No observations, however, are at present so valuable for the purpose in view as those made on the intensity of the shock. For many years, it has been the custom to regard the epicentre as coincident with the area of greatest damage to buildings; and, when the area is small, the assumption cannot be much in error. It is of course merely a rough way of obtaining a result that is generally given more accurately by means of isoseismal lines; but there are exceptional cases, such as the Neapolitan and Ischian earthquakes, when the destruction wrought by the earthquake furnishes evidence of the greater value. A single isoseismal accurately drawn not only gives the position of the epicentre with some approach to exactness, but also by the direction of its longer axis determines that of the originating fault. When two or three such lines can be traced, the relative position supplies in addition the hade of the fault (p. 219). The successful application of the method requires, it is true, a large number of observations, and these cannot as a rule be obtained except in districts that are somewhat thickly and uniformly populated, such as those surrounding the cities of Hereford and Inverness. In the Charleston earthquake, also, the position and form of the epicentres were deduced from the trend of isoseismal lines based on the damage to railway-lines and various structures within a sparsely inhabited meizoseismal area. In a few cases, of which the Indian earthquake may be regarded as typical, a fourth method has recently been found of service. The numerous after-shocks which follow a great earthquake originate for the most part within the seismic focus of the latter; and, as they usually disturb a very small area, it is not difficult to ascertain approximately the positions of their epicentres. Some, as in the Inverness after-shocks of 1901, result from slips in the very margin of the principal focus; but, as a rule, the seat of their activity tends to contract towards a central region of the focus. Bearing in mind, then, that some of the succeeding shocks originate at and beyond the confines of the focus, and that others may be sympathetic shocks precipitated by the sudden change of stress, it follows that the shifting epicentres of the true after-shocks map out, in part at any rate, the epicentral area of the principal earthquake. DEPTH OF THE SEISMIC FOCUS. It is much to be regretted that we have no satisfactory method of determining so interesting an element as the depth of the seismic focus. That it amounts to but a few miles at the most is certain from the limited areas within which slight shocks are felt or disastrous ones exhibit their maximum effects. Nor can we suppose that the rocks at very great depths are capable of offering the prolonged resistance and sudden collapse under stress that are necessary for the production of an earthquake.