August 6, 1896] 



NA TURE 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself respomible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications. ] 



Sun-spots and Faculse. 



The following account of the nature of sun-spots and of the 

 facuUt commonly found associated with them, explains these 

 phenomena by refraction through the sun's atmosphere on the 

 supposition that the centie of a spot is the centre of a high- 

 pressure area or anticyclone. 



The descending central current in a solar anticyclone is 

 caused by the exterior portion of the sun's atmosphere being at 

 a temperature less than is consistent with convective equilibrium ; 

 consequently the whole of the descending column is colder and 

 heavier than the atmosphere at the same level outside the area 

 affected. The result is that the density of the atmosphere near 

 the I'lise of the column is increased, as compared with the 

 normal density at the same level, by three different causes, 

 viz, : — 



(1) By lower temperature. 



(2) By greater pressure, resulting from the upper portion of 

 the column being colder and heavier than the normal at the 

 same level. 



(3) By additional pressure, resulting from the downward 

 motion of the column being arrested near the photosphere ; 

 hence the pressure at the base is increased by inertia. 



In the diagram annexed (a supposed section through the sun's 

 atmosphere at a high-pressure area), the surface of the photo- 

 sphere is represented by the line P P, and the successive surfaces 



6 a i 



of equal density in the atmosphere are represented by the lines 

 </], (/j, &c. These surfaces, indicating greater density in the 

 centre of the area affected, must be convex-outwards in the 

 centre, and concave-outwards near the margin, where they join 

 with horizontal equal-density surfaces in the undisturbed atmo- 

 sphere outside the anticyclonic action. 



The area in question is seen from a distant point in the 

 direction of E by the rays of light which emerge from the 

 atmosphere in approximately parallel lines, represented in the 

 diagram by the Wnesfff. These rays before emerging must 

 pass through the atmosphere by such courses as those represented 

 in the diagram ; that is to say, as they cross successive equal- 

 density surfaces, moving as they do from a greater density to a 

 less, they are refracted in a direction less perpendicular to these 

 surfaces. The greatest deflection will occur about the positions 

 of the lines ,f^, which cross the surfaces where these are most 

 inclined to the photosphere. The areas A and B B will thus 



NO. 1397, VOL. 54] 



appear dark because they are seen by the light from smaller 

 areas of photosphere, a and h h. As drawn in the figure the 

 circular area. A, is nine times the area a, and consequently its 

 mean brightness would be only one-ninth normal brightness ; 

 and the annular area, B B, being three times the area bb, would 

 appear one-third normal brightness. 



Beyond the greatest deflection lines, gg, the annular area on 

 the photosphere is of greater width and less diameter than the 

 corresponding annular area at the surface of the atmosphere. 

 .\s drawn in the figure, the annular area, c c, is equal to c c, 

 and would appear of normal brightness ; while the area, D D, is 

 about two-thirds of d d, and would appear of one and a half 

 times normal brightness. In this region one or more faculse- 

 would be seen surrounding the spot ; one only if the concave- 

 outwards curves of the equal-density surfaces were superposed 

 one on another, as in the diagram ; while if some series 

 of such curves extended beyond others, more faculse would 

 be seen. 



The occurrence of " eruptive prominences " near (but not at) 

 the position where a spot has disappeared on the margin of the 

 sun, is accordant with an anticyclonic motion round the spot ; 

 for this motion premises a rising-up of the lower atmosphere in 

 the outer portion of the area aflected. So also the greater 

 width of the absorption-lines of the solar spectrum over a sun- 

 spot, indicating that the absorbing atmosphere is thereof greater 

 pressure, is accordant with the theory here advanced. 



It should be noticed that the brightness of the surrounding, 

 facula; according to this theory arises simply from the light in 

 which the spot is deficient. James Renton. 



Observatorio Nacional, Cordoba, Argentine Republic. 



Sailing Flight. 



In NATt'RE, May 14, p. 25, you have a notice of two works 

 on flight of birds, and I am rather surprised to see that the 

 theory of upward currents in the air is still adhered to. 



In Nature, November 4, 1880, I laid a few remarks before 

 you on this subject, aided by a little diagram, and on re-perusing 

 this can see little to add to them, and nothing to alter. 



It seems to me that upward currents of air, to account for 

 sailing flight of birds, is, firstly, quite needless ; secondly, they 

 cannot be seen or proved to exist ; and thirdly, the entire 

 absence of such currents can be (at least out here) optically 

 demonstrated. 



As stated in my note (November 4, 1880), above referred to, 

 we have two steady winds out here, from N.E. and S.W. ; they 

 are not at all violent or gusty — indeed, if directed vertically they 

 could not possibly lift and sustain a 20 lb. cyrus or pelican. 



But the utter absence of vertical air currents in our N.W. 

 wind, at the very time the large birds are soaring in it, is- 

 beautifully demonstrated by the tufts of cotton, blown from the 

 burst pods of the tall cotton trees, Boiiibax malaharicum. 



For many years I have had a rather large telescope, through 

 which to study the Noga Hill villages and cultivation, at six to 

 thirty miles south, and for long was puzzled by the frequent ap- 

 pearance of small white objects, which slowly crossed the field, 

 horizontally, at all distances and elevations, and at a speed of 

 about ten or fifteen miles per hour. At last I found they were 

 cotton tufts, out of which the little seed had dropped, and the 

 beautifully steady and horizontal paths of thousands, at all 

 distances, was often remarkable, at the lime the birds were 

 soaring. Anything approaching vertical air currents must have 

 been at once detected, and easily visible. I have for hours 

 watched the sailing, at 1000 and 2Ckx) feet, of Cyrus, called here 

 Korson {Grus Antigoni): Pelican, called here Dherra {Peli- 

 canus); three Vultures, called here Hogren (Gyps Inds.), and 

 two larger kinds ; two adjutants, called here Bov Tokla 

 (Leptoftilus Argalaa.t\d Nudifrons); onejabiru, called here Telia 

 Hareng. 



Now, not one of these birds are ever seen sailing in a straight 

 line, unless when descending. They cannot rise, or even 

 sustain themselves, without flapping the wings, unless in a 

 breeze, and when moving in a curve or spiral. 



For the first 200 or 300 feet, in rising, they flap vigorously, 

 and when well above the surface eddies, begin sailing in spirals, 

 rising ten and twenty feet at each lap, wings held rigidly 

 ■extended, and the tail alone seen to move now and Ihen, and so- 

 on to 1000, 2000 and 3000 feet. 



