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 ▲ 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 corresponds to the wave; but inside 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 its motion at the bounding surface is everywhere the same as that of the 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 surface, 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 outside, 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 this motion clearer occurred to me while preparing this lecture, and to this I will now ask your attention. I have here a number of layers of cottonwool (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 wadding must slide over the surface of the body so moving through it. And this it must not do if it illustrate the conditions of fluid motion. Now there is one way, and only one way, in which material can be got through between the sheets of wadding without slipping. It must roll through; but this is not enough, because if it rolls on the under surface it will be slipping on the upper. But if we have two rollers, one on the top of the other, between the sheets, then the lower roller rolls on the bottom sheet, the upper roller rolls against the upper sheet, so that there is no slipping between the rollers or the wadding, and, equally important, there is no slipping between the rollers, as they roll on each other. I have only to place a sheet of canvas between the rollers and draw it through; both the flannel rollers roll on the canvas and on the wadding, which they pass through without slipping, causing the wadding to move in a wave outside them, and affording a complete parable of the vortex motion. I will now show by colour bands some of the more striking phenomena of internal motion, as presented by Nature's favourite form of vortex, the vortex ring, which may be described as two horseshoe vortices with their ends founded on each other. To show the surface separating the water moving with the vortex from that which gives way outside, I discharge from this orifice a mass of coloured water, which has a vortex ring in it formed by the surface as already described. You see the beautifully defined mass moving on slowly through the fluid, with the proper vortex ring motion, but very slow. It will not go far before a change takes place, owing to the diffusion of the vortex motion across the bounding surface; then 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 the 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 with the attendant wave. Not only can a ring follow a disc, but as with the plane vane so with the 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 carefully set so that the balloons are beyond the surface which separates the moving mass of water from the wave, so that 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 internal motion of fluids. Phenomenal in their simplicity they 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 the 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. [O. R.] GENERAL MONTHLY MEETING, Monday, June 5, 1893. SIR JAMES CRICHTON-BROWNE, M.D. LL.D. F.R.S. Treasurer and Vice-President, in the Chair. George Matthews Arnold, Esq. Charles Claude Carpenter, Esq. Frederick Henry Cheesewright, Esq. M. Inst. C.E. Henry Kemp, Esq. M. Inst. C.E. The Right Hon. Stuart Knill (Lord Mayor), Alexander Campbell Mackenzie, Esq. Mus. Doc. Carl Edward Melchers, Esq. Phineas Phillip, Esq. William Cuthbert Quilter, Esq. M.P. John Robbins, Esq. F.C.S. John Gorges Robinson, Esq. J.P. Thomas Thornton, Esq. George White, Esq. B.A. LL.B. were elected Members of the Royal Institution. The special thanks of the Members were returned for the following Donations to the Fund for the Promotion of Experimental Research The special thanks of the Members were returned to Messrs. Ducretet and Lejeune (of Paris) for their present of an Electric Furnace. The PRESENTS received since the last Meeting were laid on the table, and the thanks of the Members returned for the same, viz. :— FROM The Secretary of State for India-A Dictionary of the Economic Products of India. Vols. I.