Nearly all the observers, who were awake at the beginning of the earthquake, agree in asserting that the sound distinctly preceded any movement of the ground. From this, as in the case of the Andalusian earthquake, Professor Mercalli infers that the sound-vibrations travelled with the greater velocity; but, as will be shown in Chapter VIII., the general precedence of the sound admits of another and more probable explanation. THE UNFELT EARTHQUAKE. If the Andalusian earthquake first drew general attention to the distant spread of unfelt earth-waves, the Riviera earthquake showed that this was no isolated phenomenon. We know now that the propagation of such waves is only limited by the surface of the earth, but in 1887 some doubt was felt at first as to the nature of the disturbance, whether it was magnetic or mechanical in its origin. In 1884, the only observatories at which magnetographs were disturbed were those of Lisbon, Parc Saint-Maur (near Paris), Greenwich, and Wilhelmshaven. In 1887, the magnetographs registered the Riviera earthquake at these and several other observatories, the distribution of which is shown in Fig. 37. In this sketch-map, the position of the principal epicentre is represented by the small cross, while the nearly circular line shows the boundary of the disturbed area. [Illustration: FIG. 37.--Distribution of observatories at which magnetographs were disturbed by the Riviera earthquake.] Three of the observatories, those of Nice, Lyons, and Perpignan, lie inside this area. At Nice (which is thirty-seven miles from the principal epicentre), M. Perrotin states that the magnetograph curves show nothing of any interest, except a notable magnetic perturbation on the vertical force curve, the time of which, however, is not stated.[49] At Lyons (211 miles), the declination, horizontal force and vertical force, magnets were all disturbed at 6h. 25m. 47s. A.M., and Perpignan (264 miles), all three magnets, but especially those for the declination and horizontal force, were set abruptly oscillating at 6h. 25m. 20s. Elsewhere in France, the disturbances were noticed at the observatories of Parc Saint-Maur and Montsouris, near Paris (about 447 miles), and at Nantes (538 miles). At Parc Saint-Maur, all three curves show a very clear trace of the earthquake at 6h. 25m. 35s., the oscillations lasting several minutes, and at Montsouris they also began at the same time. At Nantes, the perturbations were so slight that they escaped notice on a first examination.