104: T. C. Mendenhall — Seismoscopes and 



ance, and for slight movements the pivotal point within the 

 cylinder may be regarded as fixed. Rotation takes place about 

 this point and the motion of the earth, which is that of the 

 hook upon which the link is suspended, is magnified at the 

 point of the needle as many times as the ratio of the length of 

 the needle to that of the link. A small and very light lever 

 has its short arm bent upward, and is so adjusted that when the 

 instrument is " set," the upper end, ground to a fine point, rests 

 against the pointed end of the long needle. The longer and 

 heavier arm of this lever terminates in a platinum fork, the 

 prongs of which are vertically above two small mercury cups 

 forming terminals of the electric circuit. 



The operation of the instrument is simple. A very slight 

 movement of the point of suspension is magnified at the needle 

 point ; the short arm of the lever is released and the fork drops, 

 closing the circuit. Different degrees of sensitiveness are 

 obtained by grinding the abutting points to greater or less 

 dimensions. 



A seismoscope of great simplicity of design, and offering 

 many advantages, is one originally due to Milne,* and more re- 

 cently with slight modifications, experimented with by Hayden 

 and Hallock, of the U. S. Geological Survey, by whom it was 

 also, I believe, independently invented. It belongs to the 

 family of liquid seismoscopes, of which many varieties have 

 appeared, notably several devised and used by Palmieri. In 

 all previous forms, however, the action utilized was the move- 

 ment of the whole mass of liquid in relation to the containing 

 vessel, while in this, advantage is taken of the well known fact 

 that waves are, in general, produced upon the surface of a mass 

 of liquid when it is subjected to a sudden disturbance. In a 

 cylindrical vessel these waves run from circumference to center, 

 at which point the Jiquid is sensibly elevated for an instant, and 

 through this elevation an electric circuit may be momentarily 

 closed. Mercury is the liquid used, and it is placed in a small 

 cylindrical iron box, through the cover of which a pointed screw 

 with a large divided head runs. The point of the screw, which 

 is of platinum, may be brought extremely near the surface of 

 the mercury, its position and distance being known by means 

 of the divisions on the head. A slight jar generates a series of 

 waves, the elevation at the center completes the circuit through 

 the properly insulated screw. The tendency of the mercury 

 to become oxidized or dirty upon the surface so as to become 

 inoperative was overcome by the use of a small platinum float, 

 a device previously employed for measuring the height of the 

 mercury in the cistern of a barometer by electrical contact. A 



* Trans. Seis. Soc. Japan, vol. iii. 



