and Recording Earthquake-Motions. 209 



late the position of the float. An instrument of this kind was 

 made for water as the floating medium, and gave results which 

 justify the assumption that, by causing it to write on a moving 

 plate, it is capable of giving valuable information as to the 

 magnitude of the vertical movement. This instrument has, 

 however, the disadvantage of being very large, and conse- 

 quently inconvenient; and I propose therefore to use mercury 

 instead of water. Fig. 5 indicates a form which may be 

 adopted in such a case. 



Referring to the figure, F is a varnished, or otherwise pro- 

 tected, lead float placed in a wood or iron vessel, V, filled with 

 mercury. A fine wire is fixed to the lower end of the float, 

 and, passing loosely through a hole in the plate p, serves to 

 prevent the float from turning round. A weight, W, sufficient 

 to completely submerge the float is applied to the stem above 

 the surface of the mercury. To the upper end of the stem a 

 a wire or thread, W, is attached, which is passed through a 

 small hole in the top of the vessel V, and then over the pulley 

 I, to which the index, i, is attached. With this arrangement 

 the period of vertical oscillation can be made very long; and 

 consequently each individual motion of the earth can be re- 

 corded on a moving surface, such as a drum D, relatively to an 

 undulating line clue to the slow up-and-down movement of the 

 float. By using the pulley /, the sensibility of the instru- 

 ment is made constant for any position within the range of the 

 motion. 



VII. Compensated Spring Astatic Seismograph for Vertical 



Motions. 

 The following is a description of another instrument which 

 I have contrived for registering vertical motions, and which 

 seems well adapted for that purpose. A description of it, with 

 some account of my experiments with various arrangements 

 leading to the form adopted, was given at the meeting of the 

 Seismological Society of Japan, on the 28th of April of this 

 year. The instrument is shown in fig. 6, which is a section 

 through the frame, and shows in elevation the acting parts of 

 the apparatus. A vertical spring, S, is fixed at its upper end 

 by means of a nut, n, which rests on the top of the frame F, 

 and serves to raise or lower the spring through a short dis- 

 tance as a last adjustment for the position of the cross arm A. 

 The arm A rests at one end on two sharp points, p, one resting 

 in a conical hole and the other in a V-slot; it is supported at 

 B by the spring S, and is weighted at C with a lead ring, R. 

 Over a pin at the point a stirrup of thread is placed which 

 supports a small trough, t. The trough t is pivoted at a } nas 



