Time Readings. h. m. 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.[145] 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.