TEMPERATURE OF SCOTTISH LAKES 
131 
and 49° respectively, and the mean depth of water below the discon - 
tinuity as 135 feet, an application of the formula on page 126 gives for 
the period of the temperature seiche tvv^enty hours, which is sufficiently 
near to the observed period to give colour to the theory of forcing. 
Seiches of the amplitude which is observed must produce consider- 
able currents, especially in the neighbourhood of the surface of 
separation. Assuming that the boundaries of the different layers are 
sharp, slipping between the layers is necessary, and rough calculations 
for Loch Ness show that in the case of a temperature seiche with an 
amplitude of 100 feet (this means that the range of depth at which 
water of a definite temperature is to be found is 100 feet) the 
currents due to the motion of the water particles would have a 
velocity of about 2 or 3 centimetres per second in each layer, 
or at the surface of separation a relative rate of, say, 5 centimetres 
per second. 
During the experiments in the trough containing liquids of 
different densities I was able to illustrate experimentally the nature 
of the temperature seiche. The effect of the wind current at the 
surface was to accumulate the liquid of lowest density towards the 
lee end of the trough, and so produce a tilting of the discontinuity 
layer. When the wind current was stopped or reduced, the surface 
of discontinuity could be seen to oscillate slowly up and down about 
the centre of the trough until it came to rest in a horizontal position. 
The credit for the discovery of the nature of the temperature 
seiche belongs to Mr E. R. Watson, B.Sc, at one time a member of 
the Lake Survey, and now Professor of Chemistry in the Libpur C.E. 
College, Calcutta ; but the discovery was foreshadowed by Professor 
J. Thoulet in his " Contribution a TEtude des Lacs des Vosges,"" ^ 
where he says : — " Son eau (i.e. Longemer) avail donne naissance, 
vers 8 metres de profondeur, a la couche de transition thermique 
brusque, au sein du lac, et cette couche elle-meme, sous Timpulsion de 
la masse d'eau animee du mouvement du a son courant et qui lui 
arrivait a Fune de ses extremites, s'est mise a osciller longitudinale- 
ment et transversalement, comme une sorte de seiche interieure 
provoquee par une action mecanique, et Foscillation s'est communiquee 
en s'attenuant jusqu^au fond. . . . J'ai tenu cependant a confirmer 
mon opinion par une experience synthetique. Dans ce but, j'ai 
superpose, dans une auge en verre, de Feau saturee de carbonate de 
potasse, de Falcool colore en rouge et du petrole. II m'a suffi de 
laisser tomber dans le vase des gouttes d'alcool pour communiquer un 
mouvement ondulatoire synchrone aux trois couches liquides.'" ^ The 
Lake Survey observers were at the time of observation ignorant of 
^ See Bull. Soc. Geogr. Paris, t. xv. p, 572, 1894. 
2 Op. cit, pp. 31, 32 (sep.). 
