ON EARTH TREMORS. 333- 



The general form of the true oscillation of the plumb-line is probably 

 very nearly represented by an ellipse whose large axis lies between the 

 two directions E.W. and N.W.-S.E. 



The daily oscillation of the pendulum at Strassburg is quite insig- 

 nificant when compared with the enormous changes of the zero-point. 

 During the first fortnight, when a change of 1 mm. in the position of the 

 light-point was equivalent to a deflection of 0"'027 of the plumb-line, the 

 pendulum was moving towards the north. Between April 4 and 18 it 

 travelled through G"'4,' when it stopped, and a southward motion began, 

 which was very considerable during the whole of last summer, for the 

 followino: angles were described : — 



Between April 18 and May 5 — 143G (motion south) 



May 8 „ June 1 -1312 



July 18 „ July 27 -1045 



July 27 „ Sept. 2 -10-55 



Sept. 2 „ Sept. 18 - 7-34 



The same motion continued with a varying rate and occasional 

 stoppings during the rest of the year ]892 and the beginning of 1893. 

 The total angular displacement is probably very nearly 2 minutes when 

 the intervals are taken into account during which observations were 

 missed. Since the early months of this year a reaction seems to have 

 taken place, for a slow northerly motion has commenced, and now and 

 then the pendulum remains in very nearly the same position during 

 several days. I believe that the extraordinary motion of the pendulum, 

 which far exceeds anything one could have expected beforehand, con- 

 sidering its favourable foundation, is due to two causes — a very con- 

 siderable tilt of the column from the north to the south and a large 

 annual oscillation which augments the southerly motion during the later 

 part and retards it during the earlier part of the year. 



If the tilting is due to a general motion of the ground it must afiect 

 the meridian circle, which is at a short distance to the east of the transit 

 instrument. A comparison will be made as soon as the nadir observa- 

 tions are available. In November 1892 Professor Becker had a water 

 level attached to the other side of the pillar, which was read twice a day. 

 As it is a well-known fact that water-levels are not very reliable for this 

 sort of observations, it will not be surprising to hear that the readings 

 of the level, although they agree in a general way with the motion of the 

 pendulum, differ from it in many details. 



The temperature of the cellar is read twice a day because it is subject 

 to considerable variations. Owing to the form of the building of the 

 meridian circle, in which the instruments are placed one storey high,, 

 the cellars below the observing rooms are not underground, and in 

 winter during the severe cold the temperature fell below zero (C), 

 An effect of temperature is certainly indicated, and will require a careful 

 examination. 



It may be mentioned that the pillar which carries the transit instru- 

 ment, and to which the pendulum is attached, is not massive throughout. 

 A horizontal section through the middle of it presents the following 



figure M-^. The cylindrical mantel stands on the same foundation as the 



middle part, and is probably rigidly connected with it in its lower and 



' When speaking of the angular motion of the pendulum the corresponding^ 

 motion of a vertical pendulum is always meant. 



