and Recording Earthquake-Motions. 205 



been for some time used by Prof. Chaplin at the Tokio Uni- 

 versity. I have since heard from Prof. Chaplin that he ob- 

 tained the idea from a Zollner's pendulum. Prof. Chaplin was 

 no doubt the first in this country, perhaps the first anywhere, 

 who applied this certainly valuable arrangement to the measure- 

 ment of earthquake-motion. While talking over my experi- 

 ments, Mr. Milne expressed a strong desire to have a direct 

 record of the earthquake from some such arrangement; but a 

 suitable mode of obtaining it did not then suggest itself. 

 Since that time, however, it has occurred to me that two 

 levers hinged together will not only give this arrangement, 

 but will avoid at the same time one very serious source of 

 disturbance, namely the shock which the mass receives in con- 

 sequence of the direction of motion not being at right angles 

 to the direction of the lever. Acting on this idea, I made a 

 sketch of the arrangement described in this paper, and showed 

 it to Mr. Milne, who at once offered to give the instrument a 

 trial if I did not care to do so. In consequence of this, one of 

 these instruments has been in course of construction for some 

 time, but unfortunately has not yet (March 23) been finished 

 sufficiently for exhibition to the Society. 



The principle and form of this machine will be readily un- 

 derstood by reference to fig. 3. A post, P, resembling in all 

 respects a light gate-post, is fixed firmly in the ground; and 

 to it a species of gate, A, is hinged in a manner almost iden- 

 tical in form with that usually adopted. The hinge, H, is 

 capable of being moved backwards and forwards, its position 

 being regulated by the nut, N, and from side to side, its posi- 

 tion in that direction being regulated by two screws, s, put in 

 from opposite sides of the post. These adjustments are all 

 that are necessary, after the instrument is properly constructed, 

 to allow neutral equilibrium to be obtained. The lower pivot 

 H rests against the bottom of a conical hole cut in such a way 

 that the pressure will be along the axis of the cone. The 

 upper hinge H is a knife-edge resting against the side of a 

 round hole. The hinges IP, IP are exactly similar in con- 

 struction to H, H. 



The gate A is 15 centim. long, and 60 centim. high ; the 

 central piece is a round bamboo tube of about 4 centim. dia- 

 meter, and the cross pieces hard wood firmly fixed to the cen- 

 tral piece. By adopting a symmetrical form like the above, 

 the proper positions of the hinges H, IP are readily calculated, 

 while the tubular form of the vertical piece gives great tor- 

 sional rigidity with small weight. The front gate, B, is 

 similar in form but in every way lighter, as it does not require 

 to withstand a twisting force such as acts on A. The lower half 



Phil. Mag. S. 5. Yol. 12. No. 74. Sept. 1881. Q 



