ON SEISMOLOGICAL INVESTIGATIONS. 



59 



The diurnal wandering at Bidston is small— miicli less conspicuous 

 than the tidal or semi-diurnal. At Shide, as described more fully below, 

 the semi-diurnal change is scarcely noticeable, while the diurnal is large 

 and is related to temperature. In view of the investigation which follows, 

 inquiry was made of Mr. Plummer as to what the temperature conditions 

 were. He kindly installed a thermograph and sent the trace for two 

 days, July 31 and August 1, 1915, which showed that the diurnal varia- 

 tion was on one day less than 1° F. and on the next about 1°.5, the 

 external range on the first day being 8° and on the second 14°. If this 

 may be interpreted to mean that the whole chamber is well shielded 

 from the effects of external temperature, the absence of a marked diurnal 

 effect is explained. 



At Shide the Milne -Shaw and Milne-Burgess instruments are side 

 by side on separate piers. Their booms are in opposite directions, and 

 when the lines on one are crowded together those on the other are wiclely 

 separated. There is no conspicuous tidal or semi-diurnal inequality, 

 but there is a very large diurnal inequality, which, though far from con- 

 stant in its action, usually crowds together the M.-B. trace and expands 

 the M.-S., so that it is apt to run off the drum and be lost. It will be neces- 

 sary to introduce some modification to meet this disability ; and, in order 

 to investigate its character and possibly to trace its cause, a number 

 of corresponding traces in March 1915 were measured. The quantities 

 tabulated in Table II. below are the measures corrected for the known 

 travel of the drum, so as to represent displacements of the trace from 

 its normal position. The reading at 11 a.m. is adopted as the zero, the 

 paper usually being changed at about 10 a.m. 



The numerical sums of the displacements are given at the feet of the 

 columns, as a very rough indication of the relative sensitiveness of the 

 instruments. The totals are 3,338 for the Milne-Shaw and 1,268 for 

 the Milne-Burgess, which are approximately in the ratio of the magnifica- 

 tions, viz., 300 and 100. But we shall find that this ratio is not repro- 

 duced in the systematic wanderings. 



Table II. 

 Displacements of Trace, in Units of O'Ol in. 



