146 



SCIENCE. 



[N. S. Vol. XIV. No. 343. 



to beget in the mind of the reader the strong 

 etheric conviction of his own, that he does not 

 hesitate to recall and repeat where such repe- 

 tition seems to him necessary. It must be 

 admitted that he has made out a very com- 

 mendable case for the medium as well as for its 

 magnetic, electric and luminous properties. 



The book reads easily. The style is not 

 severely didactic ; it is clear, sometimes diffuse 

 and occasionally rhetorical. Here and there 

 we meet similes and metaphors that occasion 

 surprise, partly by their unexpectedness and 

 partly by their novelty or boldness. Thus we 

 are told that when electromagnetic waves reach 

 the more tenuous strata of our atmosphere, 

 they illumine them, and as a consequence " we 

 have those brilliant auroras that cap the mag- 

 netic poles like huge candle-extinguishers" (!) 

 (p. 20). Again, "magnetic storms have over- 

 spread the continents of Europe and America , 

 at the same time, when every needle was 

 affected as with a kind of frenzy —oscillating 

 together, as if some gigantic Briareus reached 

 out his hundred arms, and with a finger on 

 every one moved them regularly or wildly as 

 the mood was upon him " (p. 6). On page 41 

 we find ' a mote of ether ' and on page 62 we 

 are confronted with ' a jungle of electromag- 

 netic manifestations.' 



The following passage, which occurs on page 

 197, is quite Tyndallian : 



From ships of war cruising in every sea ; from 

 merchant vessels plying between the ports of the 

 world ; from observatories equipped with delicate 

 instruments in various countries ; from expeditions 

 afloat and ashore specially fitted out for the purpose ; 

 and from numerous other private and public sources 

 of many kinds — have been gathered, during long 

 years, a multitude of observations of the magnetic 

 elements ; collated, classified and stripped of all dis- 

 cernible errors, they afford, when plotted on charts of 

 the globe, an excellent insight into its magnetic con- 

 dition . 



We can hardly, however, bring ourselves to 

 define the dyne as ' the unit of measure of 

 magnetic intensity ' (page 434), for we have 

 hitherto accepted it as the unit of force. On 

 the same page we read that ' ' the weight of a 

 body is the product of its mass by the force of 

 gravity ; the mass is everywhere a constant but 

 hazy (!) factor, while gravity varies slightly 



from place to place, but is always accurately 

 known. This being understood, the weight of 

 a body will be spoken of as representing it." 

 Adopting a word from this sentence, we cannot 

 but qualify the above statement as somewhat 

 ' hazy.' What we do know, and know clearly, 

 is that the weight of a lump of matter depends 

 conjointly on its mass and on the intensity of 

 gravity, so that we can write 



w = "^^mg. 



By a suitable choice of units, we can make 

 7i, = 1, and then we have 



w = m,g. 



After stating that the mass of a body is con- 

 stant, the writer might have said, in so many 

 words, that its weight depends upon its position 

 with respect to the center of the earth, and 

 thence concluded that the important property 

 of a body, both scientifically and commercially, 

 is not its weight but its mass. 



Ampere's theory of magnets is found on page 

 490 to be ' more rational than the theory of 

 magnetized particles.' We should like to 

 believe it, and consequently regret that the 

 author did not give a few reasons in support of 

 this opinion. The origin of the Amperian cur- 

 rents is no better known than that of the mag- 

 netization of the molecules in Weber's molecu- 

 lar theory, while the maintenance of the cur- 

 rents implies the further difficulty of a resist- 

 anceless circuit. 



The author is very chary in the use which 

 he makes of proper names. Doppler^ it is 

 true, gets credit on page 320 for his ' principle,' 

 and Lissajous on page 52 for his ' figures.' Why 

 not Lenz for his ' law ' (page 410) and Zeeman 

 for his ' effect ' (page 503) ? Peter the Pilgrim 

 (Peregrinus) is mentioned cum laude, but 

 Gilbert is passed sub silentio ! Yet it was 

 Gilbert, the philosopher of Colchester, who 

 first explained the behavior of the compass 

 and the dipping needle by his grand discovery 

 that the earth itself is a huge magnet: Magnus 

 magnes ipse est globus terrestris are his words. 

 Surely Commander Lyons has read De Magnete 

 either in the original Latin or in Mr. Mottelay's 

 translation ; so that his neglect of Gilbert's 

 transcendent merits is hard to explain. 



The magnetical discoveries of Columbus are 



