[ 425 ] 



XLVII. Notices respecting New Books. 

 An Introductory Treatise on Dynamical Astronomy. By H. 0. 



Plummer, M.A. Pp. 343 + xix. Price 18s. net. Oamb. Univ. 



Press. 

 'TVEEE publication of an English work on dynamical astronomy is 

 -*- a rare event, and it is with more than usual anticipation that 

 we examine Prof. Plummer s treatise ; for there can be few branches 

 of science which have suffered so much from the lack of a suitable 

 textbook. The scope of the hook is wide, the title " Dynamical 

 Astronomy " being interpreted liberally. About half the pages are 

 concerned with the subject of planetary and lunar perturbations ; 

 the remainder treat, amongst other matters, of the determination 

 of orbits, including orbits of double stars and spectroscopic 

 binaries, the libratiou of the moon, the phenomena of the earth's 

 rotation, and the formulae of numerical interpolation and quad- 

 rature. In each case the subject is developed far beyond an 

 elementary stage, and it is surprising that the author has been 

 able to keep the work within moderate compass. The general 

 design will be especially welcomed by those who have at one time 

 gained some acquaintance with the subject, and desire to revise and 

 extend their knowledge. For the university student also, it will 

 prove a valuable supplement to oral teaching, and assist in system- 

 atizing knowledge. Perhaps the reader who is trying to begin the 

 subject unaided will at first be less appreciative. Some parts 

 indeed are well adapted to his needs, and we would especially 

 commend the two chapters on the lunar theory. But in general 

 he may prefer to make his first approach to the subject through 

 the leisurely expositions of Tisserand and Klinkerfues; he would 

 certainly feel himself " hustled " by Prof. Plummer. .For a sub- 

 sequent reading — and the subject needs to be read again and 

 again — the conciseness of the present work is an advantage. 



The brevity is partly gained by the entire omission of worked 

 examples; this is referred to in the Preface, and is a deliberate 

 policy. Yet we wish that the author could have departed from 

 his rule in some places at least. Worked examples of the calcu- 

 lation of orbits and of special perturbations are essential for a 

 proper appreciation of the results obtained ; and the inexperienced 

 reader will find great difficulty in supplying these for himself. 

 It is not that we lay stress on learning the best form of com- 

 putation ; the labour-saving devices of the computing-bureau are 

 only necessary when large numbers of applications of the formula? 

 have to be made. But the unassisted reader v\ill fail to provide 

 himself with satisfactory examples, if only because he will in- 

 evitably make numerical mistakes in the lengthy computation. 

 On much the same principle the author often leaves his analytical 

 results to speak for themselves, where a few words of comment 

 might have been helpful. thus a novel theorem due to 

 E. T.Whittaker is given in § 217, but we are left in doubt 

 whether it is inserted solely for its theoretical elegance or is 

 appropriate for practical application. In some parts of the 

 subject we should have preferred a more geometrical treatment ; 

 but there may well be divergence of view as to this. The extent to 



