Oct. 7, i8So] 



NATURE 



549 



Hence, clearly, the object indicated in the title is more per- 

 fectly attained, tlie thinner the plate and the smaller and more 

 numerous the holes. Very fine wire cloth would answer the 

 purpose better than any metal plate with holes drilled through 

 it ; and very fine closely-woven cotton cloth, or cambric, answers 

 better than the finest wire cloth. The impenetrability of wet 

 cloth to air is well known to laundresses, and to every naturalist 

 who has ever chanced to watch their operations. The quality 

 of dry cloth to let air through with considerable freedom, and 

 wet cloth to resist it, is well known to sailors, wet sails being 

 sensibly more effective than diy sails (and particularly so inalie 

 case of old sails, and of sails of thin and light material). 



An illustration w-as shown to the meeting by talcing an Argand 

 lamp-funnel, with a piece of very fine closely-woven cotton cloth 

 tied over one end of it. When the cloth was dry, and the other end 

 dipped under water, the water rose with perfect freedom inside, 

 showing exceedingly little resistance to the passage of air through 

 the dry cloth. When it was inverted, and the end guarded by 

 the cloth was held under water, the water rose with very great 

 freedom, showing exceedingly little resistance to the permeation 

 of water through the cloth. The cloth being now wet, and 

 the glass once more held with its other end under water, the 

 cloth now seemed perfectly air-tight, even when pressed with 

 air-pressure corresponding to nine inches of water, by forcing 

 down the funnel, which was about nine inches long, till the 

 upper end was nearly submerged. W'hen it was wholly sub- 



Water indicated by horixontal shading ; Air by white paper. 



merged, so that there was air on one side and water on the other, 

 the resistance to permeation of air w'as as decided as it was when 

 the cloth, very perfectly wet, had air on each side of it. 



Once more, putting the cloth end under water ; holding the 

 tube nearly horizontal, and blowing by the mouth applied to the 

 other end : — the water which had risen into the funnel before 

 the mouth was applied, was expelled. After that no air escaped 

 until the air-pressure within exceeded the water pressure on the 

 outside of the cloth by the equivalent of a little more than nine 

 inches of water ; and when blown with a pressure just a very 

 little more than that which sufficed to produce a bubble from 

 any part of the cloth, bubbles escaped in a copious torrent from 

 the whole area of the cloth. 



The accompanying sketch represents the application to the 

 Navigational Depth Gauge. The wider of the two communi- 

 cating tubes, shown uppermost in the sketch, has its open mouth 

 guarded by very fine cotton cloth tied across it. The tube shown 

 lower in the diagram is closed for the time of use by a stopper at 

 its lower end. A certain quantity of water (which had been 

 forced into it during the descent of the gauge to the bottom of 

 the sea) is retained in it whUe thejgauge is being towed up to the 

 surface in some such oblique position as that shown in the sketch. 

 While this is being done the water in the wide tube is expelled 

 by the expanding air. The object of the cloth guard is to 

 secure that this water is expelled to the last drop before any air 

 escapes ; and that afterwards, while the gauge is being towed 

 wildly along the surface from wave to wave by a steamer 

 running at fourteen or sixteen knots, not a drop of water shall 

 re-enter the instrument. 



ON THE CLASSIFICATION OF BIRDS^ 



A BOUT twelve years ago Prof. Huxley had taken up the 

 ■^^ subject of the classification of birds in his usual zealous and 

 original way, and from quite a new point of view. Prof. Huxley, 

 treating birds mainly from their bones and as if they were extinct 



animals of which these parts of then- structure only were known, 

 had proposed an entirely new plan of an-angement, based mainly 

 upon the characteristic variations of the palatal bones, which had 

 passed almost unnoticed by previous writers. The author, who 

 had long been dissatisfied with the Cuvierian system, w hich with 

 certain modifications he had employed up to 1872, had in that 

 year been constrained to consider the whole subject in order to 

 decide what arrangements should be adopted in the " Nomen- 

 clator Avium Americanarum " (a joint work by Mr. O. Salon 

 and himself), then ready for publication. Prof. Huxley had 

 commenced his system with the lowest and most reptilian birds, 

 and had ended it with the highest and most specialised. But it 

 seemed to the author that by exactly reversing this arrangement 

 he would obtain a scheme which would not very far deviate from 

 that which he had previously employed for the first three orders, 

 and would offer many improvements on the Cuvierian system in 

 the remaining ones. .Such a scheme had accordingly been pro- 

 mulgated in the Introduction to the " Nomenclator " and 

 followed in that work. In the various subsequently issued 

 editions of the " List of Vertebrated Animals in the Zoological 

 Society's Gardens " a nearly similar arrangement had been 

 followed. A certain amount of adhesion having been secured to 

 this system, the author had been recently induced to devote some 

 labour to its improvement and development. As now elaborated 

 it did not profess to be in any respects original, except as regarded 

 certain small details on points to which he had devoted special 

 attention. The arrangement was in fact simply that of Huxley 

 reversed, with slight modifications consequent upon the recent 

 researches of Parker and Garrod on the anatomy and osteology 

 of little known forms. 



The author then proceeded to explain further the ' ' Systema 

 Avium " thus advocated, as shown in the subjoined table, in 

 which the approximate number of known species was added 

 after each Order. 



ORDERS OF EXISTING BIRDS 

 Subclass Carinat/e (10,121 species) 



Specit 



XIII. Gallince ... 



XIV. Opisthocomi 

 XV. Hemipodii 



XVI. Fulicariaj 



XVII. Alectorides 



XVIII. Limicolaa 



XIX. Gavite 



XX. Tubinares 



XXI. Pygopodes 



XXII. Impennes 



XXIII. Crypturi... 



Species. 



24 

 150 



60 

 250 

 130 

 100 



65 



20 



40 



Subclass Ratit.« (18 species) 



XXIV. Apteryges 4 I XXVI. Struthiones ... 4 



XXV. Casuarii 10 | 



In submittmg this arrangement, as one which on the whole he 

 was disposed to regard as the best to be adopted after many 

 years' study of the Class of Bh-ds, the author observed that it 

 should be recollected that, although a linear system is an absolute 

 necessity for practical use, it could never be a perfectly natural 

 one. It woidd always be found that certam groups were nearly 

 equally related to others in different places in the hnear series, 

 and that it was a matter of difficulty to decide with which of the 

 allied forms they were best located. But, a linear arrangement 

 being an absolute necessity, it became our duty to make it as 

 natm-al as possible. 



THE GREEN COLOUR OF OYSTERS 



IN Nature, vol. xvi. p. 397, mention was made of the fact 

 that the green colour observed in oysters in certain localities 

 is caused by a variety of navicula, to which the name A^avictila 

 ostrearid has been given. Further particulars of experiments 

 made by M. Puysegur, at Sissable, are not without interest. ^ 



" The green sUme was collected by lightly scraping the margin 

 of one of the ' clears ' with a spoon, and was put in flasks, 

 shaken for a moment and then allowed to settle, so as to get 

 rid of the mud, some admixture' of which is mevitable. The 

 coloured fluid, containing little or nothing besides diatoms, was 

 then poured off into other flasks. Care and some little dexterity 

 are requisite, as if there is too much silt or too large a quantity 

 ' Revue maritime et cuhniale, February, 1880. 



