May, 1903. 



KNOWLEDGE 



113 



that the reader knows much concerning the questions, and that 

 he is acquainted to a considerable extent with mathematical 

 processes. But, granted that, the working out of the various 

 problems is presented with clearness and judgment. Dr. 

 Moulton deals with rectilinear motion, with the i)roblem of two 

 bodies, with the problem of three bodies, with various pertur- 

 bations, and with the theory of determinations of the elements 

 of parabolic and elliptic orbits. Amougst many interesting 

 details we may mention one in particular in connection with 

 the Geaenschein. In the problem of three bodies — the sun, the 

 earth, and a third bod)' infinitesimal as compared with either — 

 assume the origin of coordinates at the centre of mass and the 

 axis of X as the moving line joining the circles of the sun and 

 earth, both of which are supposed to move in centres round 

 their common centre, the plane of x y being the plane of 

 their motion. Assuming that the infinitesimal body is also 

 moving in this plane. Dr. Moulton finds the locus referred to the 

 rotating axes, of those points in space where the velocity of the 

 particle is zero. On this locus there are three double points on 

 the .r-axis, one lying between the sun and earth, and one on 

 each side of these two bodies. Assuming the circularity of the 

 earth's orbit, the double point which is opposite the sun is 

 distaut from the earth 930,240 miles, and at this point an 

 infinitesimal body should just escape eclipse. If the infinitesimal 

 body should arrive near this point under initial conditions 

 (defined by equations too long to quote here) it will revolve in 

 an ellipse round the point of equilibrium, and the major axis 

 of the ellipse lies along the axis of y. If the initial conditions 

 are very approximately complied with, the small body will 

 move in an ellipse for a considerable time, but will eventually 

 depart far from it, and it would be possible to have any 

 number of infinitesimal bodies revolving around the same point 

 without disturbing each other. If, therefore, a great number 

 of meteors were moving in the plane of the earth's motion 

 under the required approximate initial conditions, they would 

 appear as a hazy patch of light with its centre at the anti-sun 

 and elongated along the ecliptic, and this agrees substantially 

 with the observations of Backhouse and Barnard. If Dr. 

 Moulton's solution is the true explanation then the Gegenschein 

 is not closely connected with the Zodiacal Light. True, Dr. 

 Moulton says : " It is certain that the meteors are exceedingly 

 numerous, as many as 8,000,0U0 striking into the earth's 

 atmosphere daily according to the late Professor H. A. Newton, 

 and it is only reasonable to suppose that they cause the Zodiacal 

 Light, which is very bright compared with the Gegenschein." 

 Dr. Moulton does not, however, give his reasons for assigning 

 the origin of the Zodiacal Light to meteors, and we do not see 

 that it follows from any one of the propositions here discussed. 

 "The Twentieth Century Atlas op Populau A.stroxojiy, 



COMPRISING IN TWENTY'-TWO PlATES A COMPLETE SERIES OF 



Illustrations of the Heavenly Bodies." By Thomas 

 Heath, b.a., First Assistant Astronomer, Royal Observatory, 

 Edinburgh. (W. & A. K. Johnston, Limited, Edinburgh and 

 London. 1903.) — This very handsome volume is, so far as its 

 plates are concerned, essentially a new edition brought up to 

 date, of Hind's Atlas, so popular forty years ago. The letter- 

 press is largely new, and is due to Mr. Thomas Heath, First 

 Assistant at the Boyal Observatory-, Edinburgh. Taking the 

 plates first, the old ones were very admirable at their original 

 date, and were fini>hed with a care much greater than was at 

 all usual at that time. But it may be questioned whether we 

 are not now in a position to exact a higher standard. Plate XH. 

 (views of nebula:) is by no manner of means satisfactory. 

 Apparently, the blue ground has proved utterly unsuitable for 

 the reproduction of nebular photographs, and this Plate in con- 

 sequence is wor.«e than misleading. Plate X. (of comets) is 

 not quite so bad, but here the most recent comet given is that 

 of 1882. There have been many since that date more significant 

 as to their structure than any shown here. Mr. Heath has 

 fulfilled his difficult duty of bringing the letterpress up to date 

 with considerable skill, but every hero and there a little point 

 suggests an oversight in the revision, or at least an incomplete- 

 ness. To take a few instances : — The only reference as to the 

 contour of sunspots is to Prof. Wilson's theory, illustrated by a 

 rather crude diai;rani ; Sir George Airy's determination of the 

 density of the earth is the most recent one given ; the chapter 

 on meteors contains no reforonce to anything later than the 

 shower of Ibiiii, save for the fall of a single meteorite in 1890 ; 



the Zodiacal Light is " supposed to be due to sunlight reflected 

 from minute meteoric particles crowding the region between 

 the sun and the orbit of jMercury." Still, with all these imper- 

 fections, the book is a handsome and a useful one. Chapters 

 f X. and XI. on the moon and eclipses are especially well 

 illustrated. 



