260 



Notices respecting New Books. 



was increased in numerical value, and the length of discharge 

 along which it existed was also made larger. This would 

 seem to afford ground for thinking that this appnrent negative 

 force is really due to inequalities in the distribution of the 

 discharge. In order to investigate this point further, a tube 

 was designed which allowed of the discharge being investi- 

 gated along a diameter of its section as well as its length. 

 This is shown in the fig. 8, of which little explanation is 

 necessary. 



The two disk electrodes were held a fixed distance apart 

 by a glass rod ; at A, some distance from the electrode, was 

 a piece of soft iron which allowed of the discharge being 

 moved horizontally by means of an external electromagnet. 

 The explorers, of which all but the tips were covered, could 

 be clamped in any vertical position ; for insulation purposes 

 one of these was continued down the inner tube shown in 

 the figure, the other was wrapped round the exterior of the 

 same tube, and finally made contact with the mercury column. 

 From observations with this tube it was found that although 

 the electric force was not uniform across the section, yet the 

 variations did not appear to be of themselves sufficient to 

 account for an apparent negative force being found near the 

 anode *. In conclusion my best thanks are due to Prof. 

 Thomson for the very practical interest he has taken in the 



investigation. 



Cavendish Laboratory. 



XXII. Notices respecting New Books. 



A Treatise on the Theory of Screws. By Sir Robert Stawell 

 Ball, LL.D., F.E.S. Cambridge : 1900. 



< A TEEATISE on the Theory of Screws ' is the title of a 

 -^- sumptuous volume of 544 pages published at the Cambridge 

 University Press. It is superfluous to praise the printing and 

 get-up of the new series to which this work belongs. It is 

 sufficient to remark that, so far as we have seen, the present 

 volume is singularly free from errors, clerical or of any other kind. 

 It may appear ungenerous to confess that we are somewhat 

 disappointed w T ith the frontispiece — the Central Portion of the 

 Cylindroid ; but the beauty of the book has made us fastidious, 

 and very possibly the beauty of a model formed of silver wires 

 could not be reproduced bv any photographic process. 



In 1876 appeared a small octavo volume of less than 200 pages : 

 ' The Theory of Screws : a Study in the Dynamics of a Eigid 

 Body.' The subject has grown since that date, as we find on 



* Owing to lack of time I could not investigate this as fully as I 

 wished. 



