604 Proceedings of the Poyal Society of Edinburgh. [Sess. 
upper and lower layers is sufficiently distinct, an internal (or temperature) 
seiche takes place in the lower layer, the period of which can be approximately 
calculated from the formula 
t= 2 l 
j g(p - p) 
h ii 
where p and p , h and h' are respectively the densities and depths of the 
upper and lower layers, and l the length of the lake.* In Loch Ness the 
period of this internal seiche was about three days, in Loch Garry about 
twelve hours. 
We also know from the elaborate observations on secondary undulations 
of oceanic tides carried out for the Earthquake Investigation Committee of 
Japan by K. Honda and others ( Journal of the College of Science, Tokyo, 
vol. xxiv., 1908), that in deep bays secondary undulations or seiches are of 
frequent occurrence, with a period given by the formula 
t = 
the node of the seiche being at the mouth of the bay, and the ]oop being at 
the end of the bay. 
It was suggested to me by Sir John Murray that the oscillations observed 
by Pettersson might be analogous to those observed by the Lake Survey, and 
it is not a very large stretch of imagination to suppose that Pettersson’s 
observations really show the presence, not of a long-period tide, but of a 
temperature seiche, having its node at the mouth of the Skagerak and its 
loop at the point of observation. A glance at the map will show that the 
basin is deep and well-defined, and that all the conditions favour an oscilla- 
tion of the lower dense layer of water. 
The period of such an oscillation should be given by the formula 
t = 
jk 
fi(p ~ p) 
t + i 
h K 
The line drawn on the map shows what may be taken as the mouth of 
the Skagerak, and the length of the basin is about 200 km. The Japanese 
observers found that a mouth correction had to be applied in calculating the 
period of a seiche in a bay ; and in the case of a basin of the breadth of the 
* Cp. Geogr. Journal , vol. xxiv. p. 430 ; Trans. Roy. Soc. Edin vol. xlv. p. 420 ; Proc. 
Roy. Soc. Edin., vol. xxviii. p. 1 ; ibid., vol. xxix. p. 98. 
