November lo, 1910] 



NATURE 



1 



mineralogy, mining, and metallurgy, including the library 

 of the late Prof. Hilary Bauerman, with a supplement of 

 sets of periodicals and publications of the learned societies. 



The Cambridge University Press has undertaken the 

 publication of a work entitled " Principia Mathematica," 

 bv Dr. A. N. Whitehead, F.R.S., and the Hon. B. 

 Russell, F.R.S. ; the aim of the work is to show the 

 dependence of mathematics upon logic by deducing from 

 purely logical premises the elementary propositions of 

 various branches of mathematics. The first volume, on 

 mathematical logic and prolegomena to cardinal arith- 

 metic, will be published very shortly. The second volume, 

 concerning the principles of arithmetic, is in the press. 

 In the third volume the authors have dealt with measure- 

 ment and the principles of geometry. 



We have received the first part of vol. xviii. of the 

 Journal of the Royal Institution of Cornwall. The pro- 

 ceedings at the annual and spring meetings of 1909 are 

 given at length. The annual excursion of 1909 is 

 described, and the address of the president. Dr. Richard 

 Pearce, at the spring meeting in 1909 is printed in exienso. 

 Among papers read at the meetings during 1909 may be 

 mentioned :— King Arthur's Hall on Bodmin Moor and 

 some Irish circles, by Mr. A. L. Lewis ; the fauna of St. 

 Ives Bay for 1908, by Mr. R. Vallentin ; and the inverte- 

 brate fauna of Cornwall — Hymenc^tera Entomophaga and 

 Hymenoptera Aculeata, by Mr. W. A. Rollaston. The 

 volume also contains meteorological tables for Cornwall 

 for 1909. 



OVR ASTRONOMICAL COLUMX. 



Fireball ox November 2. — A brilliant fireball was 

 observed on Wednesday, November 2, 7.46 p.m. It 

 passed from east to west over the English Channel, and 

 fell from heights of 84 to 26 miles. As seen from Corn- 

 wall and from the north of France, as well as from ships 

 in the Channel, the meteor was a splendid object, yield- 

 ing a brilliant light, as though the moon had broken out 

 from clouds. The stream of aerolites from which the 

 phenomenon was directed has its radiant point in .\ries, 

 and further observations are desirable. 



Rotation of the Moon.— A correspondent has been 

 puzzled by the perennial perplexity of non-mathematicians 

 as to how the moon can be said to rotate when she always 

 presents the same face to the earth. The answer, of 

 course, is that as we prove the rotation of the earth by 

 the fact that any meridian, such as that of Greenwich, 

 completes its circuit with respect to any fixed star in the 

 course of a sidereal day, so also the similar consideration 

 shows that the moon rotates on her axis in 275 days, 

 during which time she also completes her circuit about the 

 earth with respect to the stars. 



The moon's equator is not quite circular, since her 

 figure may be considered as possessing a solidified tidal 

 inequality of shape. Laplace examined the mechanical 

 results of this condition of affairs, and showed that the 

 moon would oscillate slightly about a mean position 

 relatively to the earth. This is called the physical libra- 

 t'lon of the moon, and in consequence of its existence we 

 see slightly more than half of the moon's surface. 



It is probable that the moon once rotated more rapidlv 

 on her axis, and that her rotation was reduced by tidal 

 friction to its present magnitude. The transition from a 

 slow rotation to a libration would present a problem of 

 consideiable mathematical difficulty. We can, however, 

 see what would be the several stages through which the 

 changes would pass. There would first be unequal speed 

 in the several parts of the rotation ; this inequality would 

 increase until at two moments in one rotation that rota- 

 tion would nearly cease ; then there would occur an actual 



NO. 2 141, VOL. 85] 



stoppage, and the direction of motion would reverse itself 

 for half a rotation, constituting a very large libration ; 

 finally, the amplitude of libration would diminish • ■ '- 

 actual insignificant magnitude. 



