June 7, 1906] 



NA TURE 



125 



prubleins and exercises are likewise provided. The 

 style of the author is attractive, and the course as a 

 whole has great educational value; in fact, we know 

 of no text-book which presents the subject in a way 

 more suited to the natural capacities of the youthful 

 reader, or which is better adapted to impart a 

 thorough knowledge of concrete geometry, and at the 

 same time to develop the reasoning faculties in a 

 legitimate manner. 



There is a chapter describing the vernier, sphero- 

 meter, callipers, and the micrometer screw gauge, 

 and also treating briefly of the mensuration of the 

 simpler geometrical solids. There are selections of 

 recent examination papers, four-figure logarithms 

 and trigonometrical ratios, answers to numerical pro- 

 blems, and a very useful general index. 



If a draughtsman were to criticise the book he 

 would probablv say that in measuring and setting 

 of!' lengths the scale should be directlv applied with- 

 out the intervention of dividers; that a line to be ac- 

 curatelv measured should have its ends clearly defined 

 by short cross-lines; and that diagonal scales, being 

 of litlle or no practical use, are made rather too much 

 of in the chapter devoted to them. But these are very 

 minor matters, and do not detract from the general 

 t;xcellence of the work. We know of no text-book 

 of eJementary geotnetry which can be more con- 

 fidently recommended to teachers, and none from 

 ivhich students are likely to derive more profit. 



hcs ,Proc^dis de Commande a Distance au Moyen de 

 VElectricitd. By Captain Regis Frilley. Pp. vii + 

 K)o. (Paris : Gauthier-Villars, igo6.) Price 3.50 

 francs. 

 Tub problem considered in this volume is that of 

 communicating to a distant mechanism a movement 

 the magnitude, direction, and sense of which are de- 

 finite functions of those of a transmitting mechanism. 

 The character of the movements which it is desired to 

 transmit varies very much in degree from the simplest 

 of all (traction), in which the three " commands " — 

 forii'ards, backwards, stop — are alone the orders to 

 be obeyed. The author classifies the different me- 

 chanisms employed, not according to their complica- 

 tion, but according to the methods that are charac- 

 teristic of them. These form seven groups — (i) direct 

 action apparatus, (2) apparatus using relays, (3) ap- 

 paratus employing rotating fields, (4) Wheatstone's 

 bridge devices, (5) apparatus based on the use of 

 induction sparks, (6) escapements, (7) Hertzian waves. 

 The various devices that have been used from time 

 to time are very clearly described under these head- 

 ings with the aid of diagrams. In chapter viii. an 

 account is given of the coinmutating device of Lieu- 

 tenant-Colonel Rivals, by which the sending and re- 

 ceiving instruments can be used as either in turn. 

 Altogether the book forms a verv useful and sug- 

 gestive summary of this very important branch of 

 modern military practice. 



Das Radium und die radioactivcn Stnffc. Bv Karl 

 Frhr. von Papius. Pp. viii + go. (Berlin: Gustav 

 Schmidt.) Price 2 marks. 

 This book contains a semi-popular account of radio- 

 active phenomena. The leading experimental facts 

 and the conclusions of their discoverers are described 

 clearly enough, but with little in the way of sug- 

 gestive comment. The printing and illustrations are 

 good, but we notice a serious error in Fig. 10, which 

 suggests that the 0-rays of radium, when deflected 

 by magnetic force, lie in the same plane as the poles 

 of the deflecting magnet. The contrary is, of course, 

 the fact, and such a mistake cannot but suggest 

 serious doubts as to the competence of the author's 

 general scientific knowledge. R. J. S. 



NO I 9 10, VOL. 74] 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 



expressed by his correspondents. Neither can he undertake 



to return, or to correspond with the writers of, rejected 



manuscripts intended for this or any other part of Saivre. 



No notice is taken of anonymous communications.] 

 lonisation and Temperature. 



