I. I ntroduction . T he essential characteristic of a twin-earthquake is the existence in the shock of two maxima of intensity connected by weaker tremulous motion, or the division of the shock into two parts separated by a brief interval of rest and quiet. This feature, however, is not entirely peculiar to twin-earthquakes; for, occasionally, one earthquake is succeeded by another so rapidly as to simulate a twin-earthquake in this respect. A closer investigation of the phenomena shows, as will be seen, that the two parts or maxima of a twin-earthquake originate in two detached, or practically-detached, foci; whereas, in a double earthquake, the foci are either coincident or overlapping. A further distinction, partly dependent on the former, may be noted. In all parts of the disturbed area the member of a double earthquake which occurs first is felt first. In a twin-earthquake, on the other hand, the second impulse may, but does not necessarily, occur before the vibrations from the first focus have reached the other; so that, over most of the disturbed area, the vibrations first felt are those which come from the nearer focus, whether that focus was first in action or not. In a double earthquake the second shock is a consequence of the first; in a twin-earthquake each is independent of the other. In other words, a double earthquake is the result of successive impulses; a twin-earthquake is due to a single generative effort. As examples of twin-earthquakes, may be mentioned the Colchester earthquake of April 22nd,