330 REPORT— 1893. 



taken in the east-west plane, and not in the plane of the meridian, 

 ■which is nearly at right angles with the mountains and the coast line. 

 For this reason, perhaps, no effect was either noticed of the heavy surf 

 which is for ever beating against the rocky coast. 



At Strassbnrg tremors were also found to be considerably smaller than 

 at either of the first two stations. During the warmer season the light- 

 point appears to be much more steady than in winter, and altogether 

 strong tremors are a rare occuri'ence. A special investigation will be 

 made when the observations have come to an end. 



For an investigation of the earth pulsations and tremors little only can 

 be done with the present arrangement of the apparatus, which only offers 

 a means of studying the statistics of these phenomena. It is necessary to 

 use quickly moving paper and plates and a highly sensitive pendulum in 

 order to make the details visible. As the soil is often perfectly steady 

 through long intervals of time, to avoid taking useless photographs a 

 telescope ought to be added, through which it is possible to see whether 

 the pendulum is at rest or not. 



(4) Sudden Deflections of the Fendulum. — I have already mentioned 

 that the curves at Wilhelmshaven are so full of irregularities that some 

 parts may be compared with declination photographs of magnetic storms. 

 But whilst these are seen more or less every day, the following disturbances 

 are amongst the rarest occurrences. 



On April 9, 1889, at 6.36 p.m., the light-point at Potsdam suddenly 

 travelled through 8^ mm., which is equal to a deflection of 0"'34 

 of the plumb-line towards the west, thus forming the following figure 



It is evident that this deflection was not instantaneous. 



for if it had been so, the curve would have appeared broken, the second 

 part beginning with swings, as is the case whenever a deflection of the 

 pendulum is caused artificiallj^ But here there is no trace of swinging, 

 and the dark appearance of the line joining the two parts of the curve is a 

 proof that the light-point moved sufiiciently slowly to leave an impression 

 on the paper. 



Many small deflections are seen on other days at Potsdam as well as 

 at Wilhelmshaven. Amongst those observed at the latter place the 

 most important took place at 11 a.m. on August 9, and at 6.30 p.m. 

 on September 24. They are similar to the case described above, but the 

 line which joins the two parts of the curve is not so well marked, and 



the following figure is formed .W-L • -^iiother remarkable case 



was observed at Puerto Orotava on February 2, soon after 9 p.m., the 

 figure being as follows ^^/^^^ . The angular value of the deflec- 

 tion is 0""17, but in this case it lasted 1 hour 20 minutes before the light- 

 point had gained its new position, and the middle part of the figure is 

 just as dark as the rest. 



Whilst the above deflections were permanent, passing deflections 

 occurred at Wilhelmshaven, and were especially marked on July 12 

 at 7 a.m., and on September 12 at 6 p.m. In the latter case the 



