492 SCIENCE PROGRESS 



Vapour ; Atmosphere and Physics of the Stellar Bodies ; The Chemistry of the 

 Atmosphere ; The Planet Mars ; Mercury, the Moon, and Venus. 



There are not many statements in the book which can be criticised ; we think, 

 however, that there are few astronomers who will agree to the explanation of the 

 dark holes in the Milky Way as due to the sweeping away by star-throngs of all 

 matter in their courses. Barnard's hypothesis of dark nebulae has much more to 

 commend it. 



The chapter on the planet Mars is well worth reading. Dr. Arrhenius discusses 

 the theory that intelligent men exist on Mars in a critical and broad-minded 

 manner, and his conclusion that the theory must take its place in the shadowy 

 realm of dreams will be acceded to by most unbiassed thinkers. " The trouble 

 with these -explanations is that they explain anything, and therefore in fact 

 nothing." 



Dr. Svante Arrhenius is a writer who possesses in a marked degree the gifts 

 of lucidity of exposition and of an excellent literary style. These qualities have 

 lost nothing in the translation from the Swedish of Mr. Fries, who is to be 

 congratulated upon its excellences. The book is well illustrated by numerous 

 figures and recent astronomical photographs. 



H. S. J. 



An Introductory Treatise on Dynamical Astronomy. By H. C. Plummer, 

 M.A., Royal Astronomer of Ireland. [Pp. ix + 343.] (Cambridge : 

 University Press, 1918. Price 18s. net.) 



There is a singular deficiency of works in the English language dealing with the 

 important branches of astronomy which are comprised under the general heading 

 of dynamical astronomy. This is surprising and regrettable when the important 

 contributions which have been made to the subject by English and American 

 astronomers are considered. It is sufficient merely to mention the names of 

 Adams, Darwin, Hill, Newcomb, Cowell, Brown, and Leuschner, amongst others. 

 The publication by Prof. Plummer of the volume under review is therefore an 

 event of importance. 



The amount of ground which is covered is surprising. The contents of the 

 book may be briefly summarised as follows : Chapters I to VI are devoted to 

 preliminary matters mainly concerned with the undisturbed elliptic motion of two 

 bodies. Chapters VII to XI are concerned with the methods for the deter- 

 mination of orbits, including the orbits of double stars and of spectroscopic 

 binaries. Chapters XII to XVIII contain an outline of planetary theory, 

 including general dynamical principles, absolute, secular, and special pertur- 

 bations, and secular inequalities. Chapter XIX is devoted to the restricted 

 problem of two bodies which serves as an introduction to lunar theory, an 

 outline of which is contained in Chapters XX and XXI. The rotation of the 

 earth and moon is discussed in Chapters XXII and XXIII, whilst the last, 

 Chapter XXIV, is devoted to formulae of numerical calculation. 



On reading the book we cannot but feel that it suffers from undue compression, 

 and that an omission of some of the matter such as, e.g., most of the chapter on 

 precession and nutation, an account of which is contained in Newcomb's well- 

 known Compendium of Astronomy, would have been an advantage, enabling the 

 remaining matter to be treated at somewhat greater length. Although com- 

 pleteness is not aimed at, it is to be regretted that an account of Prof. Leuschner's 

 admirable method for the determination of an orbit, which enables an accurate 

 approximation to be rapidly made, was not included. 



