ON BAKTir TREMORS. 325 



soil. Tbis, like the surface of a lake, is sometimes at rest, and at other 

 times subject to undulations and vibrations, which have received the 

 general name of mioroseismic movements. The horizontal pendulum in 

 its present foi'm ofi'ers the same difficulty which is often felt by earth- 

 quake observers, viz., that it is often impossible to say whether the dis- 

 turbances visible on the curves have been produced by vibrations setting 

 the pendulum swinging, or by a repeated tilting. The latter, when ib 

 takes place slowly, and when the period of the tilt does not coincide with 

 that of the pendulum, would only produce deflections ; but as all micro- 

 seismic movement appears to be very complicated and variable, probably 

 deflections and swinging of the pendulum generally act together. Thus 

 we may explain the great variety of figures which are seen on the photo- 

 graphs and sometimes extend over several hours, thereby indicating that 

 earthquake motion, when it travels over large distances, spreads out more 

 and more on account of the difference in the rate of propagation. It is 

 well to remember hereby that in a few cases in which the effect of distant 

 eartliquakes was observed by astronomers, when they were occupied with 

 the levels of their instruments, the motion was nearly always seen to be 

 of an undulatory character, whilst at the centre of an earthquake vibratory 

 horizontal motion generally prevails. 



(1) Ohservations of Earthquakes. — The comparison of the curves ob- 

 tained simultaneously first at Potsdam and Wilhelm shaven, and later at 

 Strassburg and Nicolaiew, has shown that a very large percentage of the 

 observed disturbances was common to both places. In fact, it is a com- 

 paratively rare occurrence that when an earthquake figure, however 

 small, appears on one of the photographs, it is not equally visible on the 

 other. It often happens that the curves are not sufficiently distinct, 

 owing to variations in the intensity and figure of the light-point and 

 faults in the paper, or when a general microseismic movement is more 

 pronounced at one of the stations than at the othex. In such cases 

 small disturbances may at first escape detection, but are often found 

 when notice is given from the other station. 



The difference of time is well marked in many cases during the 

 observations in 1892-93, whilst in 1889 the distance was too small, con- 

 sidering the uncertainty connected with the readings. The distance 

 between Strassburg and Nicolaiew is nearly 2,000 kilometres, and yet 

 many of the observed disturbances appear to have lost none of their 

 intensity in passing over this distance. We are thus quite justified in 

 expecting that strong earthquakes will be observed by the horizontal 

 pendulum, wherever their centre may be on the earth. Of course it is 

 quite possible that the conditions for propagation are more favourable in 

 certain places than in others, and that, for instance, earth-waves travel 

 more easily across continents than when they have to pass over broad 

 oceanic tracts. 



The publication of the entire list of nearly 200 disturbances will, I 

 hope, lead to establish a greater number of relations between some of 

 these and observed earthquakes than I have yet been able to find. 

 Nevertheless, amongst the latter there are some cases of great interest, 

 of which I mention the following : the Japanese earthquakes of Tokio, 

 April 17, 1889, and of Kumamoto, July 28, 1889; the earthquakes of 

 Wjernoje (Central Asia), July 12, 1889, and of Patras in Greece, 

 August 25, 1889; the earthquake of San Francisco, April 19, 1892; 

 several of the Zante earthquakes, and the great Levant earthquake 

 which took place in the town of Malatia on February 9, 1893. 



