April io, 19 19] 



NATURE 



ii5 



of the August Perseids. It is rather an event with 

 ^lossibilities which cannot be definitely predicted be- 

 ciuse it is affected by irregularities not fully understood. 

 I sually it must be confessed that the shower pro- 

 \ides few meteors and disappointment. However, 

 meteoric astronomers anticipate its brilliant revival at 

 anv time, and watch the spring skies with a keenness 

 which merits success. 



The meteors are due on the night of April 21, when 

 the moon will be at her last quarter, and does not rise 

 until nearly an hour after midnight. But it will be 

 advisable to watch on the preceding night also, and 

 the hours after midnight are likely to be the most 

 productive, the radiant point at 271° + 33° being at a 

 much greater altitude than in the evening hours. The 

 really active stage of the shower is limited to a few 

 hours, but the whole duration is much longer, and 

 certainly extends from April 18, when radiation is 

 from 266° + 33°, to April 26, when it has advanced 

 to 278°+33°. 



Unification of the Astronomical and Civil Day. 



The Lords Commissioners of the Admiralty have 

 given instructions to the Superintendent of H.M. 

 Nautical Almanac Office that in the Almanac for 1925 

 the day shall be considered as beginning at midnight, 

 to make the astronomical agree with the civil day. 

 This change has been resolved on after consultation 

 \vith the Royal Astronomical Society, which issued a 

 circular to the superintendents of the ephemerides of 

 other nations and to the representatives of other bodies 

 asking for opinions and suggestions. It appears that 

 the change is to be made chiefly in the interests of 

 seamen, who will find it more convenient to have the 

 same time system in use for purposes of navigation 

 and for ordinary life on board ship. It mav be remem- 

 bered that a vigorous attempt to secure this unification 

 of the civil and astronomical day was made about the 

 year 1885. 



The Evolution of Binary Systems.— Mr. J. H. 

 Jeans, in the Monthly Notices of the Royal Astro- 

 nomical Society for December, 19 18, examines some 

 of the problems of double-star orbits. While in the 

 solar system the angular momentum is too small for 

 the system to have broken up through rotation, in 

 the majority of binary systems it is too large for 

 this to have happened. Tidal action cannot increase 

 the latus rectum by more than some 60 per cent, in 

 the case of equal masses (Russell). Large alterations 

 of latus rectum, and hence of period, cannot, there- 

 fore, arise from the mutual action of the stars. 

 Either the periods have retained approximately their 

 present values throughout the star's career (this hypo- 

 thesis is rejected), or there must have been sensible 

 disturbances from other stars. This leads Mr. Jeans 

 to the interesting conclusion that the stellar system 

 was initially of about i/iooo of its present volume. 

 He suggests that the outward movement may still 

 be in progress, and notes the observed excess of 

 positive radial velocities as evidence of this. In its 

 earlier compressed condition mutual encounters of 

 stars would have been frequent. Incidentally, he 

 finds 0637 as a mean value of eccentricity of orbits 

 as produced by encounters. This accords well with 

 observed facts. 



It is advisable to direct attention to one sentence 

 of the summary. Mr. Jeans says : — " The dwarf 

 M stars have velocities which show no preference for 

 particular directions in space, and there seems to he 

 no correlation between the magnitude of their veloci- 

 ties and the parts of the universe they occupy." But. 

 in fact, we are acquainted only with those dwarf 

 M stars that are in close proximity to the sun ; for 

 such stars are intrinsically so faint that thev do not 

 appear in our catalogues at all if they are distant. 

 NO. 2580, VOL. 103] 



AERIAL PHOTOGRAPHY. 

 PHOTOGRAPHY from the air reached a wonder- 

 ■»■ ful . degree of excellence during the war, as is 

 demonstrated by the pictures that have been published 

 and shown at various exhibitions ; but for obvious 

 reasons the instruments used for this work have onlv 

 quite recently been made public. The experts who have 

 compared the various lenses suitable assure us that 

 those made by English opticians were found to be not 

 only equal to those of Zeiss and Goerz, but markedly 

 superior to them. With regard to cameras, the editor 

 of the British Journal of Photography has had an 

 opportunity of seeing the whole range of cameras 

 used by the Royal Air Force, and describes them in 

 qn article in his journal of March 21. Within a few 

 months of the beginning of the war the value of 

 aerial photographs began to be recognised, and 

 specially made cameras were first used early in 1915. 

 The first camera was of a very primitive type, and 

 fitted with a Mackenzie-Wishart adapter for 5x4 

 plates. Early in 1916 a magazine-changing arrange- 

 ment was used with the plates in metal sheaths, the 

 foremost — that is, the lowest — plate being pushed 

 sideways after exposure into the receiver by a hori- 

 zontally moving metal plate. So far the cameras were 

 of wood, but in 1917 a metal camera was introduced, 

 and the changing done by pulling a cord instead of 

 pushing a metal plate. 



The next improvement (early in 1917) was to pro- 

 vide a mechanical method of changing, the motive 

 power being produced by a small propeller, which 

 was brought into action by simply releasing a Bowden 

 lever, the shutter being automatically actuated at the 

 same time and by the same means. In 1918 this 

 camera was further improved in several ways. The 

 shutter was made replaceable by another, if necessary, 

 as on account of derangement, and lenses of focal 

 lengths from 4 in. to 20 in. might be used on the 

 same camera. Among other patterns was one, first 

 used in 1916, which would take a continuous series of 

 photographs, up to 120, on a roll of film. The ex- 

 posures were made automaticallv at intervals corre- 

 sponding with a certain number of revolutions of the 

 propeller, and by means of a small supplementary 

 lens each negative had recorded on it the height of 

 the machine and its compass bearings. Major C. W. 

 Gamble, of the R.A.F., in a lecture before the Optical 

 Society on March 13, after describing the various 

 cameras used, said that, although the most rapid 

 plates were desirable so that exposures might be 

 made late in the day and when the light was poor, 

 it was found that the density-giving capacity of the 

 plate was of at least equal importance. As time pro- 

 gressed the tendency was to use panchromatic rather 

 than orthochromatic plates, and, finally, three-fourths 

 or more of the plates used were panchromatic, a suit- 

 able light-filter being employed. 



NEW KNOWLEDGE OF A PUZZLING 



GROUP OF GYMNOSPERMS. 

 'X'HE abundance of large fronds in Rhaetic, Jurassic, 



^ and Wealden rocks, closely resembling in habit 

 those of some recent Cycads, and the occurrence of 

 hundreds of petrified trunks in Jurassic and Neo- 

 comian strata in North America and, in smaller 

 numbers, in many other parts of the world, have led 

 palaeobotanists to speak of these periods as the "age 

 of Cycads." It is, however, a remarkable fact that 

 the reproductive shoots of these Cycad-like plants 

 differ very widely from the corresponding organs in 

 the true Cycads; had we possessed no knowledge of 

 the vegetative organs, the reproductive shoots would 



