July i i, 1901] 



NA TURE 



253 



would bave done well had he made his reasoning a little 

 more clear and detailed. The method adopted is the 

 graphical method, a diagram being given for each 

 machine, &c., considered. The diagrams are given with- 

 out, in some cases, any of the reasoning which leads up 

 to them. This to the engineer who thoroughly under- 

 stands the subject does not matter, and to such we would 

 recommend the book. Besides dealing with the general 

 calculations concerned with single and polyphase motors, 

 one chapter is devoted to the special design of a three- 

 phase motor of 200 h.p. ; and two chapters, the first and 

 the last, deal with the theory of the alternating current 

 transformer. In appendix ii. a graphical method is 

 given for integrating some of the equations given in the 

 body of the book. We think it is possible for this so- 

 called "non-mathematical" treatment to be carried a 

 little too far. The electrical engineer who does not wish 

 to be severely handicapped in his profession must be 

 able to work out an integration without haying recourse 

 to a roundabout method to avoid it, which is most likely 

 only applicable to the particular case under consideration. 

 Bullet in of the Philosophical Society of Washington. 



\'ol. xiii. 1S95-1899. Pp. xxvi -I- 507. (Washington, 



D.C. ; Judd and Detweiler, 1900.) 

 The subjects of papers included in this volume are : — 

 Central American rainfall, a transcontinental series of 

 gravity measurements, cloud classifications, steel 

 cylinders for gun construction, the latitude-variation tide, 

 Alaska, graphic reduction of star places, chemistry in the 

 United States, the transcontinental arc, a century of 

 geography, the comparison of line and end standards, 

 recent progress in geodesy, secular change in the direc- 

 tion of the terrestrial magnetic field at the earth's surface, 

 and the function of criticism in the advancement of 

 science. In addition, there are a number of obituary 

 notices of members of the Society. 



Several of the subjects of the papers have already 

 been referred to in these columns, and as the papers go 

 back to March 1895, it is a little late to describe them in 

 any detail. The volume is of particular interest to 

 students of geodesy and physical geography, the papers 

 on the measurement of arcs for the determination of the 

 size and shape of the earth, and on gravity observations, 

 being full of information. The results of a series of 

 gravity measurements, made by Mr. G. R. Putnam, lead 

 to the conclusion that " general continental elevations 

 are compensated by a deficiency of density in the matter 

 below sea-level, but that local topographical irregularities, 

 whether elevations or depressions, are not compensated 

 for, but are maintained by the partial rigidity of the 

 earth's crust." Gravity measurements made on the 

 summit of Pike's Peak and at Colorado Springs, near the 

 base, give the value 563 for the mean density of the 

 earth. A discussion of Mr. Putnam's gravity observa- 

 tions leads Dr. C. K.Gilbert to agree that they "appear 

 far more harmonious when the method of reduction 

 postulates isostacy than when it postulates high rigidity." 



At the close of a paper on the transcontinental arc 

 measured by the U.S. Coast and Geodetic Survey, 

 Mr. E. D. Preston refers to the accuracy of the observa- 

 tions, and remarks : " The quality of the triangulation 

 is best shown by a comparison of bases. The Fire Island 

 one, nearly 9 miles long, was determined in five difterent 

 ways through 1800 miles of triangulation, and the extreme 

 range of the results is only two-tenths of a metre. The 

 value from Kent Island base, 5 miles long and 263 

 miles away, only differed from that given by the Atlanta 

 base, nearly 6 miles long and 8^8 miles away, by one 

 centimetre." 



The paper on the secular change in the direction of 

 the terrestrial magnetic field at the earth's surface, by 

 Mr. G. W. Littlehales, contains a number of valuable 

 plates showing curves of the secular motion of the mag- 

 netic needle for twenty-nine different places. 



NO. 1654., VOL. 64] 



LE ITERS TO THE EDITOR. 

 { The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to cot respond with the writers of, rejeei 

 jiannscripls intended for this or any other part of NaTL'KE. 

 No notice is taken of anonvinoiis communications.'\ 



On the Theory of Temporary Stars. 



In a note read before the Roy.il .\stronomical Society on 

 May 10, Father Sidgreaves offers a suggestion regarding the dis- 

 placement of the dark bands in the spectrum of Xova Persei 

 which seems to obviate the serious difficulty felt by astro- 

 physicists in the explanation of the shift of the lines on 

 Doppler's principle. 



The ingenious idea set forth in this note — emanating, 

 apparently, from so high an authority as Lord Kelvin — certainly 

 goes far to e.\plain the singular fact that the displacement of 

 the.se dark bands in the Nov.x" should ahvays be towards the 

 more refrangible side. But Father Sidgreaves remarks that the 

 suggestion does not "help us over the second difficulty: the 

 great breadth of the bright lines, some of which seemed to have 

 lost nothing in width up to the last days of April." The 

 following remarks may perhaps contribute towards an explana- 

 tion of this second phenomenon, and may thus form a theory 

 supplementary to that proposed in Father Sidgreaves' note. 



First of all, it ought to be remarked that the structure of the 

 bright bands,' when seen with high dispersion, is extremely com- 

 plicated. In Nova Aurigx, as well as in the present new star, 

 the bands were observed to consist of several bright maxima 

 separated by darker interstices. Sir Norman Lockyer, in his 

 communication to the Royal Society on March 28, presented 

 some exceedingly interesting diagrams, exhibiting the intensity 

 curves of the bright hydrogen bands in Nova Persei. Sir 

 Norman shows that these bands consisted of at least three, and 

 in the case of Hp of even four, maxima. The very same 

 structure appears in the chief nebula band at A = 501, as is 



shown by the measurements made with the Cooke spectroscope 

 of this Observatory, an account of which will shortly be 

 published. 



In the note referred to, Sir Norman Lockyer has already sug- 

 gested that we have here " indications of possible rotations or 

 spiral movements of two distinct sets of particles, travelling 

 with velocities of 500 and 100 miles per second." It appears, 

 therefore, that the extreme width of the bright bands is not 

 caused by a continuous broadening of the line — such as, for 

 instance, increased pressure would produce — but by the juxtapo- 

 sition of several lines belonging to the same substance, but of 

 somewhat different wave-lengths owing to motions in the line of 

 sight. An explanation of the width of the bright bands is thus 

 equivalent to giving a sufficient reason for the production of 

 displacements such as would conduce to the peculiar grouping of 

 the maxima in the bright lines of the spectrum of new stars. 



Father Sidgreaves starts from the assumption of " a collision 

 between two stars." We shall here proceed from the hypothesis 

 propounded by Prof. Seeliger, of Munich, that the Nova is due 

 to the phenomenon of a dark body impinging upon and pene- 

 trating into a mass of nebular material. 



Now it seems extremely unlikely that the density of the 

 matter composing the nebula should be the same throughout. 

 There will in all probability be a condensation of this matter 

 round the centre, or centres, of gravity of the mass, so that the 

 density must be assumed to decrease outwards from this centre. 

 I consider an assumption of this kind to be warranted, if not 

 demanded, by our modern views regarding the evolutions of 

 stellar systems. But if a body flying through space shoiild 

 approach such a mass, the probability is very small that its line 

 of motion would pass directly through the centre of gravity. 

 Hence we are fairly warranted in assuming that the path of 

 the body through the nebula will lie somewhere between its 

 centre and its boundary (Fig. I). 



