September ii, 1890] 



NATURE 



463 



were generally as fragmentary and one-sided as he 

 thinks." 



Without attempting to add to words so weighty, we 

 shall follow the excellent example of him who had to 

 speak after Mr. Burke, and shall simply "say ditto "to 

 Prof. Marshall. 



It is not be supposed that our remarks are calculated 

 to disparage the truth and importance of Prof. Bohm- 

 Bawerk's own theory of interest. It would argue a very 

 slight acquaintance with economical literature to suppose 

 that the worth of an author's own work is to be measured 

 by the worth of his criticisms upon the work of others. 

 Prof. Bohm-Bawerk is not the less right because those 

 from whom he differs are not wrong. We are not pre- 

 cluded from expressing unqualified admiration o{\.\i& posi- 

 tive theory which forms the sequel to this too " critical'' 

 history. F. Y. E. 



The Affinities oi Heliopora caruUa. 

 Owing to absence from Dublin I have but just seen the cor- 

 respondence on the affinities of Heliopora ccerulca in two recent 

 numbers of Nature (pp. 349, 370). Dr. S. J. Hickson has 

 undoubtedly given the true explanation of Mr. Saville Kent's 

 "curious mistake." Knowing that the polyps of this Alcyo- 

 narian had never before been observed in expanded condition, 

 I took every opportunity when in Torres Straits of examining 

 living specimens in situ. Only on one occasion was I rewarded, 

 ! and then I distinctly saw the small extruded polyps with their 

 ! eight flat fringed tentacles ; they were nearly colourless, but 

 I had a whitish tinge, in fact, they precisely resembled the 

 ' polyps of the common Alcyonium digitattim. I was unable to 

 sketch them at the time, as my bodily position was unfavour- 

 I able, and the tide was rising, In no case did the polyps exhibit 

 any sign of vitality when kept in a vessel of water in my 

 laboratory. The only published drawing (so far as I remember) 

 I purporting to be the polyp of this form is that given by Quoy 

 ! and Gaimard, but whatever it may be, it represents neither the 

 I polyp itself nor the annelid described by Mr. Saville Kent. 



Alfred B. Haddon. 

 Royal College of Science, Dublin. 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part op Nature. 

 No notice is taken of anonymous communications.\ 



The Soaring of Birds. 



In Nature of August 21 (p. 397), a new solution has been 

 oftered of the problem of the soaring of birds, which will hardly 

 be accepted as satisfactory ; for it is based on the radical mis- 

 conception that the absolute velocity with which a bird soars, or 

 flies, partly with the air, and partly through the air, can be 

 converted, wholly or in part, into a motion through the air, at a 

 swifter rate than before. This is only possible when the bird 

 passes from one current into another, or the current itself is 

 changed in rate or direction. It is strange that anyone 

 should fail to see that when a body of air is moving uniformly 

 over the land or sea, it can no more sustain a bird flying or 

 gliding within it than if it were motionless. 



Referring to your correspondent's first diagram, "It is 

 evident," he says, " that the velocity of the bird at the point c, 

 if the initial velocity of the bird and the velocity of the wind 

 are properly adapted" (whatever that may mean) "can be 

 greater than at a. He appears to think that the length of the line 

 a c'\% some measure of the velocity ; whereas, in fact, this velocity 

 will be the same, or rather greater, indefinitely near to a than it 

 is at^; the velocity of the bird through the air being then at its 

 maximum, and that of the wind uniform. Nothing is gained by 

 the length oi ca. A movement in the direction c d might be at 

 least as advantageously made close to « as further along the line. 



It is hardly worth while to press any other objection to the 

 explanation offered. But it may be noted that if the diagrams 

 given are intended to represent the actual facts the soaring bird 

 is being rapidly carried to leeward, which is a misrepresenta- 

 tion. I>et me offer what I believe to be an apt illustration. 

 While a billiard ball is rolling across a table, its direction being 

 again and again changed by its rebounds from the cushions, the 

 table is moved for\yard through space at the rate of about a 

 thousand miles a minute. This movement ought, according to 

 your correspondent, to accelerate or retard the motion of the 

 ball across the table, or at least ought to do so if the two move- 

 ments were ' ' properly adapted. " 



Reginald Courtenay. 



Tean Vicarage, Stoke-on-Trent, September i. 



[P.S.— In Nature for September 4 (p. 457), just received, are 

 remarks on the soaring of birds, by two of your correspondents, 

 which seem to me to be perfectly just ; only I would add that 

 the upward currents of which soaring birds avail themselves are 

 commonly due to obstacles to the uniform motion of the wind. 

 Also, that a soaring bird, if turning abruptly, will in general not 

 merely uplift, but strike once, with the outer wing.] 



Occurrence of a Crocodile on Cocos Islands. 



With reference to Mr. Ridley's account of the occurrence of 

 a crocodile on the Cocos Islands, I was quartered in Barbados 

 in the beginning of 1885, when a very fine alligator, over 15 

 feet in length, was washed on shore. As it was on the point of 

 crawling up the beach it was noticed by a sergeant of engineers 

 and some sappers who shot it, and afterwards exhibited it in the 

 town. 



The nearest river the alligator could have come from was the 

 Orinoco, a distance of 300 miles. 



This is improbable, as the set of the ocean currents would 

 have sent the alligator much to the west of Barbados if it had 

 come from the Orinoco. It is much more probable the alligator 

 came from the mouth of the Amazon or from the Essequibo, 

 many hundred miles further to the east. 



Dr. Mitchell, of Trinidad, told me he had seen an alligator 

 on a small log attacked by sharks in the Gulf of Paria. 



A. L. Caldwell. 



A.S. Corps, Chatham, September 6, 



THE BRITISH ASSOCIATION. 



Leeds, Wednesday Morning. 

 ''F*HE Leeds meeting has been small and quiet, the 

 •^ attendance only numbering over 1700. Happily the 

 weather has been excellent, and brilliant sunshine has 

 cast a glow over the ugliness of the place, and rendered 

 the excursions to the Aire and the Wharfe valleys de- 

 lightful. The usual social accompaniments of British 

 Association meetings have not been so plentiful as at 

 former meetings, but every one seems satisfied with the 

 hospitality displayed. 



In the proceedings of the meeting there have been one 

 or two matters of excitement and some marked successes. 

 Sir Frederick Abel considerately cut down an address 

 which was more suited for the study than the platform. 

 It is universally admitted that no more successful lectures 

 have ever been delivered at an Association meeting than 

 those of Mr. Poulton on Friday, on " Mimicry," and 

 Prof. Boys on Monday, on " Quartz Fibres." The large 

 audiences were really entranced. 



It can hardly be said that any paper of high and wide 

 scientific importance has been read this year in any of 

 the Sections. There have, however, been several most 

 important discussions on the reorganization of some of 

 the Sections, which attracted much attention, and several 

 changes are to be made. 



Next year's meeting of the Association will take place 

 at Cardiff, when Dr. Huggins will preside. The 1892 

 meeting will be held at Edinburgh, and the 1893 at 

 Nottingham. 



NO. 1089, VOL. 42] 



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