220 



SCIENCE. 



[N. S. Vol. XV. No. 371. 



Tinder such uniform conditions would be 

 •exceedingly valuable, not only in giving 

 the highest order of results in timekeeping, 

 but also in developing the peculiarities and 

 comparative merits of the clocks themselves. 

 The extreme accuracy with which two 

 blocks, one keeping sidereal and the other 

 mean time, can be compared by coinci- 

 •dences of the beats, which take place every 

 six minutes, is familiar to every astronomer. 

 Again, the more rapid minor variations in 

 the rates of clocks could perhaps be de- 

 tected and their periodicity determined by 

 comparison with the vibrations of a pendu- 

 lum swinging in vacuo. 



Improvement in perfox'mance of astro- 

 nomical clocks is of special importance in 

 fundamental astronomy. An independent 

 redetermination of the positions of the 

 fundamental stars is necessary, and for this 

 the most accurate possible timekeeping is 

 needed because, in order to be of value in 

 the present state of astronomy, such work 

 must be of the highest degree of accuracy. 

 All this has long been recognized by astron- 

 omers, and during the past forty years 

 efforts in the direction of improved time- 

 keeping have been made in all the principal 

 observatories of Europe where fundamental 

 work is attempted. 



Commenting on the bad effect of varia- 

 tions in the rates of astronomical clocks 

 due to the diurnal changes of temperature. 

 Professor Foerster, the distinguished 

 astronomer, who has been for 38 years 

 director of the Royal Observatory at Berlin, 

 wrote in 1867 : 



' ' How detrimental to accuracy such a 

 large and changeable irregularity is, is evi- 

 dent since it operates like- a variable divi- 

 sion error. 



" It is therefore necessary, in order that 

 ■a clock may be of service in absolute deter- 

 minations of star places, to have it protected 

 from the daily temperature change, and 

 also from all sudden changes of tempera- 



ture. That is, it should be mounted in a 

 place of nearly constant daily temperature 

 so that it will remain for the compensation 

 of the pendulum to effect only the last re- 

 maining fine adjustment. 



' ' The air-tight confinement is safe in un- 

 derground rooms or in heavy masonry 

 against injury to the clock-work, because 

 in the hermetically enclosed space any 

 moisture present can be done away with 

 by known means and the coming in of new 

 moisture is impossible." 



Milton Updegraff. 



U. S. Navajl Obseevatokt, 

 Washington, D. C. 



SCIENTIFIC BOOKS. 

 The Stars, A Study of the Universe. By 

 Simon Newcomb. Pp. v + 333. New York, 

 G. P. Putnam's Sons ; London, John Mur- 

 ray. 



This is professedly a book written to order, 

 as a part of the science series now appearing 

 under the editorial supervision of Professor 

 Cattell, and its author states plainly in his 

 preface that he has found the task, 'to sketch 

 in simple language for the lay as well as the 

 scientific reader the wonderful advances of 

 our generation in the knowledge of the fixed 

 stars,' much more onerous than he had antici- 

 pated, on account of 'the extent and com- 

 plexity of the subject and the impossibility of 

 entering far into technical details in a work 

 designed mainly for the general use.' 



If one may judge the extent of systematized 

 Ivnowledge concerning the fixed stars by the 

 space allotted to its presentation in the most 

 approved text-books of general astronomy, 

 from that of Arago to the present time, it ap- 

 pears that this branch of astronomy has grown 

 during the century from about one eighth to 

 one sixth part of the entire science. But the 

 indexes to recent volumes of the principal 

 astronomical periodicals show that about one- 

 third of the articles there appearing relate to 

 problems of stellar astronomy and thus mark 

 an accelerated growth of interest in and 

 knowledge of the remoter parts of the visible 

 universe. The author who attempts to digest 



