ON EARTH TREMOBS. 321 



ment. Bat, duinng tlie observations afc Wilhelmsliaven, Professor 

 Boero;en at my request had readings taken twice a day of the level of the 

 meridian circle to see if the daily oscillation coald be noticed there as 

 well. The result of one month's observations was that there was no 

 appi'eciable difference. 



The pier of the meridian circle rises from a mass of sand, which 

 forms the subsoil of the marshy ground round Wilhelmshaven, down to 

 a depth of moi-e than 250 m. The walls of the observatory as well as the 

 column of the pendulum have their foundation in a layer of clay of 

 2 m. or 3 m. thickness, which is divided from the sand by a layer of 

 turf-like material, about 1 m. thick. Perhaps, if one does not prefer to 

 attach no importance to the indications of the water level when treating 

 of so small angular quantities, we may conclude from the above result 

 that only the upper layer of clay, which presses on the elastic turf, par- 

 takes of the oscillation. 



III. The Motion of the Zero-point. — l^ext to the daily oscillation a par- 

 ticular interest is attached to the motion of the zero-point, for by studying 

 it we may hope to learn something about the slow secular changes which 

 we know take place constantly all over the earth, though the number 

 of places is small where they are noticeable to the eye within a small in- 

 terval of time. For this purpose it is desirable to have an uninterrupted 

 series of observations, or at least, where interruptions are unavoidable, 

 as will probably always be the case with such a delicate instrument, to 

 determine by other means as well as possible the motion of the zero-point 

 during the interval. 



I have pointed out before that the present observations, with the 

 exception of those made at Puerto Orotava, are incomplete in this 

 respect, for they were interrupted once in June, and at Wilhelmshaven 

 many days were missed in May. Notwithstanding, some results of 

 interest have been gathered. The daily oscillation having been deter- 

 mined, two numbers were interpolated from the readings for every day, 

 corresponding to the moments when the periodical deflection is equal to 

 zero, and curves were drawn and compared with the curves of tempera- 

 ture and barometric pressure.' 



(1) At Wilhelmshaven a remarkably strong effect is produced by the 

 changes in the barometer. When the pressure increases the pendulum 

 moves towards the east, thus indicating a sinking of the soil in the same 

 direction, and a change of 1 mm. in the former corresponds with a change 

 of 0"-29 in the direction of the plumb-line. Apparently both changes 

 take place .simultaneously, or, if the changes in the position of the pen- 

 dulum lag behind those of barometric pressure, the difference must be 

 very small. 



Thus, the horizontal pendulum in this particular case acts as a most 

 delicate barometer, and might be used as such if it were not subject to 

 other influences. As the atmospheric pressure presents a daily oscilla- 

 tion, it is necessary in discussing the daily period of the plumb-line to 

 take the former into account. 



(2) The correspondence between the pendulum and the barometer is 

 most pronounced when the mean temperature remains pretty constant 

 during a number of days, but with sudden changes of temperature the 

 effect of the latter is seen to prevail over the former. From all the 



' See the reproduction of a part of these curves in my paper, Bas Horizontal- 

 pendel, &c. 



1893. T 



