518 OF THE PHYSICAL STRUCTURE 



possess the concurrent testimony of the best observers and histo- 

 rians of these events, particularly Michell, Dolomieu, Lyell, and 

 Darwin. Michell, writing on the subject of " The Cause and Phe- 

 nomena of Earthquakes," in the Philosophical Transactions for 

 1760, says, that the motion of the earth is partly tremulous, and 

 partly propagated by waves, which succeed one another at larger 

 and smaller distances, the undulation extending much further than 

 the tremor. At Jamaica, in 1687-8, a gentleman saw the gi-ound 

 rise, like the sea, in a wave, as the earthquake passed along, and 

 lie could distinguish the effects for some miles, by the waving of 

 the ti-ee-tops on the hills. The §a.me was witnessed in New Eng- 

 land, November 18th, 1755. The wave-like motion of the great 

 Lisbon earthqualce, which happened on the fu-st of November, 

 1755, was perceived by the motion of water, and the hanging 

 branches in churches through all Germany, amongst the Alps, 

 in Denmark, Sweden, Norway, and all over the British islands. 

 This tremendous movement even reached the West Indies, a 

 distance from the seat of principal violence, of nearly three thou- 

 sand miles. A comparison of the times at which the first shock 

 was felt at Lisbon and at other places, shows the undulation to 

 have ti-avelled at the rate of more than tiventy miles per minute. 



Dolomieu, in his dissertation on the great Calabrian earthqualve, 

 states, according to JVIr. Lyell, that " the surface of the country 

 often heaved like the billows of a swelling sea, which produced 

 a swimming in the head like sea-sickness," and he further men- 

 tions as " a well-known fact, that the trees sometimes bent during 

 the shocks to the earth, and touched it with then* tops."* This 

 rocking motion of the surface was likewise experienced by Darsvin, 

 in South America, who states, on the authority of Acasto, that 

 the earthquakes of that country extend three hundred, six hun- 

 dred, nine hundred, and some of them even one thousand five 

 hundred miles along the coast.t 



That this motion is of the nature of an actual bUlowy oscilla- 

 tion of the crust, is likewise plainly indicated by the attendant 



* See Lyell's Principles, Boston edition, vol. 2. p. 330. 



t See a paper by Darwin, in Transactions of Geological Society of London 