TuE discourse by Prof. J. J. Thomson, published in 

 Nature of March 22 (vol. Ixxiii., p. 495), was of import- 

 ance from several points of view. The explanation of the 

 method of ionisalion which he sufjgests was of especial 

 interest to myself, and I should be pleased if I might be 

 allowed to raise one query concerning it. 



Prof. Thomson does not regard the temperature of the 

 gas as having any etTect upon the ionisalion. It has, 

 indeed, never been shown that high temperature alone 

 would produce ionisalion. On the other hand, is there any 

 reason for supposing that ionisalion by impact may not 

 take place much more easily at high temperature than at 

 low, and that this is the explanation of the discharge 

 observed by Prof. Thomson? That the gas in this case 

 must have a very high temperature would seem exceedingly 

 probable, for the amount of electrical energy lost in the 

 discharge is very great when compared with the thermal 

 capacity of the gas through which the discharge occurs. 

 Thus in one case when the discharge became luminous the 

 current was 0045 ampere, the potential difference 50 volts, 

 the distance between the electrodes 5 mm., and the pressure 

 of the gas o-oi mm. The dimensions of the tube are not 

 given, but if we assume the volume of the gas to be 2 c.c, 

 the residual gas to be atmospheric gas, and that the whole 

 electrical energy is used in heating the gas, we should 

 conclude that it would raise it 7-4x10' degrees. It is, of 

 course, not to be supposed that the temperature does reach 

 any such value, but we have reason to believe that it 

 reaches a very high temperature, and may it not be that 

 this has a very great effect upon the production of the 

 ions? ' C. D. Child. 



Colgate University, Hamilton, N.Y., May 11. 



The average temperature of the gas when the discharge 

 first became luminous was comparatively low ; for example, 

 a fine platinum wire immersed in it did not become hot 

 enough to be visible. The figures quoted by Prof. Child 

 refer to the current after the luminous discharge had been 

 well established ; the current when the transition from 

 dark to luminous discharge took place was very much 

 smaller, generally less than 10-'' ampere. 



J. J. Thomson. 



A Horizontal Rainbow. 



]'.M ^tudi(5 r^cemment un arc-en-ciel horizontal qui se 

 montrait a la surface d'un petit etang dans les premieres 

 heures de la matinee. On I'observail, comme celui dont 

 Mr. \V. R. M. Church a envoy^ la description k Nature 

 (April 26, p. 608), en tournant le dos au soleil ; et il dis- 

 paraissait quand la hauteur du soleil ^tait de 44° environ. 

 II avail la forme d'un arc d'ellipse dont un foyer se serait 

 trouv^ a peu prfes dans 1 'ombre de la tele de I'observateur. 

 Ses caract^ristiques ^taient les memes que celles de 1 'arc- 

 en-ciel ordinaire : ouverture angulaire de 42° sur le bord 

 rouge, largeur de 2°, apparition a 53'' (plus rare) d'un 

 second arc plus faible et plus large avec les couleurs 

 dispos^es dans I'ordre inverse, obscurity de I'espace compris 

 entre les deux arcs. 



Tout invitait done i chercher I'origine du ph^nom^ne 

 dans des spherules d'eau, qui ne pouvaient etre que 

 repandues sur la surface calme. C'est effectivement ce 

 qu'une ^tude attentive m'a fait d^couvrir. Les spherules 

 en question ont gen^ralement quelques dixiemes de milli- 

 metre de diam^tre. Elles sont tr^s nettement visibles quand 

 on se penche sur I'^tang, mais la moindre agitation les fait 

 disparaitre. Je les attribue !i la ros^e d^pos^e h la surface 

 de la nappe tranquille, laquelle est un peu grasse par suite 

 de I'existence de nombreuses colonies d'animalcules et de 

 v^g^laux dans ses eaux stagnantes. L'arc-en-ciel observe 

 par Mr. Church me semble du k la meme cause : d^p6t 

 du brouillard A I'elat spheroidal sur la surface calme du 

 lar. V. SCHAFFERS. 



Louvain (Belgium), rue des R^collets, 11. 



