ON THE FACTS OF EARTHQUAKE PHZ NOMENA. 19 
δὲ some considerable depth below the surface of the earth will produce a 
_ vibratory motion, so a very large quantity (whether it be generated almost 
instantly or in any small portion of time) will produce a wave-like motion: 
the manner in which this wave-like motion will be propagated, may in some 
_ measure be represented by the following experiment :— 
“Suppose a large cloth or carpet (spread upon a floor) to be raised at one 
edge, and then suddenly brought down again to the floor, the air under it, 
being by this means propelled, will pass along till it escapes at the opposite 
side, raising the cloth in a wave all the way as it goes. In like manner a 
_ large quantity of vapour may be conceived to raise the earth in a wave as it 
passes along between the strata, which it may easily separate in a horizon- 
_ tal direction, there being, as I have said before, little or no cohesion between 
_ one stratum and another; the part of the earth which is first raised being 
bent from its natural form will endeavour to restore itself by its elasticity, 
_ and the parts next to it beginning to have their weight supported by the 
vapour; which will insinuate itself under them, will be raised in their turn, 
till it either finds some vent, or is again condensed by the cold into water, 
_ and by that means prevented from proceeding any further.” 
_ Several successive waves, he then proposes to show, might be thus gene- 
rated, and their height will be greater the nearer they are to the point of 
their origin. 
In the third part of the paper he endeavours more minutely to describe 
_ the mechanism of the focus, as to how the water gains access; why the roof 
_ should fall in, &c., and applies some of the facts or fancies to the recorded 
secondary conditions of earthquakes, and to the fluctuations of the sea, 
which result from them. And in the seventh section, he shows, that by in- 
stigating the point of departure of various great sea-waves, when observed at 
nt points of arrival, after any great earthquake, whose origin is (as he 
ses that of all great earthquakes to be) under the sea, we may find 
point vertically over the focus of original disturbance. This he does as 
ects the great Lisbon earthquake, and shows a most remarkable percep- 
n of the nature of the motion of waves of translation, far more than the state 
exact knowledge of the subject at the time would have made us suppose 
ible: Lastly; he inquires whether it be then possible to determine any- 
= as to the depth of the focus of disturbance below the surface, but 
it can be only guessed at; but that, if we could carefully observe and 
ekon the thickness of upturned strata at some great volcanoes, we should 
ve at it. 
Such is Mitchell’s paper; which I have analysed at some length, from its 
mportatice. It contains much that is useful, mixed with the leading fallacy 
as to the nature of the earthquake wave of shock. 
Wo other works on the facts of earthquakes require to be mentioned, 
— The History and Philosophy of Earthquakes,’ and ‘ Mémoires Histo- 
et Physiques sur les Tremblemens de Terre,’ par Mons. Bertrand, ἃ 
yes 1757. 
n the former; the facts of ten great earthquakes are recorded, and in 
atter; those of Switzerland; and all such others as the author could col- 
ouguer, in his ‘Voyage en Peru,’ whither he accompanied the French 
micians to measure an are of the meridian, conceives volcanoes and 
hquakes as one and the same; and “due to gaseous inflammations and 
losions. The weakest shocks are those from the earth already shaken ; 
strongest, those caused immediately by the inflammation, which are ana- 
us to the roarings of the volcanoes, and which are repeated more or less 
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SER LS TS πα τ σα 7 
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