and eipecially in Scotland, 275 



nearer the source of violence, but more solid in geological 

 structure. The reason is probably to be found in the greater 

 effect which the vibrations have upon materials which are not 

 firmly attached to the earth's crust, and are of a soft and 

 pliable texture. If a table be struck from below with the hand, 

 all loose objects upon it will start up. On this principle it is 

 easy to see why slates on the tops of houses should be heard 

 to clatter, how articles of furniture should be knocked over, 

 and the crew in a ship struck with the shock of the Lisbon 

 earthquake should have been thrown off the deck. 



But a question may here be asked, are the undulations 

 of the earth's surface produced by vibrations, or (as many 

 eminent men maintain) are they produced by corresponding 

 undulations in the liquid nucleus on which the earth's crust is 

 supposed to float "? This is a question of which no direct so- 

 lution is afforded by the register, and which demands obser- 

 vations much more exact than any hitherto made. It is, 

 however, not immaterial to observe, that, whilst the one of 

 these theories involves a purely hypothetical and very doubt- 

 ful cause, the other is ascribed to an agent which is known 

 not only to exist, but to accompany the undulations. That 

 the undulations in Holland during the earthquakes of 1st No- 

 vember 1755 and 31st March 1761 were accompanied by strong 

 vibrations, is evident from what has been already said, and 

 likewise from the following statements : — " At Harlem, for 

 near four minutes together, not only the water in the rivers, 

 canals, &c., but also all manner of fluids in smaller quantities, 

 as in coolers^ tubsy vats^ «Sjc., were astonishingly agitated and 

 dashed over the sides, notwithstanding no motion was per- 

 ceptible in the containing vessels. In such small quantities 

 also, the surface of the fluid had apparently a direct ascent."* 

 At Leyden " the same motion Avas perceived in the water of 

 the backs (vats?) of two brew-houses.'"t 



In considering the possibility of an undulation being pro- 

 duced by a mere vibration transmitted upwards, it must be 

 remembered that the vibration operates at any particular mo- 



* Discourses ou Earthquakes, p. 309. t Ibid. p. 310. 



