May 5, 1892] 



NATURE 



zebras even at moderate distances, although there was nothing 

 to hide them, the black and white stripes blending so completely 

 that the animals assume a dull brown appearance quite in 

 harmony with the general colour of the locality in which they are 

 found, and in which, for instance, Rooi Rehbok (PeUacapreolata) 

 is also well protected on account of its peculiar brownish coat, 

 A member of our party, who on another occasion gave proof that 

 he is possessed of excellent eyesight, and who has frequently 

 hunted in similar localities, saw a zebra which was wounded in 

 one of the front legs at a distance of about 400 yards, and 

 strange to say he mistook it for a big baboon. In a letter which 

 I received from him a few days ago, he said, " It galloped like 

 a baboon from me, and I could only see that the colour was 

 greyish-brown. At about 500 yards from me it ran on to a 

 little krantz, and mounting the highest rock, drew its body 

 together just as a baboon does when its four feet are all together 

 on the summit of a little rock." His remark as to the greyish- 

 brown colour of the animal is the more valuable, as I believe 

 this gentleman, Mr. Wrench, A.R.M. of Cradock, is quite 

 unprejudiced. In my own letter to him, which drew forth these 

 remarks, I had only asked him for the distance at which he saw 

 the zebra, and I did not ask him how it was that he mistook a 

 black and white zebra for a brown baboon on a perfectly clear 

 South African day. My own observations also confirm that the 

 stripes of the zebra are of protective value. Riding along a 

 slope I suddenly saw four zebras within lOO yards above me. 

 They were galloping down the hill, but stopped when they caught 

 sight of me. As soon as they stopped I saw their stripes pretty 

 distinctly. After I had fired and wounded one of them, they 

 started again galloping down the hill round me in a semicircle 

 at a distance of about 70 yards. All this time they presented a 

 dull brown appearance, no stripes being visible, although I had 

 my attention fixed on this point. They disappeared beyond a 

 ridge, went down a little valley, and I heard afterwards that 

 they ascended the next slope, which was not more than 1500 

 yards away from where I stood with a native servant. Yet even 

 this lynx-eyed native could not see them going up this slope. 

 They had vanished from us. 



Perhaps it may interest some of your readers that zebras are 

 still fairly plentiful on the rugged hills west of Cradock. A 

 troop of forty-one individuals was seen, on the very ground over 

 which we hunted, a short time before we arrived. Our party 

 saw eleven in two days, but I believe three were seen on two 

 if not on three different occasions. This would reduce their 

 number to eight, if not to five. They are protected by Govern- 

 ment, and also by the farmers themselves, but I am afraid that 

 in spite of that their days are numbered. They are said to be 

 very destructive to wire fences, and as the inclosing of farms with 

 wire fences is steadily on the increase in this colony, many a 

 farmer will have, though perhaps reluctantly and in defiance of the 

 law, to take up his gun and clear them off his property. There 

 will then probably be an outcry by people who know the 

 difficulties of South African farming only from books written by 

 travellers who hurry through South Africa in a first-class railway 

 carriage j but those who really know South Africa well will say 

 it is a great, great pity, but it cannot be helped, unless Govern- 

 ment provides speedily an abode for these and other animals 

 threatened with extinction. The first step in the right direction 

 would perhaps be the establishment of a Government Zoological 

 Garden, but I hope others who are more competent than I am 

 will stir the people of Cape Colony up before it is too late, so 

 that something more than mere game-laws may be done to 

 preserve them. S. ScHoNLAND. 



Albany Museum, Grahamstown. 



The Protective Device of an Annelid. 



In September last I forwarded to Nature the description of 

 an effectual protective device adopted by a small tubicolous 

 Annelid which had been sent to me from Jersey ; the device 

 consisting in the coiling-up of the end of the tube. I have 

 recently been able to submit specimens to Prof. VV. C. Mcintosh, 

 of St. Andrews, who has kindly identified the builder as Sabella 

 saxicava, a form which he tells me is common in the Channel 

 Islands, and occurs also on our southern coast. So far as I 

 can learn, this peculiar and interesting habit of an Annelid had 

 not previously been observed. Arnold T. Watson. 



Sheffield, May 1892. 



NO. II 75, VOL. 46] 



The General Circulation of the Atmosphere. 



In that excellent lecture by Dr. Pernter, delivered before the 

 Scientific Club at Vienna, published by you in Nature (vol. 

 xlv. p. 593), the theory of the trade winds being occasioned by 

 the rising of the rarefied air at the equator causing an upward 

 current, while cold air from north and south flows in to supply 

 its place, coupled with the earth's rotation to the east, i> 

 attributed to Dr. Dove. " Dove was the first person . . . ' 

 But that theory will be found distinctly enunciated by Sir John 

 Herschel in his "Treatise on Astronomy" (1833), where he 

 attributes it to Captain Basil Hall, "where this is distinctly, and, 

 as far as I am aware, for the first time reasoned out." Herschel 

 was not aware that it had been distinctly reasoned out by Geoi^c 

 Hadley, F.R.S., in the thirty-ninth volume of the Philo 

 sophical Transactions, a century before Basil Hall. 



J. Carrick Moore. 



rHE SURFACE-FILM OF WATER, AND ITS 

 RELATION TO THE LIFE OF PLANTS AND 

 ANIMALS> 



IT is necessary to the exposition of my subject that } 

 should begin by reminding you of some well-known 

 properties of the surface of water. These are familiar 

 to every student of physics, and are set forth in rnany 

 elementary books. They are well explained and illus- 

 trated, for instance, in Prof. Boys's deservedly popular 

 book on " Soap-bubbles." But there may be some per- 

 sons here who have not quite recently given their thoughts 

 to this subject, and it will only cost us a few minutes to 

 repeat a few simple experiments, which will establish 

 some fundamental facts relating to the surface-film of 

 water. 



The following experiments were then shown :— 

 (i) Mensbrugghe's float. Proves that the surface-film 

 of water offers resistance to the passage of a solid body 

 from beneath. 



(2) Aluminium wire made to float on water. Proves 

 that the surface-film of water offers resistance to the 

 passage of a solid body from above. The resistance is 

 proportional to the length of the line of contact of the 

 solid with the water. 



(3) Copper gauze made to float on water. Here, a 

 number of intersecting wires are employed instead of a 

 single wire, and the consequent increase in the length of 

 the line of contact greatly increases the weight which can 

 be supported. 



(4) Frame with vertical threads, carrying a light plate 

 of brass. The threads hang vertically at first, but when 

 the whole is dipped into soapy water, the adhering film 

 exerts a pull upon the sides of the frame, draws the 

 threads into regular curves, and raises the brass plate. 

 When the film is broken, the threads resume their pre- 

 vious vertical position, and the plate falls. 



(5) Aluminium wire supported by vertical copper wires. 

 Each end of the aluminium wire forms a loop, which fits 

 loosely to one of the copper wires. When the apparatus 

 is dipped into soapy water, the contraction of the film 

 draws the aluminium wire upwards. After puUing it 

 down with a thread, the wire can be again drawn up. 

 This is another illustration of the tendency of the film 

 to contract. We use soapy water, because the film lasts 

 for a considerable time, but the surface-film of pure 

 water, though less viscous than that of soapy water, is 

 even more contractile. We have already seen that the 

 surface-film clings with considerable tenacity to any 

 solid body introduced into it, and that its hold increases 

 with the length of the line of contact. It is for this 

 reason that fine meshes offer so great a resistance to the 

 passage of the surface-film. Air can pass through the 

 meshes with perfect ease ; water also, if not at the sur- 

 face, can pass through readily enough, but the surface- 

 film in contact with air will only pass through with 



' Lecture given at the Royal Institution, March 4, 1892, by L. C. Miall, 

 Professor of Biology at the Yorkshire College, Leeds. Some passages were 

 omitted in delivery, for want o( time. 



