22 EARTHQUAKES. 



needle, capable of sliding freely up and down, marked the 

 relative horizontal motion of the earth and the pendulum 

 bob on a smoked glass plate. 



It generally happens that at the time of a moderately 

 severe earthquake the whole of these forms of apparatus 

 are set in motion, due partly to the motion of the point 

 of support of the pendulum, and partly to the friction of 

 the writing point on the plate. 



Among these pendulums may be mentioned those of 

 Cavallieri, Faura, Palmieri, Rossi, and numerous others. 

 It is possible that the originators of some of these pen- 

 dulums may have intended that they should record by 

 swinging. If this is so, then so far as the determination 

 of the actual nature of earthquake motion is concerned, 

 they belong to a lower grade of apparatus than that in 

 which they are here included. 



A great improvement in pendulum apparatus is due to 

 Mr. Thomas Gray of Glasgow, who suggested applying so 

 much frictional resistance to the free swing of a pen- 

 dulum that for small displacements it became ' dead beat.' 

 By carrying out this suggestion, pendulum instruments 

 were raised to the position of seismographs. The manner 

 of applying the friction will be understood from the 

 following description of a pendulum instrument which is 

 also provided with an index which gives a magnification 

 of the motion of the earth. 



B B B B is a box 113 cm. high and 30 cm. by 18 cm. 

 square. Inside this box a lead ring K, 17 cm. in diameter 

 and 3 cm. thick, is suspended as a pendulum from the screw 

 S. This screw passes through a small brass plate P P, which 

 can be moved horizontally over a hole in the top of the 

 box. These motions in the point of suspension allow the 

 pendulum to be adjusted. 



Projecting over the top of the pendulum there is a 



