46 D 



NATURE 



[Sept. 15, 1 88 1 



" started from Chrisfiania at about 2 a.m. .in the Ny/and 

 steamer bound for Christian«and. At Krujero the steamer forced 

 its way through the ice for half an hou- till within about a mle 

 of the land, where sleighs met it on the ice. The passenge s and 

 cargo were discharged or taken up oa the ice, out of whicli we 

 Ijacked in close proximity to the Kong Hacoii, which steamer 

 had followed us in. A beautiful sunset and Arctic winter view, 

 clear air, and rich sky, also a distant ship fast in the ice. The 

 A^'/a«rt' -stop ped at Arendal for the night, having got to the quay 

 through much ice. We observe often phosphorescence or phos- 

 phorescence-like sparks and flashes in the ice as it is broken up 

 by the steamer." 



1 think that the average thickness of the ice might have been 

 abo.it eight inches. I cannot give the temperature, but on the 

 previous day at Christiania the thermometer indicated about 8° 

 or 10° belo V zero Fahrenheit (about 40° Fahienheit of frost). 

 The diary from which the above extract is taken was kept jointly 

 by myself and my travelhng-companion, Mr. Winter, of the 

 Indian Civil Service, who of course also saw the flashes referred 

 to. I should like to have been able to talk the matter over 

 again with him, but he is now in Ind'a. J. Allen Allen 



[The question raised in this letter is a very interesting one. 

 The phenomenon i-. possibly analigcus to the electric Hashes 

 which are pro:luced when loaf-sugar is crushed or when mica is 

 rrpidly split. It ap.^ears very improbal)le that it can be due to 

 phosphorescent creatures in the water under the ice. — Ed.] 



i.Tidal Currents vcrsm Wind Waves 

 In Nature, voh xxiv. p. 286, a writer on "sea-shore 

 alluvion " positively asserts that the travelling of sea beaches 

 ii due to wind- waves, and not to tidal currents, and calls a 

 writer in the Engineer to task for having stated the latter. 

 Notwithstanding this assertion, I would sng^gest that the writer 

 in the Engineer is right. Twenty-five years ago, when an 

 engineering student, I was taught lh.it sea-beach travelling ^vas 

 due to wind-waves. Afterwards, whi'e 1- nocking abint during 

 fifteen years in the vicinity of the south and west coasts of 

 Ireland, I noted facts that went to show that such a theory was 

 not universally correct. This led me to study wind-sction on 

 the sea and lakes, also all I could find that had been written on 

 the subject ; the result being that as £ood evidence was so con- 

 tradictory, no opinion could be come to from the evidence of 

 others. But it was not till about ten years ago, when I was so 

 circumi-tauced that I could properly study wave-action, and after 

 .-ix years' careful observati >n on the south-east coast of Ireland, 

 that I found that tidal currents were the principal motive power ; 

 and on again reading what had been written on the ;u'ject, 

 tliat I found that nearly all the advocates for the drifiage of 

 sea beaches by wind-waves had stndied on beaches iv/nre the 

 most continuous and powerful winds acted in conjunc'ion with 

 the floio-tide current. As the results of my obsei-yations have 

 been published in the Proceedings of the Royal Irish Academy, 

 English and Irish Institutions of Civil Engineers, the Geological 

 Societies of London, Dublin, &c., during the last six or eight 

 years, it is unnecessary to repeat them here. I would, however, 

 point cut that when there are only wind-waves and no tidal 

 currents, the beaches as a general rule rre banked up, but do 

 not travel (the writer in Nature seems to have observed this, 

 but does not appear to see the importance of it). This may be 

 seen in the tideless Mediterranean, as pointed out by the late 

 Dr. Ansted in his paper on the Lagoons at the Delta of the 

 Rhine ; it may also be seen in Malcombe, or any other bay 

 where there is a "head of the tide" but no tidal cuiTent ; and 

 in the different freshwater lakes, when the wind-waves are the 

 only motive power. But wherever there are tidal cm'rents acting 

 on a coast the btacli must travel. Such tidal currents are those 

 that most perplex the erectors of groynes. If there was only 

 the travelling augmented by wind-waves, the erection of 

 groynes would be very simple ; bu^ as a general rule, they 

 are most necessary where there are strong tidal currents (or 

 conflicting currents) due to the regular "flow" tide, "half 

 <; lunter " tides, or "on-shore " lides ; which conflicting currents, 

 combined with the action of wind-waves, let them be direct or 

 as "ground swells," make up all the " cutting-out tides." The 

 greater the complica'ion? the greater the " cutting out," and the 

 more ingenious have to be the groynes. ' ' Fulls " accumulate with 

 the wind-wave~, but rapidly disappear when the wind ceases. 

