Fig. 37.—Distribution of observatories at which magnetographs were disturbed by the Riviera earthquake.[ToList]
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.
In Austria, disturbances were observed at Pola (295 miles) and Vienna (506 miles), beginning at 6h. 28m. 35s. and 6h. 30m. 35s., respectively. They reached Brussels (522 miles) at 6h. 29m. 27s., and Utrecht (600 miles) at 6h. 28m. 38s.[50] At Wilhelmshaven (690 miles), only the vertical force magnet was affected, the oscillations beginning at 6h. 30m. 35s., and lasting for fourteen minutes. At 6h. 27m. 55s., the declination and horizontal force magnets of Greenwich observatory (642 miles) were set vibrating, but no similar disturbances were revealed by the vertical force curve or by the two earth-current registers. At Kew (652 miles), the horizontal force magnetograph was moved by the earthquake at about 6h. 29m. 55s. The curves at Stonyhurst and Falmouth show no sign of any disturbance, nor do those at Pawlovsk in Russia, or Seville. At Lisbon (951 miles), however, the three curves indicate disturbances at 6h. 32m. 35s., but so feeble are they that they would have escaped discovery if the occurrence of the earthquake had been unknown.
The effects registered on the magnetograms are quite different from those which correspond to ordinary magnetic perturbations; but they are not unlike those produced by the action of the momentary currents which are used for making the hour-marks, except that the earthquake-oscillations lasted several minutes (see Fig. 21). In each case, then, the magnetic bars must have received a succession of several or many impulses.
Now, the effect of these impulses on each magnet must depend on the relations which exist between the period of oscillation of the magnet, the rate of damping of such oscillations, and the interval between the successive impulses. Also, the apparent commencement of the phenomena may be delayed if two impulses of contrary sense should follow one another before the bar is perceptibly displaced. It is therefore to be expected, as M. Mascart points out, that the disturbances of the three instruments need not be of the same order of magnitude, that with different forms of apparatus the effects may be very variable, and that the deflection of one instrument may precede that of another at one and the same place.
In all the magnetographs, the record is made on photographic paper, which travels so slowly that the time of a movement can only be ascertained to the nearest minute. As the disturbances on the French curves were apparently almost simultaneous, and as no two of the others differed in time of occurrence by more than five minutes, there is thus some colour for M. Mascart's contention that the magnetic apparatus registered, not the movements of the ground, but the passage of electric currents produced in the ground at a certain epoch of the earthquake.[51]
On the other hand, it is important to notice that, in the central part of the disturbed area, at Nice, two, if not all three, of the magnetographs were unaffected at the time of the earthquake.