4th. Nature of the material upon which the place stands, and of that, extending from it towards the focal point and in the opposite direction.
All these, as to the actual effects produced in any given instance, being modified by the character, of the buildings, &c., of the place, and by their structural capability of endurance. Future seismic surveys are therefore brought within limits of condition, greatly more simple, than when we imagined that the chief modifying conditions of the effects of the wave, would be found, in the intimate or molecular nature, of the rocky formations deep beneath the surface. These conditions are now shown to be such, as admit of application and deduction, with equal facility and certainty, by the observer who having once understood their principles, will but keep his eyes open.
In such surveys, nothing is so important as good maps, showing with accuracy, the physical configuration of the surface of the country. Contoured maps would be of inestimable value, and for the consideration and discussion, of facts obtained in the field, clay models, even of a rude character, if made approximately to scale, would be found to guide the eye, and assist the mind to correct conclusions, upon the apparently complicated phenomena that are often presented to us.
Geological maps, though highly desirable, are less indispensable; the earthquake observer in the field, can generally make out, en passant, enough of the geology to enable him to record his facts correctly, with reference to change of formation or boundary.
