot pass, but will come 

 up and carry the disc forward ; but the disc, although it does not 

 destroy the ring, disturbs the motion. 



If I send a more energetic ring, it will explain the phenomenon 

 I showed you at Ihe beginning of this lecture ; it carries the disc 

 forward as if struck with a hammer. This blow is not simply 

 the weight of the coloured ring, but of the whole moving mass 

 and the wave outside. The ring cannot pass the disc without 

 destruction wilh the attendant wave. 



Not only can a ring follow a disc, but, as wilh the plane 

 vane, so with Ihe disc ; if we start a disc, we must start a ring 

 behind it. 



I will now fulfil my promise to reveal the silent messenger I 

 sent to those balloons. The messenger appears in the form of a 

 large smoke ring, which is a vortex ring in air rendered visible 

 by smoke instead of colour. The origination of these rings has 

 been c.-^refully set so thai the balloons are beyond the surface 

 which separates the moving mass of water from the wave, so thai 

 they are subject to the wave motion only. If they are within 

 this surface they will disturb the direction of the ring, if they do 

 not break it up. 



These are, if I may say so, the phenomenal instances of in- 

 ternal motion of fluids. Phenomenal in their simplicity, ihey 

 are of intense interest, like the pendulum, as furnishing the clue 

 to the more complex. It is by the light we gather from their 

 study that we can hope to interpret the parable of Ihe vortex 

 wrapped up in the wave, as applied to the wind of heaven, and 

 the grand phenomenon of the clouds, as well as those things 

 which directly concern us, such as the resistance of our ships. 



UNIVERSITY AND EDUCA 

 INTELLIGENCE. 



TIONAL 



Oxford. — The Curators of the University Chest have been 

 authorised by Congregation to pay the following sums. 



To the Delegates of the Museum, a sum of .^^^^140 for each of 

 Ihe years 1S94, 1895, for Ihe grneral expenses of the .Museum. 



To the Curators of the H ilanic Carden a sum not exceeding 

 £100, to provide for expenses incurred in connection with Ihe 

 erection of new houses in the garden. 



The Curators of Ihe University Chest have been authorisi I 

 to expend a sum not exceeding /'700 in making Ihe rooms in 

 the south corridor in the Museum, and the lolls over them, 

 available for the use of ihe Hope I'rofessor of /oology, and I" 

 pay to the Mope Professor of Zoology, in addition to the stain' 

 able grant, the sum of £\o:> for each of ihe years 1894 l8'r 

 to provide for the salary of ihe allendani and other expenses > 

 the Department. 



The Delegates of Local Examinations have approved of the 

 inlroduclion of a new examination in the course lor junior can- 

 didates in elementary physiology and hygiene. 



Amongst those on whom il is proposed to confer the honorary 

 degree ol D.C.L. at the Eucxnia, is the name of Mr. Krancis 

 Gallon, F.R.S. 



One or more Natural Science Demyshi|)s, and Natural 

 Science Exhibitions will be awarded by M.agdalen College in 

 October this year, the examination to commence on Tuesday, 

 October 9. 



Al Wadham College, in the Scholarship examinations which 

 will begin on Thursday, November 29, no papers in natural 

 .science will be set, but in ihe election to one of ihc Kxhibilions 

 preference will be given lo any candidate who shall undertake to 

 read for honours in natural science, and lo proceed lo a degree 

 in medicine in the University of Oxford. 



At Keblc College an election will be held lo one Scholarship 



