34 



NATURE 



{May 9, 1889 



and we see that, if A = B = C = R, as is the case except in a 

 crystalline medium, the ellipsoid is — 



R(x2 + j2 + ^1) + (^^x + Qjf' + R-)' + («-^ + ^y + 7-)" = d ; 



so that, if Vx + Qj + Rz = L, ox + )8j/ + 72 = X. 



L + ?\ and L - /\ are the imaginary circular reactions of the 

 ellipsoid,^ and consequently theintersection of Land A, whosedirec- 

 tion cosine-; are proportional to Q7 - RyQ, &c., is the major axis of 

 the ellipsoid, when the above signs are attributed to L- and A.'^. As 

 any ellipsoid can be expressed in this form by referring it to its 

 circumscribing sphere and the corresponding planes of circular 

 section, it is apparent that any polarized state of the turbulent 

 motion can be built up of P,Q,R, and a, 0, y, polarizations. The 

 axis of the ellipsoid mentioned above represents the flow of 

 energy in the medium during the propagation of a disturbance. 



I am inclined to think that Sir William Thomson's fear that 

 diffusion would vitiate these investigations would be avoided 

 either by supposing the turbulent liquid to consist of interlocked 

 vortex rings, or of infinite intercrossing lines ; and in either 

 case a natural hypothesis would be that matter consisted of free 

 vortex rings. Geo. Fras. Fitzgerald. 



Trinity College, Dublin, April 26. 



The New Eruption of Vesuvius. 



On April 29, 30, and May i, a constant series of explosions 

 {boati) and rumblings ace )mpanied by earthquakes, which shook 

 the southern foot of Vesuvius, were very noticeable at Resina. 

 About 2 a.m., on May 2, part of the new cone of eruption 

 (formed during the last ten months) fell in, showing that the 

 internal support of the lava column had been removed, in con- 

 sequence of this filling the new dyke, the formation of which 

 had given origin to the preceding sonorous and mechanical dis- 

 turbances. On the same day at 3 p.m. the lava sank still lower 

 in the conduit on account of the dyke reaching the surface at the 

 upper part of the great cone. More of the eruptive cone crunbled 

 in, and of course simultaneously a considerable outpour of lava 

 took place from the dyke fissure which is situated on the south- 

 east side of the great cone. This outflow soon formed a long 

 tongue of lava reaching to the Pedimentina or lower. 



My friend Mr. George Bidder, Jan., who is studying at the 

 Zoological Station here, was fortunately able to visit the moun- 

 tain yesterday (May 3), and much of the information in this 

 letter I have to thank him for. Unfortunately the extremely 

 bad weather has prevented the upper part of the mountain being 

 examined, so that accurate information as to the position and 

 length of the fissure has not been obtainable ; I hope, however, to 

 make the ascent to-morrow, and will then send a more detailed 

 account for your next issue. So far as the facts at hand are 

 available, it would appear that this eruption is of small im- 

 portance, being an analogue of that of May 2, 1885, and that, 

 therefore, the lava will hardly reach cultivated ground. 



A short glimpse of the summit of the mountain this evening 

 shows that much of the eruptive cone still exists, whilst the re- 

 flection from the flowing lava is much feebler than yesterday 

 evening. Lastly, a single faint glimmer this evening at the vent 

 demonstrates the fact that the lava has not sunk very deep in the 

 chimney. The eruption, or more properly disruption, was 

 coincident with a marked barometric depression. 



Naples, May 4. H. J. Johnston-Lavis. 



The Sailing Flight of the Albatross. 



I have been much interested by the letter of Mr. A. C. Baines 

 (Nature, May 2, p. 9) upon this subject. In the year 1883 

 ("The Soaring of Birds," Nature, vol. xxvii. p. 534) I sug- 

 gested that the explanation of these puzzling performances might 

 be found in the increase of wind with height. To take advant- 

 age of this, the bird must rise against the wind and fall with it ; 

 but, at the time referred to, I had before me only the observa- 

 tions of Mr. Peal, in Assam, on the flight of pelicans, in which 

 this feature is not alluded to. In Mr. Baines's observations, 

 the omission is supplied, and there seems little reason to doubt 

 that the true explanation of the flight of the albatross has been 

 arrived at. In the case of the pelican soaring to a great eleva- 



' This was remarked to me by Prof. Lyie, of Melbourne, while I was 

 recently speaking to him upon this subject. 



tion, it is less easy to understand how the differences of horizontal 

 velocity can be sufficient. 



