Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/411

 A horizontal velocity, as low as even three or four feet per second, is sufficient to fracture some sorts of bad, ill-laid, and ill-cemented, masonry; the above difference of velocity in the two semiphases, would be in such an instance, a considerable fraction of the entire fracturing velocity.

To those accustomed only, to estimate force and velocity, by experience and the senses, the low mean velocity assigned in these pages to the earthquake wave, will probably give rise to sensations, of surprise, or of doubt. It may enable the unmathematical reader, therefore, better to estimate tactilely, the effects of a low velocity, to state, that the shock that such a velocity as that of this earthquake wave would communicate to him, if he were standing upon a solid floor, and the wave-path were vertically upwards, would feel the same, as if he had jumped down upon the floor from a height of 34 feet, alighting upon his heels and with his knees stiffened.

Fifteen feet per second of velocity, is above 10 English miles per hour. Let him imagine to himself what effect would be produced, upon a wall of 1 or 2 feet thick, and 8 or 10 feet high, if built transversely across a railway truck, which, moving along at the rate of 10 miles per hour, was suddenly stopped, by a fixed obstacle; such is quite analogous to the case of a like wall submitted to the shock of a normal wave of the earthquake of December, 1857.