And, moreover, he shows that the water oscillates according to the two principal dimensions of the lake; thus, giving rise to longitudinal _Seiches_ and transverse _Seiches_.

They occur in series of tautochronous oscillations of decreasing amplitude; the first wave produced by the action of a given cause having a maximum amplitude.
_Causes_.

The disturbances of hydrostatic equilibrium which generate _Seiches_ may be produced by a variety of causes.

Among these, the following may be cited: (a) Sudden local variations of atmospheric pressure on different parts of the lake.

(b) A descending wind, striking the surface of the lake over a limited area, (c) Thunder-storms, hail-storms, and water-spouts; and especially when the accompanying winds act vertically.