July 4, 19 18] 



NATURE 



353 



dark lines were apparent in the preliminary examina- 

 tion. The band at A 464 appears double, and is fringed 

 by a band of lower intensity on its less refrangible side. 

 On June 30 the nova was considered to be visually 

 ' qual to ^ Scuti, which is of magnitude 45. The 

 lour of the star was brick-red. 



'IR¥. METEOROLOGICAL UNIT OF 

 PRESSURE. 



A MEMORANDUM recentlv circulated by Prof. 

 C. F. Marvin, Chief of the U.S. Weather 

 Bureau, raises the question of an appropgate unit 

 of pressure, especially for ijieteorological usage. The 

 measure of pressure by a barometric height, in milli- 

 metres or inches of mercury, even when reduced to 

 standard temperature, is not an absolute statement at 

 all, for its meaning depends on the local value of 

 gravity. On the other hand, theC.G.S. measure of one 

 megadyne per square centimetre, besides being absolute, 

 happens to express quite closely the mean atmospheric 

 pressure at about 100 metres above sea-level. The 

 advantage that could be taken of this fact • has long 

 been obvious ; it is referred to in early editions of 

 Everett's "C.G.S. Units," and so long ago as 1888 

 the adoption of the unit of pressure as one dyne per 

 square centimetre, under the name of a barad, was 

 recommended by a committee of the British Associa- 

 tion. But nothing very definite followed; and Prof. 

 Marvin gives the history, which is not without its 

 moral, of the way in which the natural appropriate- 

 ness and utility of this unit re-noticed, reported upon, 

 or brought into partial use upon inconsistent systems 

 by Guillaume, Bjerknes, and various others, including 

 international committees. 



All this most people will be content to forget, if 

 possible, but two or three simple cardinal points 

 remain, and an appeal is made that we should assess 

 them and settle down to uniformity of practice for 

 the future. The first of these is : Can one unit be 

 adopted for the whole range of physical, including 

 meteorological, pressures, from high vacua to extreme 

 compression, with the help only of the familiar C.G.S. 

 prefixes of mega-, rnilli-, micro-, and convenient 

 numerical factors? Secondly: What is this unit? Is 

 it a dyne or a megadyne per square centimetre? 

 Thirdly : How far, up to the present, has actual prac- 

 tice gone to fix and ratify the answers to these two 

 uuestions? Finally: What is the name to be? 



On these points meteorologists, at any rate, may be 



id to have made up their minds. The bar, of lo* 

 livnes per square centimetre, is to be thf unit. One 

 millibar is approximately equal to the pressure of 

 075 mm. of mercury, and the mean atmospheric 

 pressure is approximately 1000 mb. or i bar. One- 

 tenth of a millibar is riot far from the accuracy with 

 which the barometer can be read. The range of the 

 barometer is included within 100 mb. In increasing 

 degree in recent years the unit has been brought into 

 use in the publications of the British, French, and 

 United States meteorological services. One may say 

 that it would now be very difficult to dislodge the 

 millibar from meteorological use. In supplement. Prof. 

 Marvin has prepared a table that shows that it is 

 entirely convenient for expressing the range of physical 

 pressures from very high vacua at o-oi microbar to 

 pressures of a megabar, at, say, the bottom of the 

 ocean ; while the dyne per square centimetre, which 

 the C.G.S. system first offers, entails a much more 

 cumbrous set of factors. 



If there are any substantial objections to the bar of 

 one megadyne per square centimetre as the unit of 

 pressure — and there do not appear to be any — the 

 wide acceptance it has already won in use should go 



NO. 2540, VOL. lOl"] 



far to outweigh them. If physicists could resolve to 

 adopt it, it would seem pretty sure of general and 

 complete acceptance, and therefore offer one more piece 

 of the difficult and contentious "No Man's Land" of 

 conflicting units won over for the right side. 



R. A. Sampson. 



DEEP-SEA NEMERTINES. 



■p\URING the forty years which have elapsed since 

 ■'--' the first two deep-sea nemertines were taken b} 

 the Challenger Expedition, a few examples have been 

 collected by various other expeditions, but deep-sea 

 nemertines have never been other than rare. Prof. 

 Brinkmann, whose monograph on pelagic nemertines 

 has recently been issued (Bergens Museums Skrifter, 

 Bd. iii., No. i, 1917, 194 pp., 16 plates), has, how- 

 ever, had a rich collection at his disposal, chiefly from 

 the Michael Sars Expedition, so that he has been able 

 to investigate the structure of most of the species 

 described. He has also subjected the previously 

 known species to careful revision, and concludes that 

 five of them are so imperfectly described that they, 

 must be labelled as "uncertain." The rule that the 

 single type specimen of a species should be kept intact 

 is, in the opinion of the author, unsound, for the 

 external features often give little help to the sys- 

 temalist, and therefore investigation by rcieans of 

 serial sections is indispensable. 



The known pelagic nemertines, all of which belong 

 to the order Hoplonemertini, are referred to eighteen 

 genera and thirt}-seven species. Bathynemertes is the 

 most primitive genus, and in its external features 

 resembles the bottom-living forms. Among the 

 pelagic nemertines two types have been evolved : — 

 (i) By an increase in the size of the gut diverticula, 

 and therefore of the body surface — without a corre- 

 sponding increase of tissue — some became specially 

 adapted to a floating life, and in these a marked 

 reduction of the musculature of the bodv-wall took 

 place; (2) from the floating forms arose the swimmers, 

 in which a tail-fin was formed, with strengthening of 

 the parts of the musculature necessary for swimming. 

 Two specimens of Nectonemertes tnirabilis were 

 observed swimming by means of undulations of the 

 body and energetic strokes of the tail. These are the 

 first recorded observations on the swimming of pelagic 

 nemertines. 



We have not space to give an adequate summary 

 of the account of the structure of these animals, but 

 reference may be made to the general reduction of 

 sense-organs, to the presence of penes in Phallo- 

 nemertes, to the reduction in the number of eggs to 

 three or four in each ovary, or in the more modified 

 genera to two, or even one, and to the presence of 

 cephalic tentacles in Balaenanemertes in both se.xes, 

 and in Nectonemertes in the male only, in which thev 

 probably act as claspers. 



The author discusses the horizontal and vertical 

 distribution of pelagic nemertines, some of which cer- 

 tainly, and the rest probably, are bathvpelagic. Most 

 of the species will probably be found to have a wide 

 distribution, e.g. Nectonetnertcs viirabilis occurs in the 

 tropical parts of the Atlantic and Pacific Oceans and 

 in Davis Strait, the conditions as to temperature, etc., 

 being uniform over a wide area in deep water. In 

 spite of the enormous mass of water transported by 

 the Gulf Stream into the North Sea, no example of 

 any .Atlantic species of pelagic nemertines has be<'n 

 taken in the North Sea. There is also a total absence 

 of reconds from the Mediterranean, explicable by the 

 fact that the pelagic nemertines in the Atlantic five 

 in deep water, while the entrance to the Mediterranean 

 at Gibraltar is comparatively shallow. 



