May 25, 191 1] 



NATURE 



417 



of water fall on any part of it where it showed white 

 when breathed on. Now tilt the plate to make the drop 

 flow, and note the resistance to its flow, and how it draws 

 itself up in the rear, leaving the plate dry. When, how- 

 ever, the moving drop comes to the part acted on by the 

 flame, all resistance to flow ceases, and the drop rapidly 

 spreads itself over the whole track, and shows a decided 

 disinclination to leave it." 



The impression thus produced lasts for some days 

 or weeks, with diminishing distinctness. A permanent 

 record may be obtained by the deposit of a very thin 

 coat of silver by the usual chemical method. The 

 >ilver attaches itself by preference to the track of the 

 tlame, and especially to the edges of the track, where 

 presumably the combustion is most intense. It may 

 be protected with celluloid, or other, varnish. 



The view, expressed by Mr. Aitken, which would 

 attribute the effect tft very fine dust deposited on the 

 glass from the flame, does not commend itself to me. 

 And vet mere heat is not very effective. I was unable 

 to obtain a good '-esult by strongly heating the hack 

 of a thin glass in a Bunsen flame. For this purpose 

 a long flame on Ramsay's plan is suitable, especially 

 if it be long enough to include the entire width of the 

 plate. 



It seems to me that we must appeal to varying 

 degrees of cleanliness for the explanation, cleanliness 

 meaning mainly freedom from grease. And one of 

 the first things is to disabuse our minds of the idea 

 that anything wiped with an ordinary cloth 

 can possiblv be clean. This subject was ably 

 treated many years ago by Quincke (Wied. Ann. II., 

 p. 145, 1877),' who, however, seems to have re- 

 mained in doubt whether a film of air might not give 

 rise to the same effects as a film of grease. Quincke 

 Investigated the maximum edge-angle possible when 

 a drop of liquid stands upon the surface of a solid. 

 In general, the cleaner the surface, the smaller the 

 maximum edge-angle. With alcohol and petroleum 

 there was no difficulty in reducing the maximum angle 

 to zero. With water on glass the angle could be 

 made small, but increased as time elapsed after 

 •cleaning. 



As a detergent Quincke employed hot sulphuric acid. 

 \ few drops may be poured upon a thin glass plate, 

 which is then strongly heated over a Bunsen burner. 

 When somewhat cooled, the plate may be washed 

 under the tap, rinsed with distilled water, and dried 

 over the Bunsen without any kind of wiping. The 

 parts wetted by the acid then behave much as the 

 track of the blow-pipe flame in Aitken 's experiment. 



An even better treatment is with hydrofluoric acid, 

 ^^ hich actually renews the surface of the glass. A few 

 drops of the commercial acid, diluted, say, ten times, 

 may be employed, much as the sulphuric acid, only 

 without heat. The parts so treated condense the 

 breath in large laminae, contrasting strongly with the 

 ordinary deposit. 



It must be admitted that some difficulties remain in 

 attributing the behaviour of an ordinary plate to a 

 superficial film of grease. One of these is the com- 

 parative permanence of breath figures, which often 

 survive wiping with a cloth. The thought has some- 

 times occurred to me that the film of grease is not 

 rntirelv superficial, but penetrates in some degree into 

 the substance of the glass. In that case its removal 

 and renewal would not be so easy. We know but 

 little of the properties of matter in thin films, which 

 may differ entirely from those of the same substance 

 in mass. It may be recalled that a film of oil, one 

 or two millionths of a millimetre thick, suffices to stop 

 the movements of camphor on the surface of water, 

 and that much smaller quantities may be rendered 

 evident by optical and other methods. RAVi.noii. 



NO. 2169, VOL. 86] 



NORTH SEA FISHERY INVESTIGATIONS.' 

 "1^ HE third Report on the Fishery and Hydrographic 

 J- Investigations conducted by the Marine Bio- 

 logical Association under the international organisa- 

 tion is the most interesting yet published. It contains 

 statistical papers dealing with the abundance and 

 migrations of edible fishes in the North Sea ; the age 

 and growth-rate of plaice from the same region ; and 

 the fishing action of trawl nets with respect to the 

 size of mesh. There are also accounts of the distribu- 

 tion of fish-eggs in the North Sea during the summer, 

 and of the physical conditions of the water of the 

 English Channel, and its contained plankton through- 

 out the year 1906. 



The first, and most lengthy, paper in the Report 

 is one of unusual interest and value. In it Miss 

 R. M. Lee has summarised and discussed a series of 

 records kept by the masters of nine Grimsby steam 

 trawling vessels, which fish over the whole of the 

 central part of the North Sea. These records cover 

 the period 1904-8, and represent the results of 14,543 

 hauls of the otter trawl net, which means about 53,995 

 hours' fishing. The statistics have been treated most 

 clearly and concisely, and show for each of twenty- 

 three areas into which the whole North Sea has been 

 divided, the monthly average quantities of each of the 

 more important species of marketable fishes caught 

 by the vessels per ten hours' trawling. In order to 

 render these results comparable with those obtained by 

 other statistical workers (Fulton, D'Arcy Thompson, 

 Redeke), the author has calculated factors which 

 enable one fishing unit to be converted into any of 

 the others used. 



From these data (which are based on the practical 

 knowledge acquired by a number of very experienced 

 fishermen) Miss Lee has deduced the general distribu- 

 tion of each of the species of fish considered over the 

 whole area of North Sea represented, the migrations 

 from one subarea to others, the spawning movements 

 of the fishes, the variations in density from month to 

 month, and the general change in the productivity of 

 the fishing grounds from year to year. Her paper, 

 and a former one which dealt in a similar way with 

 records kept by a number of Lowestoft sailing-trawler 

 skippers, form a picture of the present condition of 

 the North Sea fishing area which must prove to be of 

 inestimable value for the fishery administrators of the 

 future. If, in the early 'seventies, before steam trawl- 

 ing had become the predominant method of fishing, 

 such a summary by a trained statistician had been 

 made, how much controversy and trouble might have 

 been avoided ! Even as it is, a comparison of Miss 

 Lee's results with those imperfect records which we 

 possess of fishery operations in the North Sea in the 

 'seventies and 'eighties shows most strikingly the 

 chanr^e which has taken place, and enables us to 

 realise, to a degree hitherto unattainable, how very 

 great must have been the diminution of the stock of 

 fish inhabiting the North Sea. It is not a question of 

 the decadence of a fishing ground, but rather the 

 fishing-out of an accumulated stock, and the estab- 

 lishment of a new equilibrium, on a lower level, be- 

 tween the natural powers of recuperation of a fish 

 population and the catching power of the British and 

 Continental fishing fleets. And there can be little 

 doubt that, in the absence of concerted measures for 

 cultivation, this equilibrium must settle down to still 

 lower levels. Tt is sincerely to be hoped that this work 

 mav be continued by Miss Lee. 



Dr. Wallace has continued his former work on the 

 natural history of the plaice and subjects to detailed 

 analysis, a mass of data representing individual deter- 



1 Third Rep-Tt (Southern Aren) on Fishery and Hydro>rraphir.qllnvesli- 

 gations in the North Sea and Adjacent Water*, tgofi-S. (Cd. 5546% igit. 



