In addition to variation in the bubbles of levels which come on more or less gradually, we have many recorded instances of _sudden_ alterations taking place in these instruments.
Examples of what may have been a slow oscillating motion of the earth’s crust are referred to by Mr. George Darwin in a Report to the British Association in 1882.
One of them was made by M. Magnus Nyrén, at Pulkova, who, when engaged in levelling the axis of a telescope, observed spontaneous oscillation in the bulb of the level.
This was on May 10 (April 28), 1877. The complete period was about 20 seconds, the amplitude being 1·5″ and 2″. One hour and fourteen minutes before this he observes that there had been a severe earthquake at Iquique, the distance to which in a straight line was 10,600 kilomètres, and on an arc of a great circle, 12,500 kilomètres. On September 20 (8), in 1867, Mr. Wagner had observed at Pulkova oscillations of 3″, seven minutes before which there had been an earthquake at Malta. On April 4 (March 23), 1868, an agitation of the level had been observed by Mr. Gromadzki, five minutes before which there had been an earthquake in Turkestan. Similar observations had been made twice before. These, however, had not been connected with any earthquakes—at least, Mr. Darwin remarks—with certainty.
_Phenomena analogous to the pendulum and level observations._—As examples of phenomena which are analogous to those made on pendulums and levels, the following may be noticed. On March 20, 1881, at 9 P.M. a watchmaker in Buenos Ayres observed that all his clocks oscillating north and south suddenly began to increase their amplitude, until some of them became twice as great as before. Similar observations were made in all the other shops. No motion of the earth was detected. Subsequently it was learnt that this corresponded with an earthquake in Santiago and Mendoza.[146]
Another remarkable example illustrating the like phenomena is furnished by the observations which were made on December 21, 1860, by means of a barometer in San Francisco, which oscillated, with periods of rest, for half an hour. No shock was felt, nor is it likely that it was a local accident, as it could not be produced artificially. On the following day, however, a violent earthquake was experienced at Santiago.[147]