Jan. 30, 1890] 



NATURE 



297 



covered with material that renders them almost indistinguishable 

 from the stones and gravel in which they are found than if they 

 were naked. 



As regards the use of the peculiar hind legs in the Anomoura 

 and Dorippe, perhaps the enclosed extract from a paper read by 

 me on December 12 before the Chester Society of Natural 

 Science may be of interest. It will shortly be published in vol. 

 iv. of the Transactions of the Liverpool Biological Society. 



Alfred O. Walker. 



London, W., January 17. 



" An interesting fact, illustrating the ingenuity shown by more 

 than one species of Crustacea in concealing themselves, came 

 under my notice last summer. Having dredged a number of 

 Amphipoda, I placed them in a vessel of sea water till I could 

 examine them. Amonsi them I noticed what seemed to be a 

 piece of dead weed swimming rapidly about and occasionally 

 falling to the bottom. Examination with a lens showed that the 

 piece of weed was carried by an Amphipod (Atylus swanimev' 

 damii), which grasped it by the two first pairs of walking legs 

 (peraeopoda). When it came to the bottom the animal con- 

 cealed itself beneath the weed, which was much larger than 

 itself. 



" In connection with this habit of A. swammerdamii, it may be 

 mentioned that another species, Atylus falcatus (Metier), re- 

 sembles the first-named minutely in every respect but one, viz. 

 that the first peraeopod has the claw (dactylus) immensely de- 

 veloped, while at the base of the next joint are two or three 

 strong blunt spines or tubercles into which the point of the claw 

 fits. This would appear to give the latter species a great ad- 

 vantage over its congener in grasping an object for purposes of 

 concealment. It is a rare species, but I have met with a few 

 specimens this summer : I am not aware of its having been 

 recorded as British yet. 



" In some of the Podophthalmata the same instinct has been 

 observed, and especially among the Anomoura. All these have 

 the last or hindmost pair of legs of a shrunken and apparently 

 almost abortive form. They never appear to be used for walk- 

 ing, and are generally carried turned up on the back ; but they 

 are utilized by some species of curiously shaped, flat-bodied 

 crabs {Dorippe) to carry the valve of a bivalve mollusk over 

 their backs, under which they can squat and hide. From this it 

 is an easy transition through various stages to the hermit crabs 

 [Paguridcc), which ensconce themselves altogether in a univalve 

 shell, and use the curiously abortive hind limbs to cling to the 

 inside whorls. My friend Surgeon-Major Archer has seen 

 crabs of the genus Dorippe protecting themselves (probably from 

 the scorching tropical sun), at low tide, on the mud flats at 

 Singapore, by carrying large leaves over their backs (Journal of 

 Linn. Soc, vol. xx. p. 108)." 



I CAN corroborate Mr. Ernest Weiss's remarks on the use of 

 the modified legs of Dromia. A small one I had in an aquarium 

 would, when the sponge was removed from the back, hunt 

 about until it found something — a shell, a pebble, or even a 

 dead fish — to replace the sponge. When there was nothing in 

 the aquarium which it could seize, it was evidently in an 

 unhappy condition. 



With regard to foreign substances on other crabs, I have 

 caught spider-crabs so completely covered with sponges as quite 

 to hide their shape, and, until they moved, it was impossible to 

 say what they were. David Wilson-Barker. 



Thought and Breathing. 



With reference to Prof. Leumann's researches into the influ- 

 ence of blond circulation and breathing on mind life, referred to 

 in Nature of January 2 (p. 209), it is worthy of note that 

 regulation and suppression of the breath {Prdndydma or 

 Hatha- Vidyd), is an all-important religious observance amongst 

 Hindus. 



It is one of the eight chief requisites of the Yoga philosophy, 

 for attaining " complete abstraction or isolation of the soul in 

 its own essence," and minute instructions exist for the exer- 

 cise, which is adopted, apparently, as an immediate aid to deep 

 meditation. Some of these instructions are quoted in Prof. 

 Monier- Williams's recent work on Buddhism (p. 242), and he 

 also quotes, in connection with this subject (p. 241), Sweden- 

 borg's opinion that thought commences and corresponds with 

 respiration. 



