An earthquake, like every other operation of natural forces, must be investigated by means of its phenomena or effects. Some of these are transient and momentary, and leave no trace after the shock, and such must either be observed at the time, or had from testimony. But others are more or less permanent, and, from the terrible handwriting of overturned towns and buildings, may be deciphered, more or less clearly, the conditions under which the forces that overthrew them acted, the velocity with which the ground beneath was moved, the extent of its oscillations, and ultimately the point, can be found in position and depth beneath the earth's surface, from which the original blow was delivered, which, propagated through the elastic materials of the mass above and around, constituted the shock.
Again, certain effects, such as landslips, fissures, alterations of water-courses, &c., are produced of greater or less permanency affecting the natural features of the shaken country.
The observation of each of these classes of effects bears reference to two distinct orders of seismic inquiry.
By the first, we seek to obtain information as to the depth beneath the surface of our earth at which those
such methods of observation. Mr. Hopkins, in his mathematical résume of the laws of elasticity, as bearing upon seismology, communicated to the British Association (17th Report, Oxford, 1847), incidentally points out the geometric conditions by which, if the emergence of a shock were known, the depth of the origin might be ascertained. The procedure generally had been previously suggested by the author in his original memoir on 'Dymamics of Earthquakes.' Trans. Roy. Irish Acad., vol. xxi., Part 1. The methods employed in this work are altogether distinct from that noticed by Mr. Hopkins.
