vature, carries them upwards beyond the line of their passive position, at the same moment, that the second semi-vibration relieves the load upon them, by its inertia acting against gravity. The consequence is, the middle parts of a tolerably large floor, spring up with amazing velocity and power, beneath those who may be upon it, furniture is thrown upwards and towards the walls, tiles are projected upwards from the floor surface, masses of the concrete sometimes dislodged, and persons standing or moving on the floor are thrown upwards, and lose their balance.
Such is commonly the source of the strong impression of those who have experienced steeply emergent shocks, of an upward movement, unbalanced by any corresponding downward one, to which the title "sussultatoreo" is usually given, and which, when very violent, is called "sbalza" by the Mexicans. Of the latter, some remarkable examples will be recorded, as narrated to me.
Floors are not always, sources of increased injury however; they may occasionally act the part of props, to walls that if unsupported for their entire height, would have been prostrated, by normal or other waves of the first four classes.
This can only occur, when the line of transit is transverse to the direction of the joists, and when the end wall is in advance of the wave, and, consequently, the edges of the floors also, are intersected by a wall whose plane, is parallel with the line of transit, as in Fig. 78; or when some other such fulcrum, resists the forward motion of the floors themselves, and enables them to hold together the side walls by the insertion of the joist's ends, and so to save the wall at by cancellating the structure, in vertical and horizontal directions. The planking connect-