 I presume the writer of the article in question is aware that the 

 greatest rise of tide and the least current is at the "heads of 



the tides," while the least rise and greatest current is at the 

 "nodal or hinge lines"; and I would be interested to know 

 where permanent beachos accumulite in the latter localities, as 

 from what I have seen those that form rapidly disappear when 

 the wind cea'-es. G. H. Kinahan 



U.M. Geological Survey 



Glaciation 



In Nature, vol. xxiv. p. 364, I see a notice of a paper by 

 Dr. Woeikoffon the glaciil climate, in which it i, .shown that 

 " the difference of m ,an temperature at the lower ends of glaciers 

 (in different parts of the world) reaches fully 20°." This might 

 be expected. The extent of glaciation de, ends not at all on 

 mean or on winter te.npera'ure, but chiefly on summer tempe- 

 rature. Perpetual snow means sununer snow, so that summer 

 temperature is what determines the extent of the snow- fields 

 remaining unmelted in the summer, and consequently of the 

 glaciers W"hich are fed by the snow-fields. The extent of glacia- 

 tion is also much influenced by the amount of snow-fall. All 

 this is stated in Forbes's " Norway and its Glaciers." 



Joseph John Murphy 



Old Forge, Dunmurry, Co. Antrim 



Yellow Glass in Fog 

 Some years ago I was staying at an hotel on the Lake of 

 Constance. One morning a fog came on which completely 

 obscured the opposite shore, but looking through a strip of 

 yellow glass, which formed the border of the window, I was 

 able, to my surprise, to see it distinctly. I presume the yellow 

 glass choked the blue rays reflected by the fog, just as a Nicol's 

 prism, held at the proper angle, chokes the rays reflected from 

 the glass and enables u; to see clearly the picture 1 ehiud it. On 

 my way home I stopped in Paris, and, happening to call on one 

 of the principal opticians, mentioned the circumstance to him. 

 He forthwith showed me a nav.il telescope provide , with a cap 

 at the eye end containing a yellow glass, which could be removed 

 at pleasure, I should like to know if the sane simple con- 

 trivance has ever been used in our own navy. R. 



The New Museum of Natural History 



In your article on "The New Mu-eum of Natural History " 

 (Nature, vol. xxiii. p. 549 et sej.) it is stated that the speci- 

 men of Archaopteryx macrura in the British Museum is headless. 

 Will you permit me to draw attention to a nodule projecting from 

 the slab in which the fossil lies, which bears a striking resem- 

 blance to the cerebral portion of a bird's skull ? It is so ne years 

 since I visited the museum, but I recollect feeling satisfied at the 

 time that the nodule was the mi- sing head, and worth while 

 disinterring from its surrounding slate. E. H. Pringle 



Calicut, July 31 



[The nodule referred to by our corre pondent is well known, 

 and has been frequently criticised. Mr. John Evans, D.C.L., 

 F.R.S., drew attention to it in an article published by him in 

 the Natural History Review, 1865, pp. 415-421 = ".9" P°'- 

 tions of a cranium and of a jaw in the slab containing the 

 fossil remains of the Anhaoptcryx." Although these fragments 

 which occur in the slab in question undoubtedly belong to Archtro- 

 pteryx, yet, as stated in our article, vol. xxiii. p. 551, "The 

 original specimen described by Prof. Owen is headless," whereas 

 the newly-discovered Beriin specimen has the head entire, and 

 fairly well preserved, and still attached by the neck to the 

 trunk. — Ed.] 



On the Velocity of Light 



In view of the experiments of Young and Forbes on the 

 velocity of light, and of the article published by Lord Rayleigh 

 on the subject, it may not be out of place to state as a fact 

 which seemed at the time too evident to require special mention 

 in my paper " On the Velocity of Light," that if the velocity of 

 red and of blue light in air differed by as much as one-tenth ■ f 

 I per cent., the image of the slit whicli served as the source of 

 li^ht, instead of being white, would be spread out into a spec- 

 trum which could not fad to be observed. The total displace- 

 ment in these experiments amounted to 133 millimetres; there- 

 fore, a difference of velocity of the red and the blue rays of rS 

 per cent, would necessitate a spectrum 2-4 millimetres in length. 



