Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/366

Rh It is highly probable, from the experiments of Wertheim and Breguet, on the linear and transversal vibrations, of stretched iron wire, ('Comptes Rendu,' t. xxxii., p. 293,) in which they found the rate of propagation of the latter, greater than that of the former, in the ratio of 4634 : 3485, that these small surface transversals, will be transmitted from $$r$$ to $$p$$ faster than the direct shock can be over an equal distance, and hence the time, during which the tremors may be felt, before the shock, be proportionably prolonged.

When to all these circumstances we add, those that heterogeneity of medium must introduce, as somewhat explained in Chaps. IV. and V., there will be no difficulty in explaining the cycle of observed wave phenomena, however complex.

It follows, from what has been stated in the preceding page, that no absolutely single shock, consisting of one clear, sharply defined blow, is physically possible, to an observer situated anywhere out of the centre of impulse.

Wherever the observer may be upon the earth's surface, he must experience a more or less "blurred" and confused shock, even though it were produced by an explosion in itself absolutely instantaneous.