916 Mr. A. G. Shenstone on the Effect of an 



ordinary earthquakes occurs about or shortly before noon, 

 and that of: after-shocks about or shortly after midnight. 

 The diurnal periodicity of ordinary earthquakes is probably 

 due to more than one cause; that of after- shocks mainly to 

 the diurnal variation of barometric pressure. 



(iv.) Sena-diurnal Period. — The first maximum epoch of 

 the semi-diurnal period of barometric pressure (the amplitude 

 of which does not differ greatly from that of the diurnal 

 period) occurs about 9 a.m., and that of wind-velocity about 

 2 a.m. As the corresponding epoch for ordinary earthquakes 

 occurs about 8 or 9 a.m., it is probable that the semi-diurnal 

 periodicity of earthquakes is due to that of barometric 

 pressure. In the case of after-shocks, the epoch varies much 

 but, after the lapse of some time, the influence of the pressure 

 variations tends to prevail over other causes. 



LXXXVII. The Effect of an Electric Current on the Photo- 

 Electric Effect. By Allen G. Shenstone, M.A.* 



DURING the course of an experiment to determine 

 whether there was any relation between the Hall 

 effect in bismuth and the photo-electric effect, it was found 

 that the total photo-electric current was so much influenced 

 by the current passing through the bismuth that the original 

 experiment had to be abandoned. 



To determine the variation of this apparently new effect, 

 an apparatus, shown in the attached sketch (fig. 1), was con- 

 structed, and gave consistent results. 



The bismuth plate was kept at a negative potential of 

 14 volts, which gave practically saturation of the photo- 

 electric current. The electrometer was used with india-ink 

 resistance of 5(10) 8 ohms to give a sensitivity of, roughly, 

 (10)~ 12 ampere per mm. As only relative variations were 

 being determined, no accurate determination of sensitivity 

 was made. 



The work was all carried out at the pressure given by a 

 glass Langmuir diffusion pump, in operation throughout the 

 experiment. 



During the first part of the work it vas difficult to know 

 what sort of interdependence to expect. This first work was 

 further confused bv the fact that ihe same plate of bismuth 

 was used for a considerable number of runs, and it was 

 afterwards found that one consistent run was all that could 



* Communicated bv Prof. E. P. Adams, Ph.D. 



