426 



MR E. M. WEDDERBURN 



PR 



where I is the length of the loch, g the acceleration due 

 to gravity p, p', h and h' respectively the densities and 

 depths of the cold and warm layers, the period of the 

 temperature seiche works out at two days six hours, 

 which is certainly a far closer approximation to the 

 observed period than could have been anticipated con- 

 sidering the rough data employed. The period is not 

 constant during the year. Indeed it changes very con- 

 siderably. Take the case where the Sprungschicht has 

 sunk to a depth of 200 feet and where the temperat 

 of the upper and lower layers is respectively 49° and 43°. 

 The period then works out at three days ten hours. 

 Such a distribution is typical of late autumn. Fig. 11, 

 which is prepared from a diagram drawn by Father 

 Cyril von Dieckhoff, shows graphically observations 

 made in late autumn, and it also shows very clearly the 

 continuance of the seiche and the lengthening of the 

 period as the year progresses. It also shows how the 

 Sprungschicht gradually sinks until by the end of 

 November the temperature seiche has practically ceased 

 to affect the first 200 feet of water. Changes at this 

 depth can then be traced directly to other causes. 



These diagrams do not represent the temperature 

 seiche in the same manner as the curves drawn by a 

 limnograph represent the ordinary seiche. The diagrams 

 show the variations of temperature with time at certain 

 depths. To make a diagram more comparable to the 

 ordinary seiche record it is necessary to show the varia- 

 tions that take place with time in the depth at which 

 water at a definite temperature is obtained. Fig. 12 

 shows the depth at which there is a temperature of 50° 

 at Fort Augustus during the month of August, and so it 

 is strictly comparable to the ordinary limnogram. This 

 diagram shows that the temperature seiche has the 

 remarkable amplitude of 200 feet at times, and that 100 

 feet is quite an ordinary amplitude. The very flat top 

 which the curve has on 1st August is, I think, in- 

 structive. I believe it to be due to the influence of 

 the sun being directly felt at the depth of 25 feet, 

 at which depth there is then a temperature of 50°. 

 There is a tendency for the curves to have flat tops 



