December 24, 1891] 



NATURE 



173 



jet is returned back again ; and although the efficiency 

 is small, when compared with a pump, still the Injector 

 has the advantage of working while the engine is at 

 rest. 



The same principle is applied occasionally in the 

 Water Injector and the Ejector, where, for instance, a large 

 body of water, in the form of leakage or water ballast, is 

 to be rapidly cleared out. A somewhat similar instru- 

 ment, although quite different in principle, is that called 

 the Pulsometer, which is really a revival of the Marquis of 

 Worcester's and Savory's Fire Engine, where the pres- 

 sure of steam acts directly on the surface of the water. 

 To check the great condensation a piston was intro- 

 duced, and hence our modern steam engine. 



Hot Air Engines are described in chapter xi., and here 

 the mathematical theorems for Perfect Gases receive their 

 most beautiful applications, so that formal treatises on 

 Thermodynamics usually treat this part of the subject at 

 length. Our author dismisses it in about eight pages, 

 with a short description of the principal systems, as, 

 unfortunately, all the practical objections against the 

 use of Superheated Steam are intensified tenfold in 

 the Hot Air Engine. Ericsson once fitted a steamer to 

 cross the Atlantic with engines on this principle : they 

 were very cumbrous although the boilers were dispensed 

 with ; and the experiment did not lead to further imita- 

 tion. An exception must be made in favour of the Gas 

 Engine, as the only practical application of the Hot Air 

 Engine ; the author works out the theory, and comes to 

 the remarkable conclusion that the efficiency of the Gas 

 Engine Cycle does not depend, as in ordinary Thermo- 

 dynamics, on the difference of temperatures so much as 

 on the degree of expansion and compression. 



The author reaches the real part of his subject in 

 chapter xii., where he discusses the theory of the Actual 

 Steam Engine, as we really find it working, in the mill, 

 mine, and on the railway or steamer. 



Here Him appears as the great authority on the careful 

 records of what takes place in the actual engine (chapter 

 xvii.). 



" The measurement of quantities of heat, especially 

 when it has to be done in an engine at work, is an opera- 

 tion of great difficulty ; and it was not till 1862 that it was 

 shown experimentally by Him that A, the heat emitted, 

 is really less than H, the heat received by the engine" 

 (Maxwell, " Theory of Heat "). 



The example of Him has been followed up of recent 

 years by careful and long-continued experiments on 

 steamers and pumping engines in regular work, and the 

 results of the most important of these tests receive care- 

 ful description and analysis, in chapters xv.-xviii. ; a 

 preliminary chapter, xiv., giving a detailed account of 

 the best procedure and instruments required in Testing 

 Steam Engines. 



The book will be found indispensable, not only by 

 designers of Steam Engines, but also by writers of 

 abstract treatises on Thermodynamics, as restraining 

 their mathematical development within reasonable limits 

 of actuality, and as directing their analytical powers in a 

 useful direction. 



A. G. Greenhill. 



NO. 1 1 5 6,lvOL. 45] 



BRITISH FLIES. 

 An Account of British Flies {Diptera). By the Hon. 

 M. Cordelia E. Leigh, F.E.S., and F. V. Theobald, 

 B.A., F.E.S. Vol. I., Part I. (London: Elliot Stock, 

 1891.) 



THE reader involuntarily glances back at the title of 

 this work when the first words that meet his eye on 

 the front page are : " One of the branches of science that 

 has advanced with rapid strides during recent years is 

 geology. . . ." To commence with Fossil Diptera, and to 

 enumerate the families (and some of the genera) mem- 

 bers of which are found preserved in the earth's strata, 

 before either families or genera have been in the least 

 degree defined, is a somewhat novel way of beginning. 

 When the work is completed, students will find it useful 

 to transfer chapter i. to the end. The second chapter, 

 entitled " Classification of Diptera, with an introductory 

 account of the ancient and modern classification of 

 Insecta," contains much matter of interest to entomo- 

 logists in general, although it is questionable whether 

 the authors have arranged their material in either 

 the most attractive or the most methodical form. The 

 classification of the Diptera it is intended to follow is 

 that of Verrall, published in 1888, in which the order is 

 divided into two great sections — the Orthorrhapha and 

 the Cyclorrhapha ; the Nematocera and Brachycera being 

 included in the former, and the Proboscidea with the 

 Eproboscidea in the latter. The Aphaniptera (now in- 

 cluded in Nematocera) form the subject of the third 

 chapter, in the course of which this first part terminates. 

 The structure and metamorphosis of Pulexare discussed 

 at some length, and certain species are described in 

 detail. Some uncertainty seems to exist in the authors' 

 minds as to how many of them are engaged upon the 

 work, for they use both " we " and " I." This calls to the 

 recollection Cruikshank's picture, "In which there is 

 Antagonism of interest yet Mutuality of object." 



It is not possible from a perusal of the first thirty-two 

 pages to form a fair idea as to the general character of 

 the work. It may be stated, however, that it appears to 

 be written for those who are already entomologists, a 

 familiarity with entomological science on the part of the 

 reader being assumed by the authors. Considerable trouble 

 has evidently been taken in consulting authorities whose 

 works are accessible only to the few. That there is 

 plenty of room for a good treatise on the British Diptera 

 will readily be admitted, and if the authors should have 

 something new to tell about such genera as Chlorops, 

 Oscinis, Cecidomyia, and Hylemyia, so much the better. 

 Part I. is illustrated by five woodcuts. 



OUR BOOK SHELF. 



Principles of Agriculture. Edited by R. P. Wright, 



F.H.A.S. (London: Blackie and Son, 1891.) 

 The raison d'etre of this little volume is to be found in 

 its " tail," where are reproduced the questions set in the 

 Science and Art Department Examinations in the 

 Principles of Agriculture during the last eleven years. 

 The title-page ought to state, but it does not, that this 

 is a revised edition of a book that was published some 

 years ago. This fact is only discoverable from the 

 preface. The original edition was arranged in three 



