yune 25, 1874 



NA TURE 



H7 



follows as to the tribes of the vast region of the Amazons. 

 " There are as yet no grounds for considering that the present 

 barbaric condition in these districts is secondary, that any other 

 higher social condition had ever here preceded it, that this 

 swarming-ground of ephemera! unsubstantial hordes had ever 

 been the theatre of a cultured nation." * It is to be noticed, how- 

 ever, that this passage does not seem necessarily to involve a 

 recantation by Dr. Martius of his former opinion. lie leaves it 

 quite open that the tribes of the Amazons, though they did not 

 degenerate in this region from civilised ancestors, miglit have 

 done so elsewhere, and then migrated as savages into the forest 

 regions where as savages they remain. The context may on the 

 whole favour this vieu- of his meaning. Now this matter quite 

 deserves further looking into. It would be well worth wliile if 

 Dr. Peschel, from personal or published sources available to 

 him, would settle once for all the question whether the great 

 Bavarian ethnologist continued through life the degeneration ist 

 that we in England suppose him to have been. Some twenty 

 years ago. Dr. Prichard ("Natural History of Man," 1S43, 

 p. 497), citing Martius as to this very matter of the supposed fall 

 of the .South American tribes from an original higher state, re- 

 marked that " had he taken a more extensive survey of the 

 nations of the \\'hole continent, his opinion might have been 

 somewhat modified." As Dr. Martius did take the more exten- 

 sive survey thus recommended, it would be particularly curious 

 to ascertain whetlier it did have the effect thus foretold on his 

 mind. Edward B. Tylor 



Flight of Birds 



Allow me to return thanks to such of your correspondents as 

 have been kind enough to notice the query (vol. viii. p. 86) on 

 this subject which I made through your columns. 



As the matter seems to have excited some little interest perhaps 

 you will permit me to state in what respect the solutions pro- 

 posed appear satisfactory. 



That an " upward start " of wind ot sufficient velocity would 

 support a bird of given weight and surface of resistance is no 

 doubt the case. As in still air a bird, by holding its wings in a 

 plane slightly inclined to the horizontal, will glide with a velo- 

 city which ultimately becomes uniform, in a straight line obliquely 

 downwards, so the satne bird in the same position, but in a cur- 

 rent slanting upwards in a like direction and with a like velocity, 

 must remain at rest. Nevertheless there are difficulties in the 

 way of thus explaining the phenomenon. 



(i) It supposes the existence of .air-currents of greater rapidity 

 and at a greater angle of elevation than are likely often to be 

 met with. Taking the number of square feet in the whole resist- 

 ing surface of the bird to be equal to tlie number of pounds in 

 its weight, then a zvi/ical current of 15 miles per hour would be 

 required to support a bird Avith its tail and wings fully unfurled 

 but motionless, and a current of 30 miles per hour would be 

 recpiired if the current ascended at an angle of 30" with the 

 horizon. Now wind directed upwards by encountering the side 

 of a mountain is not likely to be inclined at a greater angle than 

 this, which is the average slope of a very steep mountam side, 

 and moreover the phenomenon of hovering without wing motion 

 may be observed where such rapid currents have no existence. 



(2) The phenomenon is sometimes observed where it is almost 

 impossible to suppose the existence of any upward air-currents 

 whatever. The first time it attracted my attention was in the 

 neighbourhood of London, towards Finchley Common, where it 

 will, I think, be admitted that there is nothing in the natural 

 configuration of the ground to determine an upward current of 

 sufficient velocity to produce the rcf|uired effect. The wind at 

 the time was certainly not boisterous, but as the bird was at a 

 considerable elevation there is still room to imagine that the 

 upper currents in which it was situated might be different from 

 those below. I was informed at the time that the bird in this 

 case was a kite ; this may have been an error, as I understand that 

 kites are now rarely seen near London. However this may be 

 I should gladly hear from such of your correspondents as have 

 the opportunity of watching the motions of the kite as to whether 

 the position of motionless hovering, which I believe this bird 

 continually assumes, can be explained always by the existence of 

 u])ward currents. I do not of course deny but what birds, while 

 hovering, avail themselves of upward currents where they can. 

 If the position is the result of considerable though imperceptible 



' vol. i. p. 375. The 

 = volume, pp. 5, 83. 



