36 



EARTHQUAKES. 



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apparatus will therefore require, not only the means for 

 stopping a clock, but also a contrivance which, at the same 

 instant that the clock is stopped, shall make a mark on a 

 record which is being drawn by a seismograph. In this 

 way we find out at which portion of the shock the time 

 was taken. 



Palmieri stops a clock in his seismograph by closing 

 an electric circuit. Mallet proposes to stop a clock by 

 the fallmg of a column which is 

 attached by a string to the pen- 

 dulum of the clock. So long as 

 the column is standing the string 

 is loose and the pendulum is free 

 to move ; but when the column 

 falls, the string is tightened and 

 the pendulum is arrested. The 

 difficulty which arises is to obtain 

 a column that will fall with a 

 slight disturbance. The best 

 form of contrivance for causing a 

 column to fall, and one which 

 may also be used in drawing out 

 a catch to relieve the machinery of a record-receiver, is 

 shown in the accompanying sketch. 



s is the segment of a sphere about 4*5 cm. radius, 

 with a centre slightly above C. L is a disc of lead about 

 7 cm. in diameter resting upon the segment. Above this 

 there is a light pointer, p, about 30 cm. long. On the 

 top of the pointer a small cylinder of iron, w, is balanced, 

 and connected by a string with the catch to be relieved. 

 When the table on which WPS rests is shaken, rotation 

 takes place near to C, the motion of the base S is magni- 

 fied at the upper end of the pointer, and the weight 

 is overturned. This catch may be used to relieve a 



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Fig. 7. 



