ON EAKTII TREMORS. 331 



following figure ' |/^ appears, and the deflection is equal to an 



angle of not mnch less than one second in the direction of the plumb-line. 



At Strassburg no strong deflections of a similar kind were noted 

 except during some earthquakes and when observations were made at 

 the transit circle, which is borne by the column to which the pendulum 

 is attached. These disturbances must be ascribed to vibrations of the 

 pillar, which are produced either by the earthquakes or by the observer 

 knocking against it, and which cause a slight change in the connection 

 between the pivots and agate cups of the pendulum. It remains, however, 

 to be explained why these displacements so often accompany earth- 

 quakes at Strassburg, whilst they were never noticed at any one of the 

 first three stations, although earth quake- motion of probably equal strength 

 was frequently observed there. Besides, it is remarkable that once when 

 a piece of iron was driven into the pillar on the side opposite to where th& 

 pendulum was placed, thus causing very considerable vibrations, the dis- 

 placement of the light-point was not larger than that which accompanied 

 one of the earthquakes. 



Observations at Strassburg. — When I returned from Tenerifle I wa» 

 anxious to continue the observations, but being myself unable to look 

 after the instrument I sent it to Professor Kortazzi, who wished to try it 

 at the Imperial Naval Observatory at Nicolaiew, and has been taking 

 observations in difierent positions of the pendulum since the spring ot 

 last year. At the same time I applied to Professor Becker, the Director 

 of the Strassburg Observatory, who very kindly oflered to take charge of 

 the other instrument which had remained at Wilhelmshaven and to place- 

 it in one of the cellars of the observatory. 



Before this was done the pendulum passed through the hands of 

 Messrs. Eepsold. It received new pivots, and the plane mirrors, which I 

 had used before, were replaced by concave silvered glass nairrors of nearly 

 1 m. focal distance. In making this change I followed the advice of Dr. 

 Eschenhagen at Potsdam, who had obtained beautifully sharp curves with 

 these mirrors. A circular mirror was cut in two, and one half was 

 ground down as much as possible to reduce its weight. But through this 

 process the focal length was slightly changed, and when the two halves 

 were afterwards placed one over the other it was impossible to make the 

 two reflected points of light equally sharp. At first the silvered surface 

 was soon spoiled through the moisture, but when the mirrors had been 

 resilvered and the ordinary precautions were taken to dry the air inside 

 the pendulum box, this difficulty was removed, and the same mirrors have 

 been in use without interruption since last July. Though the curvea 

 obtained at Strassburg are certainly finer than the older ones, I am not 

 sure that the same result might not be obtained by using the ordinary 

 plane mirrors if these were silvered on the front side. Besides it must be- 

 remembered that the distance between the mirror and the photographia 

 paper had to be reduced to more than one-half of the former distance, 

 which alone ensures a sharper image, and that it is very difficult to make 

 two such mirrors with exactly the same focal length when the latter is 

 large. 



In placing the instrument it was considered to be of special interest to 

 attach it to one of the large pillars of the observatory. The pillar of the 

 transit circle was selected and a stone table fixed into it on its east side^ 



