28 



EARTHQUAKES. 



of the bracket, magnifies the motion of the stand, in the 

 ratio of A c : c N. 



£_J 



A^ 



3 



Fig. 3. 



In an instrument called a double-bracket seismograph, 

 invented by Mr. Gray, we have two brackets hinged to 

 each other, and one of them to a fixed frame. The planes 

 of the two brackets are placed at right angles, so as to 

 give to a heavy mass supported at the end of the outer 

 bracket two degrees of horizontal freedom. 



In all bracket machines, especially those which carry 

 a pivoted weight, it is doubtful w^hether the weight pro- 

 vides a truly steady point relatively to the plate on which 

 the record is written for motion parallel to the direction 

 of the arm. 



Parallel-motion Instrument — A machine which writes 

 its record as two components, and which promises great 

 g stability, is one suggested by 



Professor C. D. West. Like 

 the bracket machines it con- 

 sists of two similar parts 

 placed at right angles to each 

 other, and is as follows: A 

 bar of iron A is suspended 

 from both sides on pivots at 

 C C by a system of light arms hinging with each other at 



d 



e <•'! 



S 



m a I I 



Fig. 4. 