Swedenborg also says: — "It is strange that this correspond- 

 ence between the states of the brain or mind and the lungs has 

 not been admitted in science." R. Barrett Pope. 



Brighton. 



On the Effect of Oil on Disturbed Water. 



Having seen the interesting article by Mr. R. Beynon on the 

 above subject (Nature, January 2, p. 205), shortly before 

 leaving England, I propose to make a few observations on the 

 theoretical aspect of the phenomena described by him. 



The simplest case of wave-motion in a viscous liquid arises 

 when two-dimensional waves are propagated in a liquid whose 

 depth is so great in comparison with the lengths of the waves 

 that the former may be treated as infinite. If at any particular 

 epoch, which we may choose as the origin of the time, the form 

 of the free surface is determined by the equation tj = Ac'"", 

 where 2icjm is the wave-length, its form at any subsequent time 

 may be represented by rj = Ae'*'+"'", and the object of a theo- 

 retical solution is to find the value of /•. The equation for 

 determining k is given in the last chapter of my " Hydro- 

 dynamics " ; and it is there shown that if the viscosity of the liquid 

 be sufficiently small, k will be of the form - a ± <)3, where o and 

 )3 are real positive constants. Hence the equation of the free 

 surface, in real quantities, may be written — 



17 = Ae-'^cosfwAT - 3^) (i) 



which represents periodic motion whose amplitude diminishes 

 with the time, and which therefore ultimately dies away, the 

 rapidity with which the motion decays depending upon the 

 magnitude of a. If, however, the viscosity be large, the solution 

 changes its character, since in this case k is a real negative 

 quantity, and the equation of the free surface becomes 



rj = Ae -"'cos/wx (2) 



which represents non-periodic motion, which rapidly dies away. 



The phenomena discussed by Mr. Beynon are somewhat 

 different from the special case of deep-sea waves, inasmuch as a 

 thin film of a highly viscous liquid, viz. oil, whose thickness is 

 very small compared with the wave-length, is spread over the 

 surface of water, which is a liquid whose viscosity is so small, 

 that it might probably be neglected altogether. The action of 

 the wind would also introduce an additional complication ; but 

 the circumstance that the thickness of the oil is small compared 

 with the wave-length, would, on the other hand, facilitate the 

 calculations which would be necessary in order to obtain a 

 theoretical solution. There can, however, I think, be little 

 doubt that the free surface would be given by equations of the 

 forms either of (i) or (2) ; where a is so large, that after a .short 

 time has elapsed after the film of oil has spread itself over the 

 water, the amplitude of the existing motion would be small 

 compared with that of the original motion. A. B. Basset. 



Hotel Beau Site, Cannes, January 11. 



Luminous Clouds. 



In the correspondence that has taken place on luminous 

 clouds, totally different classes of phenomena have been men- 

 tioned. There are self-luminous clouds entirely distinct from 

 what I have termed " sky- coloured clouds," which latter, though 

 by some deemed self-luminous, have been generally admitted to 

 shine by reflecting the direct light of the sun. 



The self-luminous clouds described by Mr. C. E. Stromeyer 

 (p. 225) appear to have been a part of the aurora which was 

 visible at the same time ; but other correspondents have men- 

 tioned self-luminous clouds which have apparently not been of a 

 truly auroral character, and the nature of these clouds seems not 

 to be understood, and requires investigation ; there may be 

 various kinds of these and causes of their luminosity. I have 

 myself not unfrequently seen what I call irregular auroras, 

 which may be one form of what others call self-luminous clouds. 

 They consist of bands which, unlike regular auroras, appear 

 indifferently in all parts of the sky, and lie in any direction ; 

 they are usually much fainter than the Milky Way, and are 

 always feebler near the zenith than near the horizon. The 

 bands composing them are generally parallel, or nearly so, and 

 3° to 10° wide. They differ from ordinary cirrus in being, so 

 tar as I can judge, perfectly transparent, and also in moving 

 extremely slowly, giving one the impression that they are much 

 higher up in the atmosphere than cirrus. Their spectrum is 



