That the changes observed in the shock were merely an effect of less or greater distance, will be obvious from Fig. 23, in which the intensity at any moment is that represented by the distance of the corresponding point on the curve from the different base-lines, the base-line _a_ corresponding to a place near the epicentre, and _b_, _c_, _d_, etc., to places at gradually increasing distances. Thus, at a place corresponding to the base-line _b_, the intensity of the tremors during the intervening pause (represented by the short line PN) was so slight that they frequently escaped notice, while the preliminary tremors observed by some near the epicentre were altogether imperceptible. At the places corresponding to the base-lines _c_, _d_, _e_, _f_, the duration of the whole shock and of each part gradually diminished, while the interval between the two parts increased owing to the gradual extinction of the final vibrations of the first part and of the initial vibrations of the second. At the farthest of these places (_f_) the first part was so weak that it sometimes passed unobserved. Lastly, at a place corresponding to the base-line _g_, the first part was imperceptible to all observers, and the shock consisted of a single series of horizontal undulations. [Illustration: FIG. 23.--Diagram to illustrate variation in nature of shock of Andalusian earthquake.] _Origin of the Double Shock._--If the double shock were observed at only a few places, we should naturally look for some local explanation of the peculiarity. The second shock, for instance, might be a subterranean echo, the earth-waves being reflected at the bounding surface of two different kinds of rock. In the case of the Andalusian earthquake, such an explanation is precluded by the almost universal observation of the double shock, the greater intensity of the second part, and the longer period of its vibrations.