April 23, 1896J 



NA TURE 



589 



lirage s A M B, that is to say, convex towards the earth (Fig. i) ; i 

 he sun in going down, at s', throws the shadow of the summit A 

 n summit B, which should therefore afterwards remain in shadow, 

 IS the sun continues to set, and as the last ray is s' a m' b'. But 

 ! in the interval the air of the valley gets sufficiently cold, the 

 1 ajectory takes an inverse curve, s" A m" b", and summit B is 

 aj;ain illuminated. i 



^Experimental Realisation of the Inversion of the Curves of 

 the Lumittous Trajectories. — With a little care it is possible to 

 superpose in a transparent cell of about 20 centimetres thick- 

 ness three layers of liquid, of which the composition is given ; 

 under Fig. 2. A movable mirror, L L, throws a stream of light j 

 through the opening, s, of a diaphragm. This beam of light, j 

 sent under different inclinations, is reflected either by the inferior \ 

 layer of chloride of zinc (dense, but less refracting), or by the 

 liyer of diluted glycerine (lighter and also less refracting than 

 he intermediate layer). 



A little fluorescene illuminates the trajectory of the streams 

 of light, and renders their curves visible ; the Alpengliihen can 

 thus be represented with a few accessory arrangements.] 



Scintillation of Stars. — -This phenomenon is also a proof of 

 the alternation of the temperature and of the movement of the 

 layers of air in the high regions. Spectrum analysis shows 

 that the scintillation is produced by a disappearance following a 

 regular order (in accordance with the variation of the zenith dis- 

 tance of the star) of the successive colours of the spectrum. ; 



\^Iinitation of the Phenomenon. — It is obtained by a very 

 brilliant experiment, which consists in throwing the image of a 

 luminous opening, o, with the help of a lens, L, on a little 

 silvered ball, B, of 3 or 4 centimetres in diameter, resting on black 

 velvet. Thus the aspect of a fixed star is obtained, with re- i 

 niarkable brightness (Fig. 3). ' 



But the luminous opening, O, is made in a card, on which is 

 liiown the spectral image of a slit, F, which is dispersed by a 

 irect vision prism, v. | 



In fact, it is the transformation of the solar energy into 

 mechanical energy which is the fundamental phenomenon ; it 

 brings all others with it. It is the only transformation which, 

 for shortness, I shall deal with here. 



The simplest mechanical phenomenon which is produced in 

 the atmosphere is the wind. The origin of the wind is the 

 difference of pressure between two points more or less distant ; 

 since the time of Pascal, it is known that the pressure of air is 

 measured by the barometer. It might be thought, according to 

 this property, that the direction of the wind is always determined 

 by the indications of this instrument ; that is to say, that the 

 wind must go from the point where the barometric pressure is 

 strongest to the point where the barometric pressure is feeblest. 



Well, this is hardly ever the case ; the real direction of the 

 wind is always oblique to this theoretical direction. This fact has 

 only been known a very few years ; it is the general meteoro- 

 logical maps, suggested by Le Verrier about thirty years ago, 

 and so universally known at the present day, which have put 

 this fact beyond doubt. 



The direction of the wind seems to titrn round the point of 

 the map where the minimum pressure is to be found, in the 

 opposite direction to the hands of a watch ; or, rather, in a direct 

 sense round the point of maximum pressure. Such is the 

 direction of the phenomenon in the northern hemisphere ; it is 

 contrary in the southern. In fact, the most ordinary movement of 

 the atmosphere is 2^ gyratory movement, which is called a cyclone. 



The whirling movement of the air has been observed for a 

 long time ; we often see it produced around us. The dust, the 

 dead leaves are lifted by the wind in a whirlwind resembling 

 eddies in rivers. Sailors know of cyclones and water-spouts, and 

 fear their dangerous effects. On the American continent there 

 are terrible hurricanes called tornados. These gyratory move- 

 ments seem only to belong to great stormy perturbations ; but 

 the more the study of the atmosphere is followed in detail, the ^ 

 more it is seen that this kind of disturbance is met with in all " 



Arrangemeiit for imitating the phenomenon of the .scintillation of stars. 



