wretched rubble masonry falls incoherent at the slightest jar.
We advance then to the churches, the barracks, or castello, the monasteries, the Casa Communale—to any of the better built and isolated, or nearly isolated buildings, and we soon discover that amongst these there are some that in every place present certain grand characteristics of partial or complete overthrow, and that these are everywhere generically much alike.
These we observe, aided by the prismatic compass, and with our previous dynamic knowledge, and soon discover that wherever such buildings, and not very dissimilar to each other, have been placed under like conditions, and so that their walls are in the same azimuths, like dislocations have affected them, and that where the directions in which the forces that we know must have produced the observed dislocations have not passed very diagonally through the walls, they have produced effects, regular and accordant with each other, and from which the directions may be inferred in which the forces themselves acted.
After a little experience we discover, that in every town (and frequently in other places) we may find rectangular buildings whose walls run very nearly north and south and east and west, and that these respond to our questions best; and finally, that buildings so posited, and having certain necessary characters of structure, when not too completely destroyed or overthrown, will enable us to discover the direction of wave transit, whatever may have been its line with reference to the walls.
It remains to describe, therefore, the effects produced upon such cardinal buildings by earthquake shock, to trace
