538 



NATURE 



[Oct. 2 1, 1875 



funnel-shaped figure and circulatory movement of the 

 entire mass and the increase of velocity towards the 

 centre will be at once seen. 



The descending movement of these whirls may be 

 examined by the preceding analysis, but observation has 

 long since placed the matter beyond doubt. Everyone 

 knows how much eddies in the current of a river are 

 dreaded by bathers ; when a swimmer has the misfor- 

 tune to be caught in one he is drawn down by a rapid 

 rotation even to the bottom of the water. There, the 

 expert swimmer, knowing how to reserve his strength in 

 place of expending it in useless efforts, extricates himself 

 by resting on the bottom, and, disengaging himself from 

 the eddy, rises quickly to the surface. Not only may men 

 be thus engulphed, but masses of floating ice, or even 

 small vessels, are drawn to the bottom by whirlpools, and 

 thereafter are extricated and rise to the surface only by 

 the obstacle afforded by the bottom, and by the contrac- 

 tion downwards, more and more marked, of the whirling 

 mass of water. 



These phenomena can be artificially produced in a 

 large mass of still water, at a part where a rapid move- 

 ment of gyration is communicated by a suitable mechani- 

 cal appliance.* If we strew dust on the surface, in order 

 to render the phenomena visible, it is ,seen that gyration 

 is propagated in the form of a cone from above down- 

 wards, even to the bottom of the vessel, drawing the dust 

 along with it. Count Xavier de Maistre, who has pub- 

 lished in the Bibliotheque Universelle de Geneve some 

 interesting experiments on this subject, has shown that a 

 layer of oil placed over the water of the funnel-shaped 

 eddy is drawn towards the bottom by a gyratory move- 

 ment ; then, when it comes in contact with the obstacle 

 presented by the base, the oil reascends in globules all 

 round the eddy which it has quitted. There is thus here 

 a double vertical movement — the first regularly descending 

 along the spires of a conical helix, the second ascending 

 and exhibiting in its ascent no geometrical figure, but 

 rising to the surface irregularly round about the eddy. It 

 is natural that the liquid thus drawn to the bottom should 

 thereafter ascend more or less, not, be it noted, in the eddy 

 down which it had been carried, but outside it, through 

 the surrounding liquid. 



This gyratory movement, which thus concentrates 

 towards the point of the eddy the sum of the moving 

 forces which the funnel-shaped whirl embraces in its vast 

 expanse, ought to produce at its base a certain amount of 

 mechanical work, and observation confirms this idea. 

 The powerful whirlpools of our rivers plough up their 

 beds and thus expend on the soil the force which they 

 have acquired near the surface at the expense of the 

 inequalities of velocity of the general current. And as all 

 currents of water possessing some little depth present like 

 inequalities of velocity among their lateral filaments, on 

 account of the friction of the water against the banks, 

 numerous whirlpools are constantly found whose action 

 consists in finally absorbing these inequalities and regu- 

 lating the flow of the water, so that the general velocity 

 of the river is perceptibly reduced. 



Vortices or Eddies with Vertical Axes in Gases. — All 

 these phenomena arc found in gaseous masses traversed 

 by horizontal currents. In currents of this sort, inequali- 

 ties of velocity will equally give rise to whirling move- 

 ments round a vertical axis, and, as may be constantly 

 observed, these gyrations will still assume the figure of a 

 truncated cone in an inverted position, which becomes 

 visible when any circumstance occurs to interfere with 

 the transparency of the air. Equally as in the case of 



* These experiments must not be confounded with the experiment of 

 water poured into a vessel to which a movement of rotation round a ver- 

 tical axis has been communicated. In this case the free surface becomes 

 hollow whilst the water rises along the sides of the vessel. A condition of 

 equilibrium is soon established totally different from the dynamical pheno- 

 mena we are here discussing. Thus the central depression is parabolical 

 and not conical, and the angular velocity of the fluid molecules is constant, 

 whereas it varies in the movements of eddies in the inverse proportion of 

 he distance from the axis of rotation. 



water, the gyration of a molecule will be the more rapid 

 the nearer it approaches the centre. The analysis which 

 confirms, or rather explains these phenomena is as appli- 

 cable to gases as to Hquids. Need it be said that water- 

 spouts, from their appearance alone, range themselves in 

 this category ? The mechanical identity of whirls formed 

 whether in liquids or in gases is found in all the details — 

 such as the descending movement of waterspouts whose 

 point is seen gradually approaching the earth, and in the 

 abrading force which whirlwinds thus exert on the 

 ground in breaking and beating down objects which 

 obstruct their course— acting thus like a plate fixed perpen- 

 dicularly at the end of a vertical axis whirling rapidly 

 round. This action evidently ceases when the lower 

 orifice of the waterspout rises a little ; it recommences 

 with energy each time that the whirling cone is lowered so 

 as to be brought into contact with any opposing object. 



We have only further to prove another characteristic of 

 eddies in a stream of water not less general, in order to 

 complete the study of the analogous phenomena in fluids. 

 At the instant when there is formed in a moving mass of 

 water one of these gyrations which are solely due to in- 

 equalities in the general current, it is evident that the 

 eddy thus formed and isolated by an invisible sheath, so 

 to speak, will follow the mean velocity of the current, be- 

 cause nothing can bear away the chief part of the velocity 

 to the molecules which compose the eddy. We shall see it 

 follow the line of the stream, preserving its axis in a 

 vertical position and continuing a longer or a shorter time, 

 or until resistances of every sort have exhausted its force. 

 It will follow the same hne of the stream as that taken by 

 a floating object without losing its circular form, and 

 without ceasing to act on the bottom, if it extend so far 

 down, as long as its store of energy is inexhausted. 



A distinction must be made between these travelling 

 eddies and the great eddies in deep still water which are 

 ceaselessly formed and re-formed at a post fixed at the 

 turning of narrows of a river. When in such places the 

 current makes itself felt it incessantly bears away with it 

 the spires thus formed ; the phenomenon is unceasingly 

 renewed, giving rise to those stationary eddies in rivers 

 which have no analogy to those of the atmosphere, and 

 which appeared to play an important part in deepening 

 the beds of rivers. 



{To be continued.) 



THE LARGE REFLECTOR OF THE PARIS 

 OBSERVATORY 



MWALLON, the French Minister of Pubhc Instruc- 

 • tion, presided on the 7th inst. at the sitting of the 

 Council of the Observatory, and at the end of the seance he 

 made an official inspection of the large refractor. On 

 the 9th the representatives of many of the Parisian papers 

 were present at the Observatory by invitation of M. Le- 

 verrier ; the weather, unfortunately, was very tempestuous. 



The telescope was left under its iron house, but every 

 detail was carefully explained by M. Leverrier, assisted 

 by M. Wolff, the chief astronomer for physical observa- 

 tions. M. Leverrier praised very highly the skill dis- 

 played by the constructors, MM. Eichens and Martin. 



The weight of the moveable part is nine tons ; the 

 mirror is 120 centimetres in diameter, with a focal distance 

 of 6'8o metres. The weight of the mirror is only half a ton, 

 instead of four tons, which would be necessary for a 

 metallic one ; its cost amounts to 2,000/. 



The telescope is suspended like a refractor in an ordi- 

 nary equatorial. The ocular is placed in front. 



On the 8th minute stars were observed by M. Wolff 

 with a magnifying power of 500, which has been found 

 to answer excellently. An ocular multiplying 1,200 times, 

 and perhaps another, 2,400, will be constructed. A micro- 

 meter is being made. 



The seeker is in front, and can be rotated with the 



