Jan. 3T, 1878] 



NATURE 



259 



molecular motions, and that it does not include the 

 potential energy of molecular configuration — the most 

 important doctrine, if true, in molecular science — is intro- 

 duced in a footnote under cover of the unpretending 

 German abbreviation " d.h," 



J. Clerk Maxwell 

 {To be continued.) 



WOLF'S HISTORY OF ASTRONOMY 



Gcschichte der Astronomic. Von Rudolf Wolf. (MUn- 



chen : R. Oldenbourg, 1877.) 



THE " History of Astronomy," by Prof. Rudolf Wolf, 

 of Zurich, a volume of 800 pages issued at a very 

 moderate figure, is a contribution to the literature of the 

 science of no ordinary value to the student. The pro- 

 duction of such a work, involving an outline of the pro- 

 gress of astronomy from the earhest times to the present 

 period, must have been a labour of great extent, requiring 

 much research, notwithstanding the assistance that might 

 be afforded by historical treatises previously in the hands 

 of astronomers, and it is only due to Prof. Wolf to 

 acknowledge the very able and complete manner in 

 which he has accomplished the heavy task he had 

 imposed upon himself some years since. 



Those of our readers who may have been desirous of 

 acquainting themselves with the general history of prac- 

 tical astronomy, and of familiarising themselves with the 

 names and the nature of the services of the principal 

 workers who have successively contributed to advance 

 our knowledge of the science, more especially during the 

 last three centuries, will, we think, have experienced 

 difficulties which the volume before us is well calculated 

 to obviate. The English reader has, it is true, Prof. 

 Grant's classical work, the " History of Physical Astro- 

 nomy," but there is much to be found in this volume, 

 which it was hardly within the scope of Prof. Grant's 

 work to incorporate. The writer of these lines very well 

 remembers the fragmentary manner in which, some 

 thirty-five years since, an English student of practical 

 astronomy was under the necessity of obtaining informa- 

 tion, more especially in private reading ; and it is one of 

 the most happy circumstances for the astronomical stu- 

 dent of the present day that this want of suitable guides 

 has been to a great extent removed, and his time there- 

 fore need not be wasted in a search for knowledge in 

 second-rate or doubtful authorities, mistakes which he 

 would be not infrequently led into thereby, being cor- 

 rected only after vexatious delay and trouble. 



Prof. Wolf divides his work into three books. The 

 first deals with ancient astronomy and progress down to 

 the fifteenth century, including theories, instruments, and 

 writings. The second commences with " the reformation 

 of astronomy" consequent on the publication of the great 

 work of Copernicus, " De Revolutionibus Orbium Coeles- 

 tium," and treats of the advances made to the time of 

 Newton ; we find therefore in this division a summary of 

 the labours of Galileo, Apian, Tycho Brahe, Kepler, 

 Fabricius, Harriot, Hevelius, Huyghens, Gascoigne, and 

 many others, including notices of the more important 

 publications of the period, which arc of interest and value. 

 The third book treats of " the new astronomy," com- 

 mencing with the discovery of universal gravitation and 



brings down the history of astronomical research and 

 discovery to the present epoch. A very great amount of 

 information is compressed into this last section of the 

 work, and it is here that the care and research of the 

 author are more particularly evidenced. There is much 

 to be found in it, for which we should look in vain in a 

 collective and compendious form elsewhere. It is well 

 and accurately put together, the few errors we have re- 

 marked being of comparatively trifling nature ; thus the 

 Saturnian satellite Tethys appears as Thetis. The 

 biographical notes, which are extended to contemporary 

 astronomers, will be a welcome feature to many readers. 



Students and others interested in the history of the 

 most ancient of the sciences, who can command a suffi- 

 cient knowledge of the German language, will find their 

 advantage in the possession of Prof. Wolf's elaborate work, 

 and we must not omit to say that that great desideratum 

 in all works of the kind — a very sufficient index, at least 

 as regards names mentioned in the history, will render it 

 of easy reference. J. R. Hind 



{To be co}ttinued.) 



OUR BOOK SHELF 



Photographic Spectra. 136 Photographs of Metallic^ 

 Gascons, and other Spectra printed by the Permanent 

 Autotype Process. With Introduction, Description, &c., 

 by J. R. Capron, F.R.A.S. (E. and F. N. Spon.) 



We gather from the author's introduction that he has 

 chiefly aimed " to popularise a subject hitherto somewhat 

 of a sealed book, confined to the laboratories of workers in 

 special research." In this he should certainly succeed, 

 though we think that his readers would not have been 

 driven away if they had found a little more reference 

 to the explanations of the various phenomena and the 

 conclusions which have been drawn from them. As it is, 

 the book is a good companion to Lecoq de Boisbaudran's 

 " Spectres Lumineux." The spectra are sharp and clear, 

 and the autotype process has lent itself well to this 

 reproduction. The results are all the more commend- 

 able because Mr. Capron has not had the advantages of 

 considerable dispersion. 



The account of the method employed is full and clear, 

 and will make the book a very useful one to beginners in 

 spectrography. 



LETTERS TO THE EDITOR 



[The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of rejected manuscripts. 

 No notice is taken of anonymous communications. 



The Editor urgently requests correspondents to keep their letters as 

 short as possible. The pressure on his space is so great that it 

 is impossible otherwise to ensure the appearance even of com- 

 munications containing interesting aitd novel facts.] 



Sun-spots and Terrestrial Magnetism 



Prof. Piazzi Smyth will no doubt welcome from any quarter a 

 satisfactory answer to his question about the discrepancy between 

 Dr. Wolf 's sun-spot period, 11 -i years, and the supposed 10-5 

 years' period for the magnetic needle. If Mr. Smyth will refer 

 to Prof. Loomis's chart of magnetic oscillations given in Prof. 

 Balfour Stewart's paper on the subject in Nature (vol. xvi. 

 p. 10), he will see that there are exactly seven minimum- 

 periods from 1787 to 1871, the mean of which is twelve years ; 

 the mean of the seven corresponding maximum-periods is ii "8 

 years. The true magnetic declination-period is then the mean of 

 these, viz., 11 '9 years. In exactly the same manner I have 

 found that the mean period of sun-spots is il'9 years. The 

 auroral displays also have the same period. 



But what is this period of 1 1 '9 years ? It is Jupiter's anomal- 



