Feb. 5. 1885] 



NATURE 



313 



q = 2-63 a \'/>, 

 whence 



a = 0-38 JJi. 



But we can always ascertain by observation the values 

 of q and h in any given case, so that the value of the 

 equivalent orifice can be easily found. 



M. Murgue has determined this value for a large 

 number of mines and has given the results in tables in 

 his second article, already referred to. The values vary 

 somewhat above and below a square metre, but a large 

 number of them are very little different from that unique 

 area. The author calls those mines whose equivalent 

 orifice is greater than a square metre, wide, or roomy, 

 and those in which it is less than a square metre, narrow, 

 or confined. 



M. Murgue has applied the same mode of comparison 

 to the resistances which the air has to overcome in 

 passing through the various kinds of ventilating machines, 

 and in this case he distinguishes the corresponding orifice 

 by the name of orifice of passage. The manner in which 

 its value is found is similar to that of the equivalent 

 orifice. 



In Mr. Brown's paper will be found a table containing 

 a summary of experiments made, with six different kinds 

 of ventilators, by a Committee of the Socie'te de l'lndustrie 

 Mindrale, in which the translator has reduced the French 

 measures to their English equivalents. He also gives 

 two diagrams : one showing the volumes of air produced 

 by the same ventilators kept running at a uniform velocity, 

 while the equivalent orifice is gradually increased ; the 

 other showing the curves of useful effect for four of 

 them. On the whole, we consider that the contents 

 of this paper deserve the careful consideration of those 

 who have not an opportunity of consulting the original 

 articles. W. Galloway 



MAGNETO- AND DYNAMO-ELECTRIC 

 MACHINES 

 Magneto- and Dynamo-Electric Machines. From the 

 German of Glaser de Cew, by F. Krohn. Specially 

 Edited, with many Additions, by Paget Higgs, LL.D^ 

 D.Sc. (London: Symons and Co., 1884.) 



I^HIS book is issued as Volume I. of "The Specialists' 

 Series," to be edited by " Dr." Paget Higgs and 

 " Professor " Charles Forbes. From what University 

 Mr. Higgs holds his degree of Doctor of Science does 

 not appear. Presumably, he is the same person as the 

 " Rev. William Higgs, M.A., D.D.," who formerly edited 

 an electrical periodical in London, and afterwards left his 

 country. Readers of the admirable volume on the 

 " Transits of Venus " in the Nature Series know the 

 name of Prof. George Forbes, and appreciate his scientific 

 standing. They are not likely to confound him with the 

 Mr. Charles Forbes who appears as joint editor. 



The present volume, translated and " specially edited," 

 gives to the public little that it did not previously possess. 

 Of books on electric lighting there are enough and to 

 spare. Dr. Schellen's work on "Magneto- and Dynamo- 

 Electric Machines" — an excellent translation of which is 

 now appearing in New York — was the first good work of 

 the kind, and it has run to a second edition. In title and 

 in matter it is greatly resembled by the present work ; 



but Schellen's work is far more elaborate and complete ; 

 whilst the one merit of the Glaser-de-Cew-Krohn-Higgs- 

 Forbes volume is that it includes a brief chapter on accu- 

 mulators—too brief, considering that the various types 

 are well and concisely explained. For the rest, the addi- 

 tions are chiefly scissors and paste work. Chapter VII., 

 on Constructional Laws, is largely taken from Prof. S. 

 Thompson's " Cantor Lectures" ; Chapter VIII. gives the 

 old set of tests executed for Trinity House in 1877 on 

 obsolete types of machine ; the only addition, relating to 

 the later and far more perfect tests made at Paris in 1881, 

 Munich and Crystal Palace (London) in 1882, and Vienna 

 in 1883, being an editorial footnote five lines in length. 

 Chapter X. is extracted from Du Moncel's book on 

 "Electromagnets"; Chapter XL (on Instruments for 

 Measurement) is apparently amplified from the price list 

 of a certain firm of electrical engineers, whose instruments, 

 exclusively, are described. Chapter XI II. is an abridgment 

 of Clausius' theory of the dynamo-machine, reprinted 

 verbatim from the abstracts from foreign journals in the 

 Proceedings of the Institution of Civil Engineers. The 

 index is most elaborate : it occupies nearly a twelfth of 

 the whole book. There are several glaring errors in the 

 work. Of these is the statement, on p. 100, that in a 

 compound-wound dynamo — in which it is desired to pro- 

 vide a current varying exactly proportionally to the 

 number of lamps that are connected to the mains — there- 

 must be maintained "a constant magnetic intensity." 

 On p. 143 it is elaborately set forth that the ratio of the 

 part of the effective electrical energy which is converted 

 into real work to the total electric energy of the current 

 can "never be greater than \ " ; and on p. 147, equally 

 elaborately, that " the maximum efficiency of an electro- 

 generator is obtained when its internal resistance is equal 

 to the resistance in the external circuit." If the latter 

 statement were true the maximum efficiency could never 

 exceed J. The fact is that both statements are untrue 

 and misleading, as are several of the statements relating 

 to efficiency on p. 144. Apparently, either the translator 

 or the editor does not understand either the English 

 meaning of the German word Nutzeffekt, or the technical 

 meaning of the English word efficiency. On p. 173 the 

 shifting of the neutral point in the rotating armature is, 

 referred to the alleged fact (?) that " the magnetism of the 

 iron core and the current in these turns of wire (which 

 have passed the poles) remain at the same intensity for a 

 few moments." The statement is misleading, and the sup- 

 posed explanation of the shifting of the neutral point is 

 well known to be a fallacy. Still more extraordinary is 

 the statement made, apparently with scientific serious- 

 ness, on p. 174, that the heating of the iron core of the 

 armature is another " consequence of the fact that the 

 maximum magnetism does not immediately disappear." 

 There are several mistakes in the definitions of the elec- 

 trical units as given in the last page of the preface. The 

 watt is given as the unit of work, instead of the unit of 

 activity ; and the extraordinary statement is made that 

 the untit of potential difference " exists between two 

 points when the unit quantity of electricity, in moving 

 from the one point to the other, requires a unit force to 

 overcome the electrical repulsion," thus making the defi- 

 nition of potential depend on force instead of work- 

 Moreover, the static units are called the " C.G.S." units 



