l62 



NATURE 



[October io, 19 12 



air, V the net wind-pressure on the wing-plane, G 

 (lie pull of gravity, the bird's weight, and F the 

 frictional resistance ' encountered by the bird in 

 moving through the air, we have indicated a solution 

 of the problem of support and of propulsion, propulsion 

 in a direction opposite to that of the horizontal com- 

 ponent of the wind's velocity. 



As the trail of smoke marks the direction of the 

 • wind with respect to the moving ship, the bird must, 

 in order to sail with the same velocity and direction 

 as the ship, have a motion relative to the air equal and 

 opposite to the motion of the smoke relative to the 

 ship. Accordingly, the bird's axis is kept parallel to, 

 and opposite to, the course of the smoke as indicated 

 by its trail from the funnel. 



With a head-wind, though there must be an up- 

 current near the bow, this current must be much more 

 narrowly local, and therefore less advantageous for 

 the bird's use in sailing, than the up-current produced 

 by a wind abeam. 



It would appear that the gulls when sailing are not 

 directly seeking food, but are merely resting, loafing 

 for the time being, with ears intent, however, for any 

 indication of a find by their more industrious brothers 

 astern. 



Having shown this discussion to a number of well- 

 known physicists aboard the Arabic, I find no oppo- 

 sition to the theory proposed for the facts as noted, 

 but a very general doubt as to whether gulls do not 

 sometimes sail on the leeward side of a ship or too 

 far astern to get the benefit of the upward air current 

 from the windward side. When I ask seafaring men 

 about the matter, they at first say confidently that the 

 birds sail on either side of the ship, but after a little 

 consideration they waver in this statement and admit 

 that they have never taken particular note of the facts 

 in the case. Accordingly I have thought it worth 

 while to write this letter, in the hope that its publica- 

 tion in N.ATURE, if room be found for it, mav stimulate 

 further observation of an interesting phenomenon and 

 perhaps prevent some inventors from wasting their 

 time and monev in vain attempts to accomplish motor- 

 less flight. 



The westward course from Queenstown may be an 

 especially favourable place for studying the question 

 here raised. Four years ago I made there some 

 observations which, so far as I can now recall them, 

 agreed in essential particulars with those which I 

 have made recently, but I did not at that time see the 

 full significance of the facts noted. 



Edwin H. Hall. 



Cambridge, Mass., September 20. 



] given would be several seconds too late. And 

 as a means of estimating the true times I 

 gave the intervals from the instant when the 

 cusps subtended an angle of specified value at the 

 sun's centre to the commencement of totality. The 

 shift of the moon's actual position relatively to its 

 tabular place would, of course, alter the predicted 

 duration of the eclipse. At the time of publication 

 of my paper (November, 1908), it was not possible to 

 give definite information as to the magnitude of the 

 errors in question, as Newcomb's latest corrections 

 were not then available. But, so far as appears from 

 these reports, the intending observers did not make 

 any further inquiries on the subject before proceeding 

 to draw up their definite programmes of observation. 

 In the circumstances this, surely, would have been a 

 wise precaution. A. M. W. Downing. 



September 27. 



Errors of the Computed Times of Solar Eclipse 

 Phenomena. 



The final reports of two of the observers of the 

 total solar eclipse of April 28, 191 1, have recently been 

 published, giving detailed accounts of their experiences 

 at Vavau, Tonga Islands. Father Cortie's report 

 appears in Proceedings, R.S., No. .\^^>S'- that of Dr. 

 \\'. J. S. Lockyer in a publication of the Solar Physics 

 Committee. In reading these reports I have "been 

 much struck by the circumstance that both 

 writers appear to have been taken aback 

 by the (to them) unexpected effect of the 

 errors of the lunar tables on the computed times of 

 the various phases of the eclipse. But I wish to noint 

 out that they were not left without warning. In 

 Monthlv Notices, R.A.S., vol. Ixix., p. 31, I stated 

 that with the existing errors of the lunar tables the 

 times of the contacts in this eclipse as there 



1 Perhaps F slioulH be taken in the direction of D, but iincenainty here 

 does not affect the main argument. 



With regard to Dr. Downing 's remarks, I was 

 aware from his published statement that the predicted 

 time of totality would probably be "several seconds 

 too late," but little thought that the error would 

 amount to so mucli as twenty seconds of time. The 

 precaution was naturally taken to observe closely the 

 diminishing cusp and to arrange to give the necessary 

 time signals from the cusp data mentioned by Dr. 

 Downing in the above letter. Unfortunately, how- 

 ever, while the cusp was visible nearly up to second 

 contact, the image of it on the screen had been oscil- 

 lating so violently (due to air tremors) that the officer 

 in charge had previously decided to give the necessary 

 signals at the computed times from the chrono- 

 meter. (See page 17 of my report for details of the 

 actual procedure.) It is true that no inquiry was made 

 by me to find out what error might be expected, but 

 it was assumed that if sucli a large error had been 

 approximatelv known .it would have been pub- 

 lished. It would be advantageous if the present direc- 

 tor of the Nautical Almanac could find his way to 

 make generally known the appro.ximate values of such 

 errors a sliort time previous to the setting out of 

 eclipse expeditions. 



The facts that Father Cortie's camp was in the 

 same clearing as that of my party and that he made 

 use of my time signals explain why he experienced the 

 same error. William J. S. Lockyer. 



Solar Physics Observatory, October i. 



NO. 2241, VOL. go] 



A Flower-sanctuary. 



It seems to me that Sir Edward Fry takes a rather 

 narrow view of the by-law under discussion ; for 

 surely it mav be held that the removal of any of 

 the special Cheddar plants, in such quantities as to 

 leave a blank where there was formerly a mass of 

 colour, would constitute a "disfigurement" or 

 "damage" within the meaning of the enactment: 

 and I suggest that any bench oT magistrates anxious 

 to preserve the beauty of the gorge should find no 

 difliculty in convicting under the by-law. If, how- 

 ever, it be held that the existing by-law is inadequate, 

 it seems clear that county councils have full power 

 to enact far more stringent and specialised by-laws. 



I remember that on the commons under the juris- 

 diction of the London County Council there used to 

 be — and doubtless still are — notices forbidding anyone, 

 under a penalty of five pounds, to pluck even a single 

 blossom of any wild flower; and I think that Sir 

 Edward Fry will find on St. Vincent's Rocks, at 

 Clifton, notices announcing a similar penalty against 

 anyone who shall gather the rare Arahis stricta : at 

 any rate, there were such notices a few years ago, 



