ON tHE MEASUREMENT OF THE LDNAU DlSTCEBANCE OF GRAVITY. 12 1 



doubt that the surface of the earth is in incessant movement, -with 

 oscillations of periods extending from a fraction of a second to a year. 



Whether it be a purely superficial phenomenon or not, this considera- 

 tion should be of importance to astronomical observers, for their instru- 

 ments are necessarily placed at the surface of the earth. M. Plantamour 

 and others have shown that there is an intimate connection between the 

 changes of level and those of the temperature of the air ; whence it follows 

 that the principal part of the changes must be superficial. On the other 

 hand, M. d'Abbadie has shown that it is impossible to explain all the 

 changes by means of changes of temperature. It would be interesting 

 to determine whether changes of a similar kind penetrate to the bottom 

 of mines, and Gruithuisen's suggestion of a catachthonic observatory 

 seems worthy of attention, although he perhaps went rather far in the 

 proposition that the observatory should be ten or fifteen miles below the 

 earth's surface. 



It may appear not improbable that the surface of the soil becomes 

 wrinkled all over, when it is swollen by increase of temperature and by 

 rainfall. If this, however, were the case, then we should expect that 

 instruments erected at a short distance apart would show discordant re- 

 sults. M. Plantamour, however, found that, at least during three days, 

 there was a nearly perfect accordance between the behaviour of two sets 

 of levels at three kilometers apart ; and during eight years there appeared 

 to be general agreement between the changes of level of the astrono- 

 mical instruments at Greenwich and Cambridge. It would be a matter 

 of much interest to determine how far this concordance would be main- 

 tained if the instrument of observation had been as delicate as that used 

 by M. d'Abbadie or as our pendulum. 



M. Plantamour speaks as though it were generally recognised that 

 one pier of a transit circle rises during one part of the year and falls at 

 another.' But if this be so throughout Europe, we must suppose that 

 there is a kind of tide in the solid earth, produced by climatic changes ; 

 the rise and fall of the central parts of continents must then amount to 

 something considerable in vertical height, and the changes of level on the 

 easterly and westerly coasts of a continent must be exactly opposite to 

 one another. We are not aware that any comparison of this kind has 

 been undertaken. The idea seems of course exceedingly improbable, but 

 we understand it to be alleged that it is the eastern pier of transit instru- 

 ments in Europe which rises during the warmer part of the year. Now 

 if this be generally true for Europe, which has no easterly coast, it 

 is not easy to see how the change can be brought about except by a 

 swelling of the whole continent. 



We suggest that in the future it will be thought necessary to erect 

 at each station a delicate instrument for the continuous observation of 

 changes of level. Perhaps M. d'Abbadie's pool of mercury might be best 

 for the longer inequalities, and something like our pendulum for the 

 shorter ones ; or possibly the pendulum when used in a manner which 

 we intend to try might suflfice for all the inequalities. 



' ' Dana roperation au moyen de laquelle on verifie I'horizontalite de I'axe d'une 

 lunette meridienne, il parait qu'on remarque bien un leger mouvement d'exhausse- 

 ment de Test pendant une partie de I'annee, mais il n'est pas aussi considerable que 

 celui qu'accuse mon niveau, et Ton n'a jamais remarque, que je sache, une oscillation 

 diurne comme celle qu'a indiquee le niveau dans le pavilion.' — Comptes Rendiis, June 

 24, 1878, vol. 86, p. 1525. 



