1905 - 6 .] The Hydro dynamical Theory of Seiches. 
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tolerably large range within which it was impossible to select any 
particular position as the most preferable. The limits of error 
thereby involved seemed too large to be satisfactory, and recourse 
was had to the other method in the hope that better results might 
be obtained. 
At a node the vertical displacement is a minimum, and, under 
ideal circumstances, would be zero. On one side of the nodal line 
the liquid falls, while it is rising at the other side ; and con- 
versely. The plane of the water in its changing position inter- 
sects the plane of the water when at rest along the nodal line. 
The position of intersection of the initial and moving plane was 
observed by marking the direction of the former by means of 
a stretched wire. Limits were noted within which the node 
appeared to lie, and the mid-point of the average of these limits 
was taken as representing the nearest approximation to the true 
position of the node. The limits were necessarily wide, since 
the angle between the two planes, depending on the amplitude of 
the seiche, was very small. This part of the experimental results 
can only furnish limits within which the nodes seemed to lie. 
The same difficulty in fixing the precise position of the nodes 
was experienced by the writers of this paper during observations 
on Loch Earn under the supervision of Professor Chrystal in the 
autumn of last year. The method adopted in that case was that 
of approaching the node from both sides simultaneously, and thus 
fixing limits within which the node was situated. These observa- 
tions could only be satisfactorily carried out when the particular 
seiche motion was not too much masked by the presence of 
another, a contingency which very rarely arose. Even in the 
most favourable experimental circumstances it cannot be said that 
any oscillatory motion is perfectly pure. 
The general results of the tabulated observed positions of the 
nodes show that in comparison with those in a rectangular vessel 
of uniform depth, the nodes are displaced towards the shallower 
part of the liquid. Thus in the case of the rectangular vessel of 
uniform depth, the binodal position is midway between the centre 
and the end, while in the case of the concave parabola the 
binodal position is nearer the end than the centre, and in the 
convex parabola the binodal position is nearer the centre than 
