142 Mr D. Milne Ofi Earthquake-Shocks felt in Great Britain^ 



fowl flying at a little distance from him." — " Finding that it 

 continued, and seemed approaching him, he stood still, lean- 

 ing on his fishing-rod, to observe the phenomenon, and heard 

 the sound distinctly approaching, until it seemed to surround 

 him, when he felt the ground tremble under him." *' He 

 states the sound as resembling the sound of muir-burn ; or the 

 rattling, crackling, and hissing noise made by a large extent 

 of heather on fire." 



From these accounts, it is plain that there are connected 

 with the earthquake-shock, sounds both in the earth and in the 

 air, and which are apparently quite distinct and independent. 

 It is true that the noise generated in the earth must produce 

 some effect on the air, otherwise it would not be heard ; and, 

 therefore, it may be thought that this explains the perception of 

 a noise in the air. But this would be an erroneous inference. 

 If the sound generated in the earth is distinctly perceived to 

 come from the earth, it cannot be confounded with any aerial 

 noise ; and it is impossible that the ear should make a mistake, 

 in regard to the place from which the sounds proceed. If 

 this be the case when the sounds are similar, much more must 

 it be the case when they are so dissimilar, as the subterranean 

 and the aerial sounds above described appear to be. 



The next question is, What is the cause of this two-fold 

 sound ? 



It is stated in almost all the reports, that the noise pre- 

 cedes as well as follows the movement of the ground ; that 

 it gradually dies away ; the moment of greatest loudness 

 coinciding with the moment of greatest movement in the 

 ground. 



It may be inferred from this, that what produces the 

 movement in the ground produces also the sound in both the 

 earth and air. The former sound may be accounted for 

 merely by the commotion of the strata of gravel and soil 

 caused by the shock. But how can the sound or report heard 

 in the air be produced X What is the particular agent which 

 produces the undulations of the earth's surface, and which, 

 when passing into the atmosphere, produces the peculiar noise 

 heard there % A satisfactory answer to this question would 



