100 KEFORT— 1882. 



he remarks, although there may sometimes be considerable agitation in 

 an earthquake, the true centre of disturbance may be very distant. 



In conclasion Rossi gives a description of a good method of making 

 a microphone. A common nail has a short piece of copper wire wound 

 round it, and the other end of the wire is wound round a fixed metallic 

 support. The nail thus stands at the end of a weak horizontal spring ; 

 but the nail is arranged so that it stands inclined to the horizon, instead 

 of being vertical. The point of the nail is then put to rest on the middle 

 of the back of a silver watch, which lies flat on a slab. The two elec- 

 trodes are the handle of the watch and the metallic support. He says 

 that this is as good as any instrument. The telephone is a seismological 

 instrument, and therefore, strictly speaking, beyond the scope of this 

 Report ; but as some details of its use have already been given, I will 

 here quote portions of an interesting letter by Prof. John Milne, of the 

 Imperial Engineering College of Tokio, which appeared in ' Nature ' for 

 June 8, 1882. Mr. Milne writes :— 



' In order to determine the presence of these earth-tremors, at the end 

 of 1879 I commenced a series of experiments with a variety of appai'atus, 

 amongst which were microphones and sets of pendulum apparatus, very 

 similar in general arrangement, but, unfortunately, not in refinement of 

 construction, to the arrangements now being used in the Cavendish 

 Laboratory. 



' The microphones wei'e screwed on to the heads of stakes driven in 

 the ground, at the bottom of boxed-in pits. In order to be certain that 

 the records which these microphones gave were not due to local actions, 

 such as birds or insects, two distinct sets of apparatus were used, one 

 being in the middle of the lawn in the front of my house, and the other 

 in a pit at the back of the house. The sensitiveness of these may be 

 learnt frora the fact that if a small pebble was dropped on the grass 

 within six feet of the pit, a distinct sound was heard in the telephone, 

 and a swing produced in the needle of the galvanometer placed in con- 

 nection with these microphones. A person running or walking in the 

 neighbourhood of the pits, had each of his steps so definitely recorded, 

 that a Japanese neighbour, Mr. Masato, who assisted me in the experi- 

 ments, caused the swinging needle of his galvanometer to close an 

 electric circuit and ring a bell, which, it is needless to say, would alarm a 

 household. In the contrivance we have a hint as to how earth- tremors 

 may be employed as thief-detectors. 



' The pendulum apparatus, one of which consisted of a 20-lb. bob 

 of lead at the end of 20 feet of pianoforte wire provided with small galva- 

 nometer mirrors and bifilar suspensions, were also used in pairs. With 

 this apparatus a motion of the bob relatively to the earth was magnified 

 1,000 times, that is to say, if the spot of light which was reflected from 

 the mirror moved a distance equal to the thickness of a sixpence, this 

 indicated there had been a relative motion of the bob to the extent of 

 1,000th part that amount. 



' The gi-eat evil which everyone has to contend with in Japan when 

 woi'king with delicate apparatus is the actual earthquakes, which stop 

 or alter the rate of ordinary clocks. 



' Another evil which had to be contended with was the wind, which 

 shook the house in which my pendulums were supported, and I imagine 

 the ground by the motion of some neighbouring trees. A shower of rain 

 also was not without its eSects upon the microphones. After many 



