ox THE MEASDREMENT OF THE LUNAR DISTURBANCE OF GRAVITY. 101 



months of tiresome observation, and eliminating all motions which by any 

 possibility conld have been produced by local influence, the general result 

 obtained was that there wei'e movements to be detected every day and 

 sometimes many times per day. . . . 



' A great assistance to the interpretation of the various records which 

 an earthquake gives us on our seismographs is what I may call a barri- 

 cade of post-cards. At the present moment Yedo is barricaded, all the 

 towns around for a distance of 100 miles being provided with post-cards. 

 Everyone of them is posted with a statement of the shocks which have 

 been felt. 



' For the months of October and November it was found from the 

 records of the post-cards that nearly all the shocks came from the north 

 and passed Yedo to the south-west. When coming in contact with a 

 high range of mountains, they were suddenly stopped, as was inferred 

 from the fact that the towns beyond this range did not perceive that an 

 earthquake had occurred. This fact having been obtained, the barricade 

 of post-cards has been extended to towns lying still farther north. The 

 result of this has been that several earthquake origins have, so to speak, 

 been surrounded or coralled, whilst others have been traced as far as the 

 seashore. For the latter shocks, earthquake-hunting with post-cards has 

 had to cease, and we have solely to rely upon our instruments. Having 

 obtained our earthquake centres, at one or more of these our tremor 

 instruments might be erected, and it would soon be known whether an 

 observation of earth-tremors would tell us about the coming of an earth- 

 quake as the cracklings of a bending do about its approaching breakage. 

 To render these experiments more complete, and to determine the exist- 

 ence of a terrain tide, a gravitimeter might be established. I mention 

 this because if terrain tides exist, and they are sufficiently great from a 

 geological point of view, it would seem that they might be more pro- 

 nounced and therefore easier to measure in a country like Japan, resting 

 in a heated and perhaps plastic bed, than in a country like England, 

 where volcanic activity has so long ceased, and the rocks are, compai'a- 

 tively speaking, cold and rigid, if an instrument, sufficiently delicate to 

 detect differences in the force of gravity, in consequence of our being 

 lifted farther from the centre of the earth every time by the terrain tide 

 as it passed between (sic) our feet, could be established in conjunction with 

 the experiments on earth-tremors.' 



The only account which I have been able to find of M. Bouquet de 

 la Grye's observations (mentioned in the last Report) is contained in 

 the ' Comptes Rendus ' for March 22, 1875, page 725. M. Bouquet 

 writes : — 



' . . . The observation of the levels of our meridian telescopes put us 

 on the track of a curious fact. Not only is Campbell Island subject to 

 earthquakes, but it also exhibits movements when the great swell falls in 

 breakers on the coast. I thought that it would be interesting to study 

 this new phenomenon. The instrument, which was quickly put together, 

 consisted of a steel wire supporting a weight, to which was soldered a 

 needle ; the movements of the weight were amplified 240 times by means 

 of a lever ; by passing an electric current through this multiplying 

 pendulum, which was terminated at the bottom by a small cup of 

 amalgamated tin, regular oscillations of yJ^y^jth of a mm. could be re- 

 gistered. I propose to repeat these observations with a pendulum of 



