ON EARTH TREMORS. 



295 



angle whose circular measure is 27i/(2« — & tan o). In the present case 

 a=14-36 inches, 6= -56 inch, o=44°, and the number of threads on 

 the screw is 100 to the inch. One complete turn of the screw will 

 therefore tilt the frame through an angle of 146-4 seconds. A fractional 

 turn of the screw may be made by the lever, f, which can be clamped to 

 it by a screw in front. The lever ends below in a rectangular plate with 

 ends projecting outwards ; and, by two screws working through these 

 ends, and abutting against the stops, G, the range of the lever's move- 

 ment can be regulated. The length of the lever is 335-5 mm., so that 



Fig. 4. 



.-O. 



with the range of movement of 144 mm. of the bottom of the lever the 

 tilt given to the frame is exactly one second. 



It will be useful to give the general arrangement of the instrument 

 now in use in Birmingham. It is erected in a cellar, the plan of which 

 18 shown in fig. 4. The pendulum is placed at A on a foundation de- 

 scribed below. The line A B runs east and west. Ten feet west of the 

 pendulum a board, b, is placed on the floor of the cellar. This board, 

 which was used in the experiments at Cambridge, contains a pair of 

 rails on which a carriage with three legs slides, bearing a gas-jet. On 

 the east side of the carriage is a screen, pierced by a circular hole, one 

 inch in diameter, having on the west side a piece of ground glass, and 