Reference may be made to a paper by Mr. H. Airy (Nature, 

 vol. xxvii. p. 590), in which the matter is further discussed. 

 Similar views have also been put forward more recently by an 

 American author, whose name I have, unfortunately, forgotten. 



Terling Place, Witham, May 6. Rayleigh. 



"Giphantia." 



In a curious little work, entitled " Giphantia," the full title 

 of which I subjoin, there is, at pp. 95-98, a passage that may 

 have some interest in connection with the early history of photo- 

 graphy, and of which I therefore subjoin a copy. 



The Camp, Sunningdale, April 29. J. D. Hooker. 



"Giphantia: or, a View of what has passed, what is now 

 passing, and, during the Present Century, what will pass, in 

 the World." Translated from the Original French, with Ex- 

 planatory Notes. (London : Printed for Robert Ilorsfield, in 

 Ludgate Street, 1761.) 



" Thou knowest that the rays of light, reflected from different 

 bodies, make a picture and paint the bodies upon all polished 

 surfaces, on the retina of the eye, for instance, on water, on 

 glass. The elementary spirits have studied to fix these transient 

 images : they have composed a most subtile matter, very viscous, 

 and proper to harden and dry, by the help of which a picture is 

 made in the twinkle of an eye. They do over with this matter 

 a piece of canvas, and hold it before the objects they have 

 a mind to paint. The first effect of the canvas is that of a 

 mirrour ; there are seen upon it all the bodies far and near, 

 whose image the Vv^h.\. can transmit. But what the glass cannot 

 do, the canvas, by means of the viscous matter, retains the 

 images. The mirrour shows the objects exactly ; but keeps 

 none ; our canvases show them with the same exactness, and 

 retains them all. This impression of the images is made the 

 first instant they are received on the canvas, which is imme- 

 diately carried away into some dark place ; an hour after, the 

 subtile matter dries, and you have a picture so much the more 

 valuable, as it cannot be imitated by art nor damaged by time. 

 We take, in their purest source, in the luminous bodies, the 

 colours which painters extract from different materials, and 

 which time never fails to alter. The justness of the design, the 

 truth of the expression, the gradation of the shades, the stronger 

 or weaker strokes, the rules of perspective, all these we leave 

 to nature, who, with a sure and never-erring hand, draws upon 

 our canvases images which deceive the eye and make reison to 

 doubt, whether, what are called real objects, are not phantoms 

 which impose upon the sight, the hearing, the feeling, and all 

 the senses at once. 



"The Prefect then entered into some physical discussions, 

 first, on the nature of the glutinous substance which intercepted 

 and retained the rays ; secondly, upon the difficulties of preparing 

 and using it ; thirdly, upon the struggle between the rays of 

 light and the dried substance, three problem^ which I propose 

 to the naturalists of our days, and leave to their sagacity." 



Geological Photography. 



In the report of the Annual Conference of Delegates of 

 Corresponding Societies of the British Association {vide Nature, 

 vol. xxxix. p. 187), reference is made to the proposed ap- 

 pointment of a Committee to collect and register photographs 

 of localities, sections, or other features of geological interest in 

 the United Kingdom. Several Societies have already attempted 

 local photographic surveys, but the need is felt to secure 

 uniformity of action by all the Societies interested, and to arrange 

 for the photographs to be available for teaching and other 

 purposes when needed. In order that steps may be taken to 

 arrange for the practical working of the proposed scheme at the 

 forthcoming meeting of the British Association at Newcastle, I 

 am desirous of invoking the kind aid of those of your readers 

 who have interested themselves in the photography of local 

 geological features (especially of typical and temporary sections) 

 in favouring me with the following information : — 



(i) A list of photographs already taken, illustrating given 

 localities or sections j and 



