February 22, 1889.] 



SCIENCE. 



151 



This seemed conclusive, but I naturally turned for further cor- 

 roboration to the passage in the Quarterly Review referred to. 



Judge of my surprise when I found, on the preceding page (p. 

 208), the following passage, which the Athenaum reviewer, had 

 evidently neglected to read when he declared that the person who 

 wrote the book was not known : — 



" Our first impression, on taking up the volume, was, that, as the 

 subject of the Arctic regions had become one of the fashionable 

 topics of the day . . . some hanger-on of Paternoster Row had 

 contrived, with the help of Egede, Fabricius, and the interminable 

 Cyclopaedia of Dr. Rees, to hash up a fictitious voyage to Davis's 

 Strait, in order to gratify the eager appetite of the public, and at 

 the same time to ' put money in his purse.' Recollecting, however, 

 that the log-book of the ship ' Thomas,' of Hull, in which this voy- 

 age is stated to have been made, was within our reach, we turned 

 to it, and found that Bernard O'Reilfy, Esq., was not, as we sus- 

 pected, a phantom conjured tcp for the occasion, but that there ac- 

 tually was a person of this name, in the capacity of surgeon, on 

 board that ship " [the Italics are mine]. 



The process of " evisceration " referred to by the AthencBiun re- 

 viewer then begins with great ferocity — too great, it seems to me, 

 even for such a ridiculous book as it is. This, however, only 

 proves, what is easily seen from reading the book, that it is quite 

 worthless from a scientific point of view, and evidently the work of 

 what we should call a " crank " nowadays, not that it is not the 

 work of the man whose name appears on the titlepage. 



If the Athenceum reviewer has no further evidence to submit in 

 regard to the authorship of the book, I do not see how we can 

 doubt that the book is genuine, even though it is not authentic 

 (which is quite another matter), and that it was perfectly proper to 

 insert the title in a bibliography of such extended scope as Mr. Fil- 

 ling's work. In fact, it would have been a mistake for Mr. Pilling 

 to omit the book from his list, in spite of all its glaring absurdities. 



John Murdoch. 



Smithsonian Institution, Feb. i8. 



The Soaring of Birds. 

 Owing to the remoteness of my present situation, I have but 

 just seen the query of Mr. Kent. The point which he makes is agocd 

 one. I see also that I made a slight misstatement in my previous 

 article. In the first place, as before remarked, we have the force 

 AB due to the weight of the bird, and the force AD due to the 



excess of velocity of the wind over the velocity of the bird. These 

 two forces may be combined into the resultant AC. This result- 

 ant is resisted by the force CA. due to the resistance of the air act- 

 ing on the wings of the bird as he wheels in circles about the point 

 A. The force CA is therefore proportional to a function of the 

 velocity with which the bird moves in describing these circles. The 

 greater the velocity of the bird, the greater the force. Now, this 

 velocity which he is capable of attaining with regard to the wind is 

 dependent solely on the absolute velocity of the wind with regard 

 to the earth, and can never be more than twice as great ; i.e., when 

 he is moving with the velocity of the wind and in the opposite 

 direction. 



Now let the velocity of the wind be x, and the excess of its velo- 

 city over the mean linear velocity of the bird (with respect to the 

 earth) in the same direction as the wind be a, then the mean linear 

 velocity of the bird with regard to the earth will be x—a. We will 

 suppose. that the velocity of the wind is such that CA=AC: the 

 bird will therefore continue to revolve about the point ^, which will 

 consequently be its mean position. It must, of course, be remem- 

 bered, that, while these forces are in equilibrium, the bird is slowly 

 drifting over the earth's surface in the same direction as the wind. 

 Its mean position would therefore describe a horizontal line with 

 respect to the earth. 



Now suppose the velocity of the wind (.r) to increase, while its 

 excess over that of the bird (a) remains the same : AC will there- 

 fore remain constant. But the velocity of the bird with regard to 

 the earth (x — a), and also his absolute velocity with respect to the 

 surrounding air, have increased, and therefore CA has increased 

 also. Accordingly, the bird will be carried above and to the right 

 of the point A. In the mean time the bird is drifting rapidly to- 

 wards the left, in the direction of the wind : he will therefore 

 describe a path lying in the same general direction as the line AG, 

 Q. E. D. W. H. Pickering. 



Los Angeles, Cal., Feb. ii. 



A RECENT number of Science (xii. p. 267) gives an account of a 

 paper by Mr. G. K. Gilbert, containing a theory of the soaring of 

 birds, which traces this phenomenon to the advantage gained by 

 the bird in gliding to and fro between contiguous horizontal layers 

 of a horizontal wind, moving at different rates. The theory pre- 

 sented is said to have been anticipated by Lord Rayleigh {Nature, 

 xxvii. p. 534); but it seems to me to rest upon what is, apparently 

 at any rate, quite a different assumption from that which Lord 

 Rayleigh made. 



Mr. Gilbert imagines a bird gliding to windward in the lower of 

 two contiguous layers, and traces the changes which his relative 

 velocity will undergo if he first pass into the upper layer, then turn 

 in it, then move to leeward, passing into the lower layer, and finally 

 complete a cycle by turning to windward. He concludes, that, after 

 the completion of the cycle, the bird's velocity will have increased 

 by twice the velocity of the upper layer relative to the lower, fric- 

 tional resistance being left out of account. This result he obtains 

 by assuming, that, after turning, the bird's velocity, relative to the 

 medium in which he turns, will be the same as before ; in other 

 words, that during the turn his velocity relative to the earth will 

 change by an amount equal to twice the velocity, relative to the 

 earth, of the medium in which the turn is made ; the change being 

 an increment in the turn to windward, and a decrement in the turn 

 to leeward. Of course, in accounting for the phenomenon of soar- 

 ing, some assumption must be made as to the bird's power of regu- 

 lating the magnitude and direction of the force exerted upon him 

 by the wind. But it should be a reasonable one, and, if not evi- 

 dently so, should be justified. Mr. Gilbert's assumption does not 

 seem evidently reasonable, and yet he does not even refer to its hav- 

 ing been made. 



In another recent number of Science (xiii. p. 31), Professor Pick- 

 ering has shown that in a uniform horizontal wind the phenomenon 

 of soaring is quite consistent with the law of the conservation of 

 energy, provided frictional resistance is not too great, but he does 

 not show how it may be accomplished. Lord Rayleigh, on the 

 other hand, has stated that a uniform horizontal wind certainly 

 cannot help us to explain this phenomenon. With so emphatic a 

 statement from so high an authority, one is fearful of rushing in 

 where angels fear to tread in attempting an explanation on this 

 hypothesis. Nevertheless I venture to submit to your readers the 

 following considerations, showing, I think, how soaring may occur 

 in a horizontal wind which has no differential motion. 



The force exerted by a horizontal wind on a bird may clearly be 

 inclined upwards ; for the wind, strikmg the lower surface of the 

 wing, is deflected downwards, and must therefore have been acted 

 upon by the wing with a downward force. The wind must there- 

 fore have exerted on the wing an upward force. What the exact 

 direction and magnitude of this upward force will be, will depend 

 upon the velocity of the wind relative to the bird, the wing area, 

 and the ingenuity of the bird in adjusting its wings. With a strong 

 wind'and a wing'area large relatively to the mass of the bird, it 



