ON THE HYDRODYNAMICAL THEORY OF SEICHES. 605 
This table must, of course, be used with great caution. In the first place, as we 
increase the number of the nodes we trench more and more upon the limits of the 
hypothesis that the waves are “long.” Then it cannot possibly represent the case of 
convex (2.e. in practice concavo-convex lakes) so accurately as the case of wholly concave 
lakes. It will be noticed, for example, that in Geneva and Ness, which are similar in 
their seiche-character, there are seiches of 20’ and 8°8’, probably trinodal or quadri- 
nodal, which do not fit into the scheme suggested. As the nodality increases, the 
periods become more and more nearly equal, and therefore more difficult to distinguish, 
either by a rough calculation, or, for that matter, by observation, from one another, and 
from progressive surface waves of purely local significance which are not seiches at all. 
Subject to these qualifications, the table is very interesting and suggestive. It 
shows the greater variety of possible periods in some lakes as compared with others. 
It shows that there is nothing surprising, from the hydrodynamical point of view, in the 
fact that the three longest periods for Constance are 55:8’, 39°1’, and 28°1’. The 
sewche & la quinte of which Foret speaks in the cases of Constance, Garda, and 
Starnberg is in all probability simply the binodal seiche ; and the seiche whose period 
is approximately half the longest period is a trinodal. Such questions cannot be 
finally settled until the phases of the seiches have been determined by simultaneous 
limnographic observations at different parts of the lake, as has been done by ENprRos 
in his admirable investigation of the seiches of the Chiemsee, Seespiegelschwankungen 
beobachtet am Chiemsee, Traunstein, 1903. 
I have included in the table as longitudinal seiches some which have been 
held by observers to be transversal. This I have done for two reasons. In the 
first place, the absolute identification of these by means of phase observations 
has not in all cases been satisfactorily accomplished.* Again, it is possible that a 
transversal seiche might coexist with a longitudinal one of nearly the same period 
throughout a considerable part of the lake. The interference of these at the ventral 
points of the longitudinal seiche would produce the phenomenon of seiche beatst at 
various points along the shore. At the ends of the lake, which are ventral points for 
all the different pure seiches, all these seiches interfere. It follows, equally from 
observation and from the analogy of the vibrating string of varying density, presently 
to be mentioned, that the average amplitudes of the seiches that occur in nature 
diminish rapidly as their nodality increases. Hence the chief features of the limno- 
graphic trace at the shallow ends of a lake will in general be the periodic configuration 
due to the interference of the uninodal and binodal seiches ; the effect of the others will 
merely be to produce an embroidery on the main outline. Also, since the periods 
Become more nearly equal as the nodality increases, this embroidery will have an 
* See Foren, Le Léman, t. ii. p. 148. 
_ + Since this was written I have noticed that EnpR6s, in his able analysis of the seiches of the Chiemsee, cites 
examples of variations in the phases and amplitudes of nearly pure seiches, which he regards as due to the inter- 
ference of seiches of the same period differing in phase. He suggests, with great probability, that such seiches are 
generated by a common but intermittent cause of disturbance. 
