The Depth of Origin of Earthquakes

It is a peculiarity of the present state of seismology that it is necessary to define the meaning which one attaches to the word. earthquake. Up to the end of the last century this was hardly necessary, the word was still used in its old-fashioned sense of a quaking, or shaking, of the earth which could be felt and, when violent enough, caused damage. When it was found that, with suitable instruments, disturbances could be detected, which could not be felt, and it was observed that some of these were evidently connected with great earthquakes, it was too readily assumed that this was true of all, and a new connotation of the word arose. This newer branch of seismology has been more and more extensively followed up, until it has almost ousted the older seismology, and some of its devotees are inclined to refuse the right, of the Disturbance which can be felt, to the name of earthquake, unless it is accompanied by long-distance records. In what follows the word is used in the old-fashioned sense, and the earthquakes dealt with are the disturbances which can be felt, which are essentially phenomena of the outer crust, and as such belong to the domain of geology. In the study of these, one of the first objects is a determination of the depth of origin, and several distinct principles are involved in the methods which have been proposed. Robert Mallet believed that that angle of emergence could be determined from the measurement