T. GRAY ON A SEISMOGRAPHIC APPARATUS. 223 



direction of motion at any one point cannot be taken as giving at 

 all reliable evidence of the direction of propagation of the shock. 



Discussion. 



The Eev. E. Hill remarked that it was desirable that observa- 

 tions of this kind should be made in this and other countries in 

 which only small earthquake-movements take place. He called 

 attention to the necessity of distinguishing the motions directly due 

 to the shock from the subsequent swinging of the pendulum. 



Prof. Judd asked if arrangements were made in this instrument 

 for recording several earthquakes in succession. 



Prof. Hughes asked whether the instrument had been tried along- 

 side of others differently constructed, and whether the results ob- 

 tained were the same. It seemed more complicated than the instru- 

 ments used in the Vesuvius Observatory ; and he would be glad if 

 Mr. Gray would explain the advantages of his arrangement for 

 dipping the metal point in the mercury over that adopted by Palmieri, 

 in which the weight carrying the point is simply suspended by a 

 spiral spring attached to the solid framework. 



The Author, in reply, stated that the swing of the pendulum was 

 of such a long period, as compared with that due to the earthquake, 

 that they could not be confounded with one another. The instru- 

 ment would record any movements between ^ - of an inch and j of 

 an inch ; the multiplication might be varied from 4 to 7. No pro- 

 vision was made for the occurrence of more than one earthquake in 

 the same night ; but such repetitions of earthquake-shocks were rare 

 in Japan. The instrument had not yet been employed anywhere 

 in its present combination. In reply to Mr. Evans he stated that 

 an instrument of this kind would cost about <£45. 



