July 6, 188:3.] 



SCIENCE. 



17 



electrician a stndj- of the fundamental mag- 

 netic measurements rather than the perusal 

 of treatises of this nature. 



LEDGER'S SUN AND ITS PLANETb. 



The sun, ils planets, and their satellites. By Rev. 

 Edmund Lkogf.r, M.A. Loudon, Stanford, 

 1882. 432 p. 12°. 



Of late a considerable number of scmi- 

 popular works have appeared on astronomical 

 subjects. They seem to meet a felt want of 

 the community, and have been verj- success- 

 ful. We call them semj-popular : because, 

 while they are not written for professional as- 

 tronomers, they are adapted, in their style and 

 mode of treatment, less to the great masses of 

 the business and laboring population than to 

 the educated people who are engaged in vari- 

 ous professional occupations. Those, for in- 

 stance, who are busy in teaching, or with the 

 practice of medicine or law. or who are pursu- 

 ing geological or biological research (in short, 

 pretty much all who would naturally subscribe 

 for Science) , generally wish to keep ati coit- 

 rant of what is going on in other than their 

 own special lines of work, and are delighted 

 to find what they want, when they can get 

 it in an attractive form. 



Mr. Ledger's book is an excellent one of 

 this class. It is less ditfuse than Mr. Proc- 

 tor's essays, and not quite so imaginative. 

 It is narrower in its scope than Professor 

 Xewcomb's Popular astronomy, but easier 

 reading, and fuller of detail in respect to the 

 subjects of which it does treat. It makes no 

 special claims to originality, hut is accurate 

 and clear, and the style is unpretentious and 

 agreeable. The book is nicely gotten up, and 

 ver\- well illustrated. Altogether, we have no 



hesitation in pronouncing it a volume well 

 worth reading and possessing. 



It is made up of fifteen lectures read in 

 1881 and 1882 in Gresham college, London. 

 Two are upon the sun, two are devoted to the 

 moon, two to the earth, and two to .Tu|)iter and 

 his satellites. Each of the other planets has a 

 chapter to itself (counting the group of plane- 

 toids as one), and there is a chapter entitled 

 • Ptolemy versus Copernicus.' Naturally, the 

 lectures are not all of equal interest and value ; 

 but none of them are poor, or could be well 

 dispensed with. The chapters upon Mars and 

 the planetoids strike us as particularly good, 

 and contain information not otherwise very 

 easily accessible. The chapters on the sua 

 and moon are also excellent, though naturally 

 enough, in the main, only an abridgment and 

 compilation from the recent books on these 

 subjects ; to which books the author hand- 

 somely acknowledges his obligations. 



There are remarkably few mistakes in the 

 work : in fact, in reading it over for this notice, 

 we have found none at all, unless we count as 

 such, a blunder in the illustration on p. 147, 

 representing the com|)arative size of the sun 

 as seen from Mercury at perihelion and aphe- 

 lion ; the ditference being represented very much 

 greater than the truth. Speaking of illustra- 

 tions, the fine Wood bury- type of the eclipse of 

 1871 deserves special mention, and several of 

 the pictures of Mars and Jupiter are unusually 

 excellent. It is rather a pity that a few pages 

 of tables were not appended, containing the 

 numerical statistics of the planetary system. 

 They would have greatly increased the value of 

 the book for those who wish not merely to read 

 it once, but to keep it on their shelves for 

 occasional reference. 



WEEKLY SUMMARY OF THE PROGRESS OF SCIENCE. 



ASTRONOMY. 

 Flexure of the broken transit. — Professor C. A. 

 Young, after alluding to tlie fact that the tlexure-cor- 

 rection of this pecuHar form of transit is not treated of 

 in any of the common te.xt-books on practical astron- 

 omy (not even in Sawltsch, who specially describes and 

 discusses the instrument itself), states the theory of 

 the correction to transits of stars ob.served with 

 the ' broken transit,' which is often so great as to 

 amount to a large fraction of a second of time at the 

 zenith. The constant of flexure must be known, and 

 its effect eliminated, before the colliraation error can 

 be determined by reversal of the instrument on a 

 circumpolar star. The correction has the same co- 



eflBcient with the level-error; and denoting this lat- 

 ter, as usually obtained, by 6, the flexure-constant 

 by /, and the pivot-correction by p, the complete 

 formula for the 'level-constant' is [h ± (f ± p\]. 

 Thus, by flexure, the time of transit of a star is af- 

 fected by/ cos 2 sec d. The .sign of/ changes with 

 the reversal of the instrument, being alway< plus 

 for eye east, and minus for eye west. Prof. Young 

 gives several methods of determining/; by observ- 

 ing zenith stars in reversed positions of the instru- 

 ment, by means of the coUimating eye-piece and 

 mercury-basin, or a vertical collimator supported 

 al)ove the instrument, and by least-square treatment 

 of equations given by repeated observation of suit- 