-VI. 8vo. 1889-92. Abbe, Professor Cleveland-The Mechanics of the Earth's Atmosphere. 8vo. 1891. Accademia dei Lincei, Reale, Rona-Atti, Serie Quinta: Rendiconti. Classe di Scienze fisiche, matematiche e naturali. 1o Semestre, Vol. II. Fasc. 7. 8vo. Academy of Natural Sciences, Philadelphia-Proceedings, 1892, Part 3. 8vo. 1892. American Geographical Society-Bulletin, Vol. XXIV. No. 4, Part 2; Vol. XXV. No. 1. 8vo. 1892-3. Astronomical Society, Royal-Monthly Notices, Vol. LIII. No. 6. 8vo. 1893. British Architects, Royal Institute of-Proceedings, 1892-3, No. 15. 4to. Chili, Société Scientifique de-Actes, Tome II. Livraison 3me. 8vo. 1893. East India Association-Journal, Vol. XXV. Nos. 3-5. 8vo. 1893. Editors-American Journal of Science for May, 1893. 8vo. Analyst for May, 1893. 8vo. Athenæum for May, 1893. 4to. Chemical News for May, 1893. 4to. Chemist and Druggist for May, 1893. 8vo. Electrical Engineer for May, 1893. fol. Electric Plant for May, 1893. 8vo. Engineer for May, 1893. fol. Engineering for May, 1893. fol. Horological Journal for May, 1893. 8vo. Industries for May, 1893. fol. Iron for May, 1893. 4to. Ironmongery for May, 1893. 4to. Journal of Geology, Vol. I. Nos. 1, 2. 8vo. 1893. Machinery Market for May, 1893. 8vo. Monist for May, 1893. 8vo. Nature for May, 1893. 4to. Open Court for May, 1893. 4to. Photographic Work for May, 1893. 8vo. Telegraphic Journal for May, 1893. fol. Transport for May, 1893. fol. Tropical Agriculturist, 1893. 8vo. World's Fair Electrical Engineering for May, 1893. 8vo. Zoophilist for May, 1893. 4to. Florence Biblioteca Nazionale Centrale-Bolletino, Nos. 177, 178. 8vo. 1893. Franklin Institute-Journal, No. 809. 8vo. 1893. Meteorological Observations of the Pennsylvania State Weather Service. fol. 1893. Geographical Society, Royal Geographical Journal, Vol. I. No. 5. 8vo. 1893. Geological Institute, Imperial, Vienna-Verhandlungen, 1893, Nos. 2-5. 8vo. Harvard University-Bulletin for May, 1893. 8vo. Institute of Brewing-Transactions, Vol. VI. No. 6. 8vo. 1893. Johns Hopkins University-American Chemical Journal, Vol. XV. No. 5. 8vo. 1893. American Journal of Philology, Vol. XIV. No. 1. 8vo. 1893. Linnean Society-Journal, No. 204. 8vo. 1893. 8vo. 1893. Manchester Geological Society-Transactions, Vol. XXII. Part 8. National Academy of Sciences, Washington, U.S.A.—Memoirs, Vols. I.-V. 4to. 1886-91. North of England Institute of Mining and Mechanical Engineers-Transactions, Vol. XLII. Part 3. 8vo. 1893. Odontological Society of Great Britain-Transactions, Vol. XXV. No. 7. 8vo. 1893. Payne, Wm. W. Esq. and Hale, Geo. E. Esq. (the Editors)—Astronomy and AstroPhysics for May, 1893. 8vo. Place, Walter, Esq. (the Author)-Notes on Natal. 8vo. 1893. Pharmaceutical Society of Great Britain-Journal for May, 1893. 8vo. Saxon Society of Sciences, Royal-Philologisch-Historische Classe, Berichte, 1892, Abhandlungen, Band XIII. No. 6. 8vo. Mathematisch-Physische Classe, Abhandlungen, Band XIX. 8vo. 1893. Berichte, 1893, No. 1. 8vo. 1893. Selborne Society-Nature Notes, No. 42. 8vo. 1893. Smithsonian Institution Contributions to Knowledge, No. 842. 8vo. 1892. Society of Architects-Proceedings, Vol. V. No. 11. 8vo. 1893. Society of Arts-Journal for May 1893. 8vo. Tacchini, Professor P. Hon. Mem. R.I. (the Author)—Memorie della Società degli Spettroscopisti Italiani, Vol. XXII. Disp. 4a. 4to. 1893. United Service Institution, Royal-Journal, No. 183. 8vo. 1893. United States Department of Agriculture-Monthly Weather Review for February, 1893. 4to. 1893. Weather Bureau Bulletin, No. 8. 8vo. 1893. United States Department of the Interior-Eleventh Census of the United States, Part 1. 4to. 1892. Compendium of the Eleventh Census, Part 1. 4to. 1892. University of London-Calendar, 1893-4. 8vo. Upsal University-Bulletin de l'Observatoire Météorologique, Vol. XXIV. 4to. 1892-3. Vereins zur Beförderung des Gewerbfleisses in Preussen-Verhandlungen, 1893: Heft 5. 4to. 1893. Wagner Free Institute of Science, Philadelphia-Transactions, Vol. III. Part 2. 4to. 1892. Yorkshire Archæological and Topographical Association-Journal, Part 47. 8vo. 1893. |