• Martius, " Beitrage zur Ethnographic Amcrik 

 other passages here reierred to will be found in the s 



muscular action they would naturally seek to economise their 

 strength as far as possible by availing themselves of whatever 

 support they could get from upward wind currents. 



As your correspondent, 1. Herschel, implies, it is difficult to 

 dissociate the hovering and the soaring of birds. That birds 

 soar, that is, that they continue suspended in the air for long 

 periods of time together, in rapid motion, with no further appa- 

 rent movement of the wings ihan is necessary to guide them, 

 and this under circumstances w here it is obviously impossible for 

 them to avail themselves of upward air slants, cannot be denied. 

 Whoever has made the voyage to the Cape must have observed 

 this in the case of the albatross. This bird appears to rise from 

 the sea with great difficulty and with the expenditure of much 

 wing power ; but, being once fairly launched in the air, its flight 

 becomes a most inexplicable phenomenon. In the open ocean, 

 during a steady wind, it soars for hours about a ship going at the 

 rate of six or eight knots an hour, without apparent difficulty, 

 and with no further wing motion than seems necessary to guide 

 it, now skimming the water in the wake ol the ship, nosv sweep- 

 ing round to the side or in front, rising and falling by what has 

 been well described as an apparent act of volition, and with no 

 perceptible loss of velocity. Now I think it must be admitted 

 that the motionless hovering and the soaring of birds are pheno- 

 mena closely allied to each other, that no explanation of the one 

 is satisfactory which does not explain the other also, and that, as 

 the theory of upward slants cannot possibly explain the soaring 

 of birds, it cannot be accepted as a satisfactory explanation of 

 their hovering. 



Besides the " upward air slant " theory, a correspondent of 

 one of your contemporaries refers me to the Duke of Argyll's 

 "Reign of Law" under the supposition that the matter is fully 

 explained in the third chapter of that work. I only refer to this 

 to point out the curious example it furnishes of fallacious reason- 

 ing. The author obviously thinks that, by a proper arrangement 

 of its wings and tail and the position of its body, a bird can 

 without muscular exertion remain suspended in a horizontal sdr- 

 cmxtxX, prmnded the latter be of sufficient velocity (see p. 170). 

 This of course requires no refutation ; but the whole of the 

 chapter in which it occurs may be read with interest as illustrat- 

 ing the curious mistakes a clever and earnest amateur will fall 

 into in writing on even the most elementary scientific subjects in 

 which he has had no exact training. F. Guthrie 



GraalT Reinet College, Cape Colony 



An Optical Delusion 



The following is an optical delusion which is none the less 

 interesting for being very easily explained. 



Let a person standing before a looking-glass look attentively 

 at the reflection of the pupil of one of his eyes, and then at th.at 

 of the other— let him look at different/;;/* of the eye, and from 

 one eye to the other, first at one and then at the other. Know- 

 ing that in thus changing the direction of his gaze his eyes must 

 move about in their sockets he will expect to see that they do so 

 in the glass. As a fact they '..oill appear perfectly still. 



If he looks at the eyes of another person trying the experiment, 

 the peculiar fixedness of his own will be still more striking, when 

 he looks at them again. 



I will not spoil the riddle by giving the answer at the end. 



J. H. 



Longevity of the Carp 



Can any of your readers give any well-ascertained proof of 

 the length of life attained by the carp ? When residing as a youth 

 at St. Germain, I was told by an aged Legitimist that his father 

 had watched the same carp throughout the whole ol his life, and 

 the son asserted that he had known the identical fish for twenty 

 and thirty years after his father's death, thus giving to them an age 

 of from sixty to seventy years. That remarkable statement is 

 more than substantiated by Lady Clementina Davies, who, 111 

 " Recollections of Society " (p. 49)> alludes to the longevity of 

 the carp in the moat of the Chateau de St. Germain, one bearing 

 in his gills a ticket proving him to be over 200 years of age ; 

 and others at Versailles, bearing silver rings through their gills 

 with the name of the courtier who had inserted it, and testifying 

 to an almost incredible longevity. What amount of truth may 

 we attribute to these statements ? 



Croydon, Suney, June 13 RoBT. Rodolph Suffield 



