ON THE MEASUREMENT OF THE LUNAR DISTURBANCE OF GRAVITY. 99 



houses and carriage-roads. It was protected against insects, and was 

 wrapped up in wool. Carpet was spread on the floor of the cave to deaden 

 the noise from particles of stone which might possibly fall. Having 

 established his microphone, he waited till night and then heard noises 

 which he says revealed ' natural telluric phenomena.' The sounds which 

 he heard he describes as ' roarings, explosions occurring isolated or in 

 volleys, and metallic or bell-like sounds ' [fremiti, scopii isolati o di 

 moschetteria, e suoni metallici o di campana] . They all occurred mixed 

 indiscriminately, and rose to maxima at irregular intervals. By artificial 

 means he was able to cause noises which he calls ' rumbling (?) ov 

 crackling ' [rullo o crepito]. The roaring [fremito] was the only noise 

 which he could reproduce artificially, and then only for a moment. 

 It was done by rubbing together the conducting wires, in the same 

 manner as the rocks must rub against one another when there is an 

 earthquake. 



A mine having been exploded in a quarry at some distance, the 

 tremors in the earth were audible in the microphone for some seconds 

 subsequently. 



There was some degree of coincidence between the agitation of the 

 pendulum-seismograph and the noises heard with the microphone. 



At a time when Vesuvius became active, Rocca di Papa was agitated 

 by microsisms, and the shocks were found to be accompanied by the 

 very same microphonic noises as before. The noises sometimes became 

 ' intolerably loud ; ' on one occasion in the middle of the night, half an 

 hour before a sensible earthquake. The agitation of the microphone 

 corresponded exactly with the activity of Vesuvius. 



Rossi then transported his microphone to Palmieri's Vesnvian ob- 

 servatory, and worked in conjunction with him. He there found that 

 each class of shock had its corresponding noise. The sussultorial shocks, 

 in which I conceive the movement of the ground is vertically up and 

 down, gave the volleys of musketry [i colpi di moschetteria], and the 

 undulatory shocks gave the roarings [i fremiti]. The two classes of 

 noises were sometimes mixed up together. 



Rossi makes the following remarks : ' On Vesuvius I was put ^ ii> 

 the way of discovering that the simple fall and rise in the ticking 

 which occurs with the microphone [battito del orologio unito al 

 microfono] (a phenomenon observed by all, and remaining inexplicable 

 to all) is a consequence of the vibration of the ground.' This passage 

 alone might perhaps lead one to suppose that clockwork was included in 

 the circuit ; but that this was not the case, and that ' ticking ' is merely 

 a mode of representing a natural noise, is proved by the fact that he 

 subsequently says that he considers the ticking to be 'a telluric 

 phenomenon.' . 



Rossi then took the microphone to the Solfatara of Pozzuoli, and 

 here, although no sensible tremors were felt, the noises were so loud as . 

 to be heard simultaneously by all the people in the room. The tickmg 

 was quite masked by other natural noises. The noises at the Solfatara 

 were imitated by placing the microphone on a vessel of boiling water. 

 Other seismic noises were then imitated by placing the microphone on a 

 marble slab, and scratching and tapping the under surface of it. 



The observations on Vesuvixis led him to the conclusion that the 

 earthquake oscillations have sometimes fixed nodes and loops, for there 

 were places on the mountain where no eflPects were observed. Hence, as 



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