32 



EAKTHQUAKES. 



from such instruments, chiefly on account of the incon- 

 venience in making a spring sufficiently long to allow of 

 enough elongation to give a long period of vibration. 

 Similar remarks may be applied to the horizontally 

 placed elastic rods, one end of which is fixed to a wall, 

 whilst the opposite end is loaded with a weight. 

 Such contrivances, furnished with pencil on the weight 

 to write a record upon a vertical surface, were used in 

 1842 at Comrie, and we see the same principle applied in 

 a portion of Palmieri's apparatus. Contrivances like 



these neither give us the true 

 amplitude of the vertical mo- 

 tion, insomuch as they are 

 readily set in a state of oscil- 

 lation ; nor do they indicate 

 the duration of a disturbance, 

 for, being once set in motion, 

 they continue that motion in 

 virtue of their inertia long 

 after the actual earthquake 

 has ceased. They can only 

 be regarded as seismoscopes. 

 The most satisfactory in- 

 strument which has yet been 

 devised for recording vertical 

 motion is Gray's horizontal- 

 lever spring seismograph. 



This instrument will be 



better understood from the 



accompanying sketch. A 



vertical spring s is fixed at 



its upper end by means of a nut n, which rests on the 



top of the frame F, and serves to raise or lower the 



spring through a short distance as a last adjustment for 



Fig. 6. 



