August 20, 1908J 



NATURE 



567 



arrive at a figure nearly as good as that of the matrix, of 



rather greater focal length, it may be, but nearly equally 

 iengthened in all its parts. RossE. 



Birr Castle, Parsonstown, August 14. 



The Form of a Dirigible Balloon. 



Although it is not likely that dirigible balloons will be 

 of much practical value e.Kcept for purposes of amusement 

 and to a limited extent for military observations, it seems 

 a pity that the designs of such as are made should not 

 embody existing knowledge as to the conditions which 

 o^overn the resistance and stability of submerged bodies. 



The resistance of a submerged body is a far less complex 

 quantity than the resistance of a ship (inasmuch as no 

 question of surface waves is involved), and can be stated 

 simply as the sum of the resistances due to surface fric- 

 tion and wake, both of which vary very nearly as the 

 square of the speed. 



The surface friction resistance cannot be reduced below 

 a certain limit, for the surface exposed cannot be less than 

 the surface of a sphere which will hold the required volume 

 of gas. 



The wake or eddy-making resistance can, however, be 

 very largely reduced by giving the balloon a shape gently 

 tapering towards the stern, a shape which is adopted by 

 all animals (birds, fish, whales, seals, &c.) the habits of 

 which require them to move rapidly as submerged bodies. 



The shape of the head of the balloon does not matter 

 much so long as the profile is a fair curve. The important 

 thing for reducing the wake is that from some position 

 in front of the maximum diameter of the balloon the radius 

 of curvature of the profile should increase, slowly at first, 

 but continuously, towards the stern. 



With regard to stability, it should be borne in mind that 



I flV£«M£S. 



A, Centre of gravity of balloon. 



B, Centre of buoyancy of balloon. 



a submerged body, whether a sphere or elongated surface 

 of revolution, is naturally unstable when moving in air or 

 other real fluid, and will not without guidance continue 

 to move in a straight line owing to the instability of the 

 motion of the wake. 



This form of instability may be got rid of by providing 

 small fins near the stern the planes of which contain, or 

 are parallel to planes passing through, the axis of figure 

 of the body. 



Their number must not be less than three, but there is 

 no objection to a greater number except on the ground of 

 extra surface friction. 



In the accoinpanying sketch a form is shown which 

 would be suitable for a dirigible balloon so far as resist- 

 ance and stability are concerned. 



If it is assumed that the total lifting power of such a 

 balloon is to be 2000 lb. (including its own weight), its 

 capacity must be something under 30,000 cubic feet, and 

 if the length is five times the greatest diameter, its 

 dimension will roughly be : — diameter, 30 feet ; length, 150 

 feet ; and superficial area, 7000 square feet. 



The surface-friction resistance at twenty miles an hour 

 will require less than 2 horse-power. I have no data for 

 the eddy-making resistance of such a form, but it would 

 probably be less than half that of the bolster-like shapes 

 generally adopted, which are hardly less resistful than a 

 flat end would be (this applies to the stern only, not to the 

 head). 



The framework necessary to give the balloon the shape 

 figured in the sketch would undoubtedly add to the dead 

 weight, but would more than " pay for its carriage " by 

 lessening the resistance, and thus allowing of the use of 



NO. 2025, VOL. 78] 



lighter engines, and I think that perhaps the most valuable 

 part of Count Zeppelin's work has been to show that it is 

 practicable to construct a balloon the shape of which is 

 moulded by internal frames. A. M.\llock. 



6 Cresswell Gardens, S.W., August 12. 



The "Sky-coloured Clouds" or Twilight Glows. 



There can, I think, be no doubt that the numerous 

 descriptions which have appeared of the recent twilight 

 phenomena relate to essentially the same phenomenon as 

 the " sky-coloured clouds," or, as the late O. Jesse called 

 them, the " luminous night-clouds." On June 30 and 

 July I, which seem to have been the principal dates, I 

 was unfavourably situated for observing the phenomenon, 

 but I may point out that I directed attention to it in your 

 issue of June 11, p. 127. 



The displays that have been noted in former years have 

 been marked by a very striking appearance as of cirrus 

 clouds, only (as O. Jesse ascertained) at a much greater 

 altitude, VIZ. fifty-one miles. From the descriptions this 

 year the cirrus-like appearance has evidently not been so 

 well marked, although it is mentioned by some observers, 

 and I noticed it myself (partly here and partly in Scotland) ; 

 but sometimes the aspect of the light was uniform, and not 

 striated like cirrus. In former years I have also some- 

 times observed the same when there was evidently a 

 tendency to the formation, and yet the clouds did not 

 appear, but only the luminosity. M. F^lix de Roy, in the 

 Gazette Astronomique d'Anvers, No. 8, p. 63, is of opinion 

 that this year the phenomenon was not the same as that 

 investigated by O. Jesse, but he calls it an " extraordinary 

 twiliaht." In either case, no doubt the cause is essentially 

 the same — i.e. reflection — some substance being at such a 

 height in the atmosphere that the sun 

 can shine upon it when far below the 

 horizon of the observer. The colora- 

 tion, although varying in intensity on 

 different occasions, is also the same in 

 either case, viz. red or red-orange 

 near the horizon, gradually chang- 

 ing through orange, yellow, and 

 green to blue above. This is the 

 ordinary coloration of a clear twi- 

 light sky, the difference between 

 which and the recent phenomenon 

 being mainly that of the height 

 of the substance reflecting the sun's 

 rays. 

 It has never been ascertained what the substance is, and 

 it may be that in different years^ it has not been the same. 

 This might account for the appearance this year being less 

 like cirrus than formerly. It has been suggested that it 

 may be meteoric dust. ' T. W. Backhouse. 



Sunderland, .•\ugust 11. 



August Meteors of 1908. 



The conditions this year were all against any bright 

 or plentiful exhibition 'of the Perseids. Apart from the 

 presence of the full moon, the sky at Bristol on August 11 

 and 12 was full of thin white clouds, amid which it was 

 only possible to distinguish bright meteors. On August 10,. 

 however, the firmament was clear, but the Perseid shower 

 was evidently influenced by the luminous atmosphere,^ for 

 only about twelve per hour were visible before midnight. 

 The radiant point was in the usual position at 43°-!- 57°. 

 A few brilliant meteors were seen from Perseus on 

 August II, but clouds prevailed to such an extent that it 

 was impossible to judge as to the strength of the display. 



Mr. J. H. Elgie, of Leeds, had a clearer sky, and 

 observed on August 10 and n a pretty numerous display 

 of bright Perseids. Watching for an hour at about mid- 

 night on August II, he saw twenty meteors. Reports 

 from other places state that although some brilliant meteors 

 were seen, they fell short in point of numbers with those 

 observed in previous years in better circumstances. At 

 Bristol the radiant point moved from 25°-|-53° on July 26 

 to 43° -1-57° o" August 10. Watching was resumed earty 

 on the verv clear nights of August 15 and 16, but meteors 

 were found to be scarce, and the Perseid shower gave little 

 sign of continued activity. W. F. Denning. 



