Chemistry and Physics. 395 



the slab is parallel to the axis of the bar and hence at right angles 

 to the pin. 



Let M denote the mass of the pendulum bob. When the sys- 

 tem is at rest the normal "pressure" on the slab is given by 

 P z= Mg sin a. When the apparatus is moving with an hori- 

 zontal acceleration x the pressure is P = Mg sin a -\~ Mx cos a. 

 In order that the pin may always remain in contact with the 

 slab it is necessary to choose the angle a so as to satisfy the 

 inequality tan a > w m /g, where w m is the greatest absolute 

 value of the acceleration for the registering of which the appa- 

 ratus is designed. Since g is relatively large the angle a is 

 usually small. Let p = P -P , and write w in place of 'x, then 

 p = Mw cos a. ' ' Thus the pressure p is always proportional to 

 the acceleration, quite independently of the form of the function 



Accordingly the problem is now reduced to that of finding an 

 experimental arrangement which would make it possible to 

 determine directly the instantaneous value of p. The phenom- 

 enon of piezo-electricity enables this to be accomplished without 

 difficulty, for, as is now well known, if a plate of quartz or 

 tourmaline be placed between two metallic sheets and subjected 

 to pressure, a free electric charge will appear on the sheets. 

 Moreover, within wide limits, the magnitude of the charge is 

 directly proportional to the pressure. The details of the elec- 

 tric connections are too simple and obvious to merit full descrip- 

 tion in this place. Suffice it to quote that : ' ' The most suitable 

 instrument for measuring the charge is Luts-Edelmann's string 

 electrometer." The motions of the string are easily recorded 

 photographically by the aid of simple optical accessories. The 

 numerical data and photographic curves given in the original 

 paper are quite consistent with the following conclusions stated 

 by the author : — ' ' The apparatus described above for the direct 

 determination of accelerations proves to satisfy fully the object 

 for which it was designed. It gives directly, without any appre- 

 ciable retarding effect, the instantaneous value of the accelera- 

 tion, however arbitrary the type of motion may be. It does not 

 introduce any oscillations of its own, it has very small inertia, 

 and does not manifest any fatigue. Its sensitiveness may be 

 regulated as desired." — Proc. Roy. Soc., 95 A, 492, 1919. 



h. s. u. 



II. Geology. 



1. Shore Processes and Shoreline Development; by Douglas 

 Wilson Johnson. Pp. xvii, 584; 73 pis., 149 text figs. New 

 York, 1919 (John Wiley & Sons).— In this good book is clearly 

 described the cycle of shore processes that develops the various 

 kinds of shorelines along the oceans. The author proceeds logi- 

 cally in his study, from the cause and internal nature of water 