In truth, the card, c o, is not in the focus of the spectrum, 

 which focus is formed further off, in the plane of the lens L. 

 The result is that the rainbow image of the slit on the card has 

 a white, part in its centre ; it is there that the opening, o, is 

 placed. Also the light thrown on the ball, B, is entirely 

 colourless. But the beam of light, on coming out of the opening, 

 expands into spectrum on the lens of projection L, which re- 

 composes it in B, as in a celebrated Newtonian experiment. 



Then by placing a screen with large meshes before the lens L, 

 certain radiations are taken away, and the star, B, appears 

 coloured. 



A divergent half lens, n, with same focus as I,, cancels its 



•effect, and the spectrum of the star, with the artificial bands 



created by the screen, appears on a white screen by the side of 



the ball. This is the imitation of the spectrum analysis of the 



cintillation of stars.] 



It is seen by these few examples that the study of the optical 

 henomena of the atmosphere, aided by physical analysis and 

 vnthesis, can, and must, teach much about the calorific pheno- 

 Miena of the regions beyond our reach. 



Dynamic Phenomena of the Atmosphere. — The phenomena 

 udied up till now are due to conditions of almost perfect 

 «|uilibrium in the atmospheric layers ; they might be called 

 static. But the calorific action of the sun, combined with the 

 cooling action of radiation in space, can produce phenomena of 

 movement representing every degree of intensity, from the ; 

 feeblest to the most violent : we call these phenomena dynamic. I 

 They make themselves apparent in very different ways. 



(i) \5v\A&x \\\& ioxva ol mechanical energy : winds, whirlwinds, 

 cyclones, water-spouts, &c. (2) Under the form of calorific | 

 energy, which makes itself felt by the formation of clouds, rain, | 

 hail corresponding to different changes of state of water, the 

 element of the atmosphere which is continually varying. (3) 

 Under the form of electric energy : lightning, thunder, <S:c. 



NO. 1382. VOL. 53] 



manifestations of displaced air. It is therefore concluded that 

 the gyratory movement is to some extent the normal condition 

 of agitated air ; it would hardly be possible to employ force on 

 a gaseous mass without developing more or less rapid rotations, 

 which tend to acquire for themselves a permanent condition. 



Experimental Proofs. — Every time a rapid jet of gas is pro- 

 duced, one or more cyclonic movements are formed at the side 

 of the jet. If the projected column is of a cylinder shape, the 

 cyclonic movement will take the form of a ring ; for example, 

 the rings of smoke which are observed after the explosion of 

 cannons, guns, &c. 



[Repetition of the well-known experiment of smoke-rings, 

 produced by striking the canvased end of a box filled with 

 vapour of hydrochlorate of ammonia, with a circular opening on 

 the opposite side. The smoke-rings are rendered visible by 

 throwing them in the line of a beam of electric light.] 



Multiple Origin, of the Gyratory Movements of the 

 Atmosphere. — Nearly all the general causes which act on the 

 movement of the atmosphere are gyratory influences ; when 

 once the movement is set going, it continues of itself, and some- 

 times increases in amount ; in the first place, the movement of 

 the rotation of the earth must be cited, which always brings with 

 it a small component of rotation for a displacement of a gaseous 

 mass in latitude or altitude ; in the second place, and as decisive 

 cause, the solar heat, which warms the air near the surface, or 

 the clouds. As the ascending tendency of the heated gas cannot 

 be equal over the whole surface exposed to the rays of the sun (as 

 much because of the nature of the ground as because of its 

 inequalities), the equilibrium is upset in certain parts, and 

 gaseous columns ascend. This is, therefore, the same ca.se as 

 the jets, quoted above, and consequently under favourable circum- 

 stances for gyrations round horizontal axes. When once the 

 gyration is established, the causes which have produced it 

 keep it up and augment it. 



