5 So 



NA TURE 



[April 23, iqo^ 



neath us? The canvas was prepared of old time, but 

 the design first traced upon it has been modified again 

 and again bv natural agents ; new pigments have been 

 absorbed by it, while others have changed their nature, 

 and have often become more beautiful and permanent 

 in their decay. The describer of a rock, the petro- 

 grapher, mav well pause before it, and proceed to fill 

 in the details with an almost affectionate care. 



Is life, in fact, so brief that the name assigned to 

 each object must express it completely in its habit as 

 it lived? 



We have no right to raise objections to the scheme 

 of these four serious and conscientious workers, on the 

 ground that it involves a considerable tax upon the 

 memory. Yet we may question the advantage of com- 

 pressing all our information into the tabloid form, and 

 leaving the reader to dissect the compound in order to 

 find out its contents. Abbreviation goes far when a 

 rock, already accepted as dosalane in class, that is, with 

 silica-a/umina minerals predominant in its norm, is 

 also found to be grano-lwrnblende-germanare. 1 In the 

 scheme proposed, there is special virtue in the last 

 syllables of such words. Yet what would be gained 

 for the correct appreciation of a work of art if it were 

 described as samsodelic angelo-italare, because it con- 

 tained (or might in other circumstances have con- 

 tained) Samson and Delilah, and belonged to the 

 Italian school of which Michael Angelo is the repre- 

 sentative? Or should we describe the House of 

 Commons for 1903 as unanim-hibern-britannare, and 

 also as dochambcrlane, on account of the predominance 

 of a particular constituent? " Is't not possible to 

 understand in another tongue? " 



Were we to comment on all the details selected as 

 a basis for rock-classification, we should unduly ex- 

 tend the present notice. The historic review is of great 

 interest and value, and the proposal (p. 180) to revert, 

 for field-purposes, to the old loose signification of 

 granite, diorite, &c, has some points in its favour. 

 Similar reasoning, however, would allow us to speak 

 of a mineral as a fossil; nor are the historic authorities 

 always correctly invoked by the reformers of petro- 

 graphy. D'Aubuisson, that is to say, Haiiy, from 

 whom he had the term, does not (p. 182) use aphanite 

 in the wide sense stated; for him, it is a compact 

 diorite, with amphibole predominant over felspar. If 

 we loosen the bonds of peridotite (p. 183), we must go 

 back to Cordier, and use it for a basalt or dolerite rich 

 in olivine. As for felsite (p. 184), the authors can 

 hardly have realised the odd mixture of materials 

 associated under the name by Gerhard. It may be 

 sufficient to mention labradorite felspar and the pitch- 

 stone o Meissen as felsitic rocks in the sense of the 

 inventor of the term. 



However, we conclude as we began ; there are per- 

 sons who desire classification in order to promote 

 accuracy of comparison. Such accuracy must be 

 welcomed by every geologist, where individual speci- 

 mens are concerned. Whether it is of so much service 

 when we consider rock-masses as portions of the earth's 



1 Mr. H. Stanley- Jevons {Geoiog ' i ine, iqoi, p. 313) has already 



attempted such term- a> ,\. 'talite and j&giau'gi-nettrijolite. It 



would ot course be similarly possiole tj ex.ires* a whole chemical analysis 

 by a skilfully constructed word some decimetres in length. 



NO. 1747, VOL. 67] 



crust is a question for the worker in this or that par- 

 ticular district. At any rate, our authors have sought 

 perfection in the domain on which they set their gaze. 

 To all of us is the mission sent, of Sir Persalane, Sir 

 Salfemane, Sir Dofemane, and Sir Perfemane — for we 

 cannot but regard these* names (p. 102) as those of 

 champions seeking for a Grail. The path is lit by 

 their high endeavour, even if we may not follow it to 

 the end. Grenville A. J. Cole. 



ALTERNATING CURRENT ENGINEERING. 

 Die Grundgesetze der Wechselstromtechnik. By Dr. 

 Gustav Benischke. Pp. 141. (Brunswick : Friedrich 

 Vieweg und Sohn, 1903.) Price 3.60 marks. 



THIS volume, the third issue of " Elektrotechnik 

 in Einzel-Darstellungen," comes as rather a sur- 

 prise after the first two highly specialised parts of this 

 series on lightning arrestors and the parallel running 

 of alternators. In order to peruse the book with profit, 

 the reader must be acquainted with the fundamental 

 theory of electricity and magnetism, and also with the 

 general laws of electrical engineering. Ability to use 

 the differential and integral calculus is also necessary, 

 in order to understand the mathematical reasoning 

 given. 



The book is divided into six parts — introduction, 

 the simple alternating current circuit, mutual induc- 

 tion, capacity phenomena, composite wave forms, and 

 polyphase currents. The arrangement and scope of the 

 book will render it of most use to the practical elec- 

 trical engineer, who, though using certain symbols 

 and' equations every day, yet is apt to lose sight of 

 their fundamental origin, and, in order to comprehend 

 new problems, needs, now and then, to refresh himself 

 in the theoretical basis of his work. Such a simple 

 thing as the measured value of an alternating current 

 is an example of what we mean. Every engineer 

 knows, of course, that what he calls the current is the 

 root of the mean of the squares of the instantaneous 

 values of the current. Why this should be, and to prove 

 the reason why, would, we think, puzzle a good many 

 men who would be very much insulted if they were told 

 that they could not do so. The why and the wherefore 

 of this matter is set forth in the introduction of the 

 book. Part ii. deals chiefly with the application of 

 Ohm's law to the alternating current circuit, the work 

 done by an alternating current and the use of vectors. 

 The third and largest part of the book is concerned 

 mostly with the laws of the transformer. No attempt 

 to treat of design is made, nor is the practical per- 

 formance of any actual machines studied. The sub- 

 ject is treated purely from the theoretical engineering 

 point of view. 



Part iv., on capacity, is very short, and does not give 

 much beyond the deduction of the formula for the 

 calculation of the effective current in a circuit contain- 

 ing resistance, self-induction and capacity, and also 

 showing the conditions under which electrical reson- 

 ance can occur. Part v. is the most useful of the book, 

 as it serves as a guide to the difficult task of dealing 

 with the irregular wave-forms given by alternators 

 and transformers. The appendix can be used in con- 



