Page:Great Neapolitan Earthquake of 1857.djvu/481

second. This, therefore, may be taken as the velocity of translation of the first shock here (through the limestone); the total time of its transit from the origin (surface velocity) is therefore

${\frac {33415}{240}}=139.230$ seconds;

and through the clays and gravels the whole time is

$139.230+1.750=140.980$ seconds.

The velocity per second of surface translation in the clays and gravels was therefore

${\frac {33415+402}{141''}}=239.84$ yards per second.

The shock through the limestone reached the Certosa from the Colline of Padula, the nearest point of rock in the path, through an intervening stratum of clays and gravels, by which there must have been some loss of vis vivâ. If we throw off the decimals for this correction, which we can only estimate, we have finally, for the surface velocity of translation, 240 yards per second in the limestone, and 239 yards per second in the clays and gravels, measures approaching so nearly to equality, as to warrant one or both of the following conclusions.

Either, the main primary or direct wave arrived, like the other through the limestone formations, deep beneath the clays and gravels of the piano, and shook the latter resting upon them, at their own rate of translation nearly; or (here at least), the bedding-joints and other breaches of homogeneity in the limestone rock produce a retardation of the wave transit therein, such as reduces the velocity to nearly that in the dense clays and gravels. In this ex-