262 Mr D. Milne on Earthquake-Shocks in Great Britain^ 



occurrence, experienced the concussion in a very remarkable 

 manner. Her *' anchors, which were lashed (in the usual place), 

 bounced up, and the men were thrown a foot and a half along 

 the deck ; and of a sudden, the ship sunk in the water as low 

 as her main chains."*'* Another ship at sea about 120 miles 

 south of the one last mentioned, was struck about the same 

 time, " and the compass was overturned in the binnacle.' 't 



The effects now referred to, must be attributed to a vibra- 

 tion propagated from the subterranean parts, probably from 

 one spot, and radiating towards the surface of the earth in all 

 directions. It may be reasonably supposed, that the part of 

 the earth's surface which is vertically above the source of this 

 vibration, will be most violently affected by it,' and that those 

 parts to which the vibrations rise obliquely, and which they 

 reach only after passing through a greater mass of strata, will 

 experience a less violent shock. 



It is evident, that if the vibration in question were to emanate 

 from the earth's centre, it would reach the surface in all places 

 about the same time, and with equal degrees of violence, suppos- 

 ing the earth's interior structure to be homogeneous throughout, 

 or at equal depths. If, therefore, one and the same shock is 

 felt at different places, at times and with degrees of intensity 

 which vary with the distance from the point where it was most 

 intense, the shock must have originated short of the earth's 

 centre ; and the greater the disparity in the times of its arrival 

 and the intensity at different places, the nearer to the earth's 

 surface the source of the vibration must be. If it were known 

 at what rate such vibrations are propagated through the earth's 

 crust, the observance of the above circumstances would lead to 

 a discovery of the depth at which, in any case, a shock origi- 

 nated. 



Some illustration of these remarks is afforded by the Lisbon 

 earthquake of 1755, and another notice in the register under 

 date 31st March 1761. As the best method of exhibiting the 

 facts now referred to, they have been thrown into the form of 

 tables. 



* Discourses on Earthquakes, London, 1757, p. 333. t Ibid. p. 331. 



