466 PROFESSOR CHRYSTAL 



pensated for the fact that 9/5 is not so close an approximation to TJT 2 as is the sixth 

 convergent, 70/39. 



In times of storm or even moderate wind there is of course a strong embroidery 

 of various kinds ; but usually the UB-dicrote configuration can be seen through all 

 the confusion, and it soon becomes the prominent feature when the weather begins 

 to settle. 



At this point we may indicate how the lake can be made to analyse its own seiches. 

 Fig. 1 shows three simultaneous limnograms, the lowest one taken at the Picnic Point, 

 about 480 yards from the eastern end of the lake, the middle one taken near the 

 binode, the uppermost one taken near the uninode. All three are somewhat embroidered 

 by the wind, but the St Fillans seiche is a UB-dicrote, the middle one a nearly pure 

 uninodal, and the uppermost one a nearly pure binodal. The figure is at once an 

 interesting confirmation of Forel's theory and a verification of the approximate 

 accuracy of the mathematical theory of Loch Earn regarded as a biparabolic lake.* 



Comparison of Earn with Tay and Lubnaig. 



The seiches on Loch Tay present the strongest possible contrast to the seiches of 

 Earn. No clear dicrote or other easily recognisable configuration is ever seen. Often 

 it is not easy even to recognise the uninodal seiche. The contrast may be partly 

 realised by looking at the two pairs of limnograms in fig. 2. In the first pair is seen 

 the beginning of the long seiche on Earn above mentioned, alongside of the simul- 

 taneous seiche on Tay — which was the most regular one found on that lake between 

 4th October and 9th November 1905. The second pair is the end of the long seiche 

 on Earn with the simultaneous one on Tay, whose irregularity is typical. 



As yet our knowledge of the seiches and meteorological conditions of Loch Tay is 

 not sufficient to enable us to explain this difference ; but we may point out here that 

 Loch Tay is relatively a shallower lake than Earn ; it is more crooked ; and the relation 

 of its axial line to the path of the minor atmospheric disturbances is different. 



This divergence of conditions occurs in an exaggerated form in the case of Lubnaig, 

 which is very shallow, has a very irregular basin, and lies across the path of the 

 atmospheric disturbances. Accordingly, we found only four cases in which we could 

 recognise a definite seiche in Lubnaig, having a period of about 24 min. ; and in each 

 case only a few undulations could be counted. One of these seiches is shown in 

 fig. 3. During the rest of the six weeks of observation nothing was found but wind- 

 embroidery and sub-permanent wind denivellation, such as would be naturally expected 

 in a shallow lake. About this negative result there seems to be little room for doubt, 



* The following method of roughly analysing a dicrote seiche which is tolerably pure and shows the 5/9 

 configuration may be mentioned here. If y be the minimum minimorum of the ordinates of the limnogram, 

 Vi ) 2/2) 2/3 the ordinates at distances |Tj , £Tj , and |