| Time h. m. | Readings. | |
|---|---|---|
| 25th. | 4 00 p.m. | 104·5 |
| 4 5 „ | 103 | |
| 4 10 „ | 102 | |
| 4 25 „ | 101 | |
| 4 30 „ | 100 | |
| 4 40 „ | 98 | |
| 4 42 „ | 99·5 | |
| 4 45 „ | 100 | |
| 4 50 „ | 101 | |
| 4 55 „ | 101 | |
| 5 00 „ | 100 | |
| 26th. | 7 00 a.m. | 105 |
Usually this level moves through about three divisions per day.
From March 25 to May 4 it travelled from 98 to 127. Since then, to June 5, it has descended to 116. During this period the east west level has been comparatively quiet. One division of the north south level equals about 2″ of arc.
Many of these changes may be due to changes in temperature, variations in moisture, and other local actions. Some of them, however, are hardly explicable on such assumptions. The fact that the general direction in change of the vertical, as indicated by a tromometer standing on the same column with the levels, showed that the change which was taking place was rather in the column than in the instruments.
The fact also that at the time of a barometrical depression a pulse-like surge can be seen in the levels, having a period averaging about three seconds and sometimes amounting to about one second of arc, is a phenomenon hardly to be attributed to sudden fluctuations in moisture or temperature, but indicates real changes in level.[146]
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.[147]