It has also been noticed that when an earth has become completely saturated or water-charged, a sudden fall of the barometer to a low pressure will liberate the pent up water which the soil cannot contain and cause it to burst out, the equilibrium having been so delicate. Under such circumstances slips are nearly sure to ensue, and to be serious from their sudden action. Extensive slips in earthwork seldom occur during the excavation, or a short time after the completion of a cutting; on the other hand, movement in an embankment frequently happens during deposition. In the case of an embankment, time may cause the earth to become consolidated, but in a cutting the disintegrating and disturbing forces, and the combined action of air and water percolating until they force forward the earth, are usually gradual in their operation, and often require a year or two to cause a state of instability; in fact, the history of slips, with a few exceptions, in soils whose condition is very readily changed by water, indicates that serious movement in cuttings does not generally occur until a cycle or two of the seasons has elapsed, during which period meteorological influences, aided by vibration and other deteriorating operations, are slowly and regularly proceeding, until at length such a change in the general condition is caused that a slip happens, apparently from some sudden agency, whereas the stability of the earth has been gradually and surely wasting away for a long time; hence the importance of continual careful observation in cuttings even of moderate depth in doubtful soil. In canals and works of a similar character constructed to contain water, if any movement or slip of earthwork takes place, it usually occurs within a short time of the water being admitted, and generally within a few months and seldom after so long a period as a year, the ground in a short time becoming consolidated, being exempt from severe vibration and many of the disturbing agencies present in railway cuttings and embankments.