70 



NA TURE 



[jMov. 1 8, iSSo 



The oblate and prolate shells were each of ihem made from the 

 two hemispheres of sheet copper which plumbers solier together 

 to make their globular floaters. By a little hammering it is easy 

 to alter the hemispheres to the proper shapes to make either the 

 prolate or the oblate figure. 



Theory had pointed out that the rotation of a liquid in a rigid 

 shell of oval figure, being a configuration of maximum energy for 

 given vorticity, would be unstable if the containing vessel is left 

 to itself supported on imperfectly elastic sup; orts, although it 

 would be stable if the vessel were held absolutely fixed, or borne 

 by perfectly elastic supports, or left to itself in fpace unacted on 



by external force ; and it was to illustrate this theory that the 

 oval shell was made and filled with water and placed in the 

 apparatus. The result of the first trial was literally startling, 

 although it ought not to have been so, as it was merely a reali- 

 sation of what had been antici|jated by theory. The frame- 

 work was held as firmly as possible by one p.-rson with his two 

 hands, keeping it a; steady as he could. The spmning by 

 means of a fine cord ' round a small V pulley of i-inch diameter 

 on the axis of the oval shell, and passing round a large fly-w heel 

 of 3 feet diameter turned at the rate of about one round per 

 second, was continued for several minutes. This in the case of 

 the oblate shell, as w.as known from previous experiments, would 

 have given amply sufficient rotation to the contained water to 

 cause the apparatus to act with great firmness like a solid 

 gyrostat. In the first experiment with the oval shell the shell 

 was seen to be rotating with great velocity during the last minute 

 of the spinning ; but the moment it was released from the cord, 

 and when, holding the framework in my hands, I commenced 

 carrying it towards the horizontal glass table to test its gyrost.Jtic 

 quality, the framework which I held in my hands gave a violent 

 uncontrollable lurch, and in a few seconds the shell stop^jcd 

 turning. I saw that one of the pivots had become bent over, Ijy 

 yielding of the copper shell in the neighbourhood of the stiff 

 pivot-carrying disk, soldered to it, showing that the liquid had 

 exerted a very strong couple against its containing shell, in a 

 plane through the axis, the effort to resist which by my hands 

 had bent the pivot. Th.e shell was refitted with more strongly 

 attached pivots, and the experiment has been repeated several 

 times. In every case a decided uneasiness of the framework is 

 perceived by the person holding it in his hands during the 

 spinning ; and as soon as the cord is cut and the person holding 

 it carries it towards the experimental table, the franiew irk 

 begins, as it were, to wriggle round in his hands, and bytne time 

 the framework is placed on the table the rotation i- nearly all 

 gone. Its utter failure as a gyrostat is precisely what was e.v- 

 pected from the theory, and presents a truly wonderful coiitrast 

 from what is observed with the apparatus and operation- in every 

 respect similar, except having an oblate instead of a prolate shell 

 to contain the liquid. 



^ Instead of using a long cord first wound on a bobbin, and finally wound 

 up on the circumference of the large wheel as described in Nature. 1* ebruarj- 

 I, 1S77. p. 297, I have since found it much more convenieni to use an endless 

 cord little more than half round the cucumference of the large wheel, and 

 less than half round the circumference of the V pulley of the siyr stai . ant 

 to keep it tight enough to exert whatever tangential lorce on the V pul ey is 

 desired by the person holding the framework in his hand. After coniinumg 

 the spinning by turning the fly-wheel for as long a time as is ju<lged pioper, 

 the endless cord is cut with a pair of scissors and the gyrostat released. 



ON A DISTURBING INFINITY IN LORD A 

 RAYLEIGH'S SOLUTION FOR WAVES IN 

 A PLANE VORTEX STRATUM 

 T N the paper in List week's Nature under this heading by Sir 

 -*■ William Thomson, the lower part of the illustration was 

 inadvertently turned round at the last moment by the printer ; 

 the cut should stand as follows : — 



SARGASSUM"^ 

 "I'lII.S paper opens with a discussion of the value of the 

 species Sargassiim hacciferum, the particular species of 

 this genus which is well known as the Gulf-weed. The author 

 considers that the floating plants to which this name, has been 

 given are simply fragments of many varieties or species of 

 Sargassum, more particularly of i'. vulgare. In support of this 

 view he points out that, from the accounts of nearly all authors 

 who have examined specimens, it appears that the lower part of 

 the stem had been broken across, and that it is therefore fair to 

 conclude that they belong to plants which are rooted under 

 ordinary circumstances. This conclusion had been already 

 arrived at by Kuuiphius, C. Agardh, Rennell, Humboldt, and 

 more recently by G. von Martens ; but of the-e writers Uennell 

 and Humboldt are of opinion that the floating fraijments continue 

 to grow, and in this they agi'ee with Thunberg, Meyen, and 

 Harvey. Dr. Kuntze contends that there is not sufficient evidence 

 forthcoming to establish the correctness of this view. He urges 

 that, even admitting that some growth takes place, it is only 

 temporary, and that it therefore affords no ground for regarding 

 these as pelagic plants. The only other cases of growth of Fuci 

 when floating are offered by Macrocystis pyrifera (Sir Joseph 

 Hooker, " Flora Antarctica," vol. i.), and hy Fiiais vesiculosus 

 (Mr. Moseley, "Notes by a Naturalist on the Challenger"), and 

 doubtless Dr. Kuntze's objections apply to these also. The 

 question naturally arises as to whether these floating plants are 

 actively living, or are dying, or dead. 



In the case of Sargassum Dr. Kui tze considers that their bright 

 yellow colour is due to changes taking place, either preliminary 

 to or in consequence of death, in the brown colouring-matter 

 of the attached forms to which he believes the floating fragments 

 belong. Mr. Mo-eley, howevei, is of opinion that this is the 

 natural colour of these plants whilst living. It does not appear 

 that any such difference in colour has been noticed in attached 

 and floating specimens of Ma.-rocyslis or of Fucus, and this is a 

 fact which is not in harmony with Dr. Kuntze's views respecting 

 Sargassum. Again, the gene al ob.5ervation that these floating 

 Fuci have no reprnduct've . r.;ans offers a further difficulty which 

 they do not explain. Dr. Kuntze endeavours to meet the difii- 

 culty by .stating that he has found receptacles occasionally in 

 free-swimming individuals, and he gives figures of two plants 

 bearing them ; hut neither f r ■ n the figures nor from his account 

 of them is it possible to conclude with certainty that the bodies 

 in question are really of a reir .ductive nature ; and he explains 

 the usual absence of thee or^'ans in the floating individuals by 

 suggesting that the receptacles, being the most fragile parts of 

 the plant, are the most reaoily destroyed, and further that, 

 owing to the small number of air-chambe's with which they 

 are provide, d, they would ink on becoming detached. In 

 this case, as in ihat of the lolour, these explanations respecting 

 Sargassum will only become valid when they are found to hold 

 good of Macrocysti- and of Fucus also. It is apparent that the 