Ei'HEMERis FOR H ALLEY 's CoMET. — Dr. Ebell publishes 

 a continuation of his ephemeris for Halley's comet in 

 No. 4450 of the Astrononiische Nachrichleti. The 

 ephemeris covers, in four-day steps, the period November 5 

 to December 31, and shows that the comet is now travel- 

 ling in a south-westerly direction through Corvus ; its 

 magnitude is about 15-5. 



Selenium Photometer Measures of the Brightness of 

 H.ALi.Ev's Co-met. — Observing at the Illinois University 

 Observatory, Mr. Joel Stebbins measured the brightness of 

 Halley's comet with his selenium photometer on fifteen 

 occasions during May, and now publishes the results ir» 

 No. 2, vol. xxxii., of the Asfrophysical Journal. The 

 selenium cell was attached to the 12-inch refractor, and, 

 through a diaphragm, light from a circle 7 minutes of 

 arc in diameter was admitted to it ; Mr. Stebbins suggests 

 that eye-estimates of the comet's brightness never included 

 a larger area. The cell is known to be especially sensitive 

 near the red end of the spectrum, and it is supposed that, 

 unless the spectrum of the comet was very peculiar, the 

 systematic error of these observations would be less than 

 visual comparisons of a luminous surface with a point 

 source of light, such as a star ; extra-focal images of stars 

 were used in the comparison, and in the morning observa- 

 tions the brightness of the sky was measured and taken 

 into account in adopting final values for the comet's 

 brightness. The range of the latter is shown by the 

 following values, given in magnitudes: — May 3, 2-0; 

 May II, 06 ; June i, 36. The second value, o-6. is 

 vitiated by bad observing conditions, but Mr. Stebbins 

 states that the comet became brighter than the first magni- 

 tude, although it never reached magnitude 0-0. 



The Apparent Diameter of Jupiter. — An earlier dis- 

 cussion of the observations of an occultation by Jupiter, 

 made at the Z6-se Observatory on May 21. 1908, led to the 

 conclusion that the apparent diameter of the planet, as 

 generally adopted, should be diminished ; the occulted star 

 was BD. + iq° 2095. 



In No. 4450 of the Astrononiische Xachrichten Father 

 Chevalier, director of the Z6-sfe Observatory, suggests that 

 the observational results were not sufficiently certain to 

 have such an important conclusion based upon them. 



Attempting to determine more trustworthy data, he 

 measured a photograph of the planet taken on Slay 19, and 

 determined the corrections to the tabular place. Then 

 applying these differences he found the jxjsition for May 21. 

 This gave the position-angle of the star as 140° 23' and 

 its distance from the centre of Jupiter as 18-7''. a value 

 greater than the semi-diameter of the planet. It is difficult 

 to reconcile this result with the data for the occultation, 

 and F"ather Chevalier urges that the observations made at 

 other observatories should be closely examined and dis- 

 cussed from this point of view. .\ number of discussions 

 such as he now publishes would possibly elucidate the 

 matter 



Curved Photographic Plates. — In No. 161 of the 

 Harvard College Observatory Circulars Prof. E. C. 

 Pickering describes some interesting experiments made for 

 ascertaining the practical efficiency of curved plates in 

 celestial photography. 



\\'ith the 16-inch Metcalf telescope employed, the differ- 

 ence in focus between the edge and the centre of the plate 

 is only 08 mm., but the experiments show that the bend- 

 fng of the plates to the focal curve is advantageous, while 

 there is little likelihood of counterbalancing disadvantages. 



Several methods were tried, such as holding the ordinary 

 photographic plate against a properly curved concave 

 surface by means of mucilage, &c., but it was found 

 that the most successful method was to have the space 

 between the plate air-tight, and then to exhaust it by 

 means of a pump. Reproductions of actual photographs 

 j illustrate the gain in definition over the whole plate. 



