140 REPORT — 1895. 



and tremors undoubtedly show a close relationship, and therefore the former 

 may, by its mechanical action upon buildings, trees, and the surface of a 

 country, produce slight tremors, and influence the character of a record. 



The points which are marked in connection with the recent 

 obsei'vations are as follows : — 



1. Belationship of Tremors to Localities and Instruments, 



Tremors have been pronounced at Station A, the instrument at which 

 station, however, was the one most sensible to changes of level. At 

 stations on the surface in Tokio they have been feeble, but have varied 

 in their intensity. Underground upon the rock they have iiever been 

 observed. This latter observation, which is based upon records obtained 

 from five instruments, is in direct opposition to the observations made at 

 Rocca di Papa in Italy, whei-e, I understand, tremors are as pronounced 

 underground as they are upon the surface. 



At Station A, an instrument which showed ti'emors even in a more 

 marked manner than the large horizontal pendulum, was a similar instru- 

 ment made of a few millimetres of aluminium wire, a small mirror, and a 

 needle point, weighing only a few grammes, a comparatively large form of 

 which is described in the Report for 1892. On account of the manner in 

 which the spots of light reflected from the mirrors of a pair of these 

 instruments placed side by side would come to rest, and then start 

 suddenly to move in the same direction, I was led to the conclusion that 

 they were actuated by an intermittent tilting, and therefore that tremors, 

 rather than being elastic vibrations, had the character of wave-like 

 undulations.^ The fact that the instrument most sensitive to changes of 

 level gave the most pronounced records appeared to strengthen this sup- 

 position, and I was led to call these movements eaith pulsations. 



The only effect produced ))y heavy gusts of wind striking the building, 

 or the beating of a steam hammer at a distance of fifty yards, is to pro- 

 duce a temporary vibration in the pointers of an instrument ; but there is 

 no angular displacement, and consequent swing, which characterises the 

 movements during a tremor storm. 



An important observation made at Station A was that tremors were 

 produced when two tons of water were taken out of a well distant about 

 thirty yards. The operation caused the ground upon the well side to rise, 

 and the horizontal pendulum was gradually displaced in an opposite 

 direction. From this it may be inferred that either the pendulum took 

 up its new position intermittently, or that the level of the ground changed 

 intermittently. Whichevei- it may have been, it may be concluded that 

 whenever a rapid change in the inclination of the ground takes place, 

 horizontal and probably other pendulums may be caused to swing, and, as 

 will be seen in the next section, at least a portion of the tremor records 

 may be explained on the supposition that they are due to such causes. 



2. Relationship of Tremors to the Diurnal Wave. 



(1) Even when a horizontal pendulum is steadily following the diurnal 

 wave and no tremors are visible, slight tremors nearly always appear 

 about 6 or 9 a.m., just at the time when its easterly excursion has been 

 completed and it turns to commence a relatively rapid motion towards the 

 west (figs. 9 and 1.3). 



' The instruments were under the same cover. See Appendix. 



