8 
Proceedings of the Koyal Society of Edinburgh. [Sess. 
To complete the cycle there only remains the transition from the third 
phase back to the first phase, and this transition is also attainable experi- 
mentally. For with the continuance of the wind-current the discontinuity 
layer in the experimental trough, as in a natural basin, gradually sinks 
deeper. Owing to the currents induced at the discontinuity layer by the 
wind-current, there is a gradual mixing of the lower with the upper layer. 
The difference in density between them diminishes, and the deepening of 
the discontinuity layer gradually results in the complete mixing of the 
whole liquid, and finally the whole liquid in the trough again becomes of 
uniform density. 
The Temperature Seiche. 
The third phase in the temperature cycle is accompanied by the tempera- 
ture seiche, and it was primarily to demonstrate the possibility of its exist- 
ence that the present investigation was undertaken. An oscillation of the 
liquid below the discontinuity layer in the experiments already described 
was easily obtained. While the wind-current was kept blowing along the 
surface there was noticeable a considerable transference of the surface 
water to the lee * end of the trough, the surface layer at that end being 
considerably deeper than at the end of the trough from which the wind 
was blowing. If the wind-current was suddenly cut off, a seiche in the 
lower liquid followed, with a period depending on the difference in density 
between the upper and lower layers. The period for the ordinary seiche 
in the trough was about 3 seconds, while the period of the “tempera- 
ture seiche ” varied from 10 to 20 seconds in most of the experiments. 
The seiche very rapidly died down, not making more than two or three 
oscillations. 
Even while the wind-current was blowing a small “ temperature seiche ” 
was usually observable, possibly due to inequalities in the wind-current. 
Another phenomenon accompanied the cessation of the wind-current, and 
it may be called a “ density bore.” A solitary wave was formed at the 
discontinuity layer at the end of the trough from which the wind was 
blowing, and slowly travelled the length of the trough, when it was reflected, 
but with great loss of amplitude. There may be a corresponding bore in 
lochs, and if there is, it might account for many apparent irregularities in 
temperature ; for the passage of such a bore would explain a sudden rise 
in temperature of short duration, and at a considerable depth. I have not, 
* The term “ windward ” is used for “leeward,” and vice versa, on pp. 416 and 418 of 
my paper on Lake Temperatures in Trans. R.S.E . , vol. xlv., part ii., p. 409. 