"Waves and Ripples in Water, Air and ^ther." By 

 Prof. J. A. Fleming, F.R.s. Pp.299. Illustrated. (S.P.C.K.) .5s.— 

 The title of this book comprehends the most important branches 

 of physics ; fur. according to modern conceptions, wive mo'ion 

 accounts for the greater part of physical phenomena. Waves 

 and ripples in the air lead to sound and music, and in the ajtber 

 they give rise to light and electricity. Prof. Fleming had there- 

 fore a wide subject to deal with when he gave the lectures at the 

 Royal Institution, which are reproduced in the volume under 

 notice. Everyone who pees this book will be glad that the 

 lectures are now available to a wider audience than that of 

 Albemarle Street, for they are full of interest and instruction. 

 The style is easy and pleasant to read, the experiments and 

 other examples are striking, and the illustrations are clear and 

 attractive. Prof. Fleming knows how to deal with a subject in 

 a manner which claims attention and commands respect for the 

 dignity of science ; for this alone his book merits many readers. 

 No better popular yet scientific account of the principles upon 

 which wireless telegraphy is based could be desired than this 

 volume affords, and among similar subjects effectively dealt 

 with are the electron theory of electricity, the relations between 

 light and electricity, motions of vessels m relation to waves in 

 water, experimental tanks for the study of motions of ships of 

 various designs, earthquake waves and the phonograph. Every 

 page contains an interesting statement of fact or principle, and 

 we urge .all who desire to enrich their knowledge to obtain a 

 copy of Prof. Fleming's book. 



"Publications of the Maharaja Takhtasin.ji Observa- 

 tory, PooNA." Vol. L "Report on the Total Solar 

 Eclipse of January 21-22, 1898, as observed at Jour, 

 IN Western India." By Kavasji Dadabhai Naegamvala, 

 M.A., f.r.a.s , etc.. Director of the Observatory. — It is 

 nearly five years since the total solar eclipse occurred, the 

 observations of which Prof. Naegamvala here reports. This 

 delay is certainly much to be regretted, since two important 

 eclipses have taken place in the interval, but in all probability 

 the delay has arisen from causes quite bej'ond Prof. Naegamvala's 

 control. The report bears strong evidence to Prof. Naegamvala's 

 energy and power of organisation in bringing together and 

 equipping so well-ordered a force for the observation of the 

 eclipse. The most important work which he undertook was 

 spectro.scopic, and his most powerful instrument a Cooke triple 

 achromatic, of six inches ajierture, used with two prisms of 45° 

 as a pi-ismatic camera, and a series of successful photographs 

 were taken with it. Of these, the first plate, taken at the 

 instant of second contact, appears of most importance, and a 

 long list is given of the bright lines measured upon it. Tliis 

 plate was specially exposed for the purpose of the study of the 

 " Flash," and it is disappointing to find that Prof. Naegamvala 

 does not ve'nture to draw any conclusion as to the relation of 

 the "Flash" to the Fraunhofer spectrum. He quotes, indeed, at 

 length Mr. Maundcr's statement of the points at issue from 

 Knowledge for August, 1898, but has nothing to say as to the 

 light which his results can throvv upon them. With some of 

 his smaller spectrographs he obtained important spectra of the 

 corona, beside photographs of the corona itself with various 

 instruments. A fine composite drawing of the corona from the 

 latter photographs made bj- Mr. Henry Cousens, Superintendent 

 of the ArchiBological Survey, to whom several of the plates of 

 spectra, and some very interesting studies of coronal types are 

 also due, forms the frontispiece to the volume. 



"The Flora of the East Riding of Yorkshire, 

 INCLUDING a Piiysiographical SKETCH.'' By Jas. Fraser 

 Robinson. (Brown & Sons. 1902.) 7s. 6d. — The North 

 Riding of Yorkshire has had an able botanical exponent in Mr. 

 J. G. Baker, and the West Riding iu Dr. .^.rnold Lees. Now 

 the East Riding has also a flora of its own, thanks to Mr. 

 Robinson an 1 the Members of the Hull Scientific and Field 

 Naturalists' Club. Among the many local floras published of 

 late years the present book will take a high place. There is 

 throughout a good sense of proportion ; the general featui-es 



