_Objections to Dutton's Method._--I have described this method somewhat fully, though it seems to me open to more serious objections than Mallet's first method which it is intended to replace. We have, in the first place, no reason for supposing that the focus is either a point or a sphere, or that the initial impulse is uniform in all directions. If the earthquake were caused by fault-slipping, both assumptions would be untrue, and it is for those who employ the method to prove their validity. But of greater consequence is the fact that, if the method were correct, all earthquakes originating at the same depth must have index-circles of equal radii. If the depth of the focus were, say, ten miles, then the index-circle must have a radius of about six miles, whether the initial disturbance be of extreme violence or so weak that it is not felt at the surface at all, much less so far as six miles from the epicentre. The law of the inverse square is of course only true for a perfectly elastic and continuous medium, and the real curve of intensity is not that of the continuous line in Fig. 31, but something of the form represented by the dotted line. In this case, the rate of change of intensity is greatest at some point C', nearer than C to the epicentre, and the application of Major Dutton's rule would give a point F', nearer the surface than F, for the focus. Thus, assuming that the method can be applied in practice--and the test involved is one so delicate that it would be difficult to apply except with refined measurements--then all that we can assert is that the calculated depth is certainly less than the true depth. _Dutton's Estimate of the Depth of the Seismic Foci._--In applying the method, the chief difficulty is to obtain a series of isoseismal lines corresponding to equidistant degrees of intensity. As already pointed out, those given in Fig. 29 are merely diagrammatic; but the index-circle of the Woodstock focus, represented by the dotted line, is made to pass through the places where the rate of change of intensity was found to be greatest. The radius of this circle being very nearly seven miles, it follows that the resulting depth of the Woodstock focal point would be about twelve miles. Major Dutton regards this estimate as probably correct within two miles. In the neighbourhood of the Rantowles epicentre, the isoseismals in both Figs. 28 and 29 are elongated in form. The _index-circuit_, as it would be called in such a case, cannot be drawn completely, but its radius parallel to the shorter axis of the curves is about 4-1/2 miles, and the resulting depth of the Rantowles focal point would be nearly eight miles. VELOCITY OF THE EARTH-WAVES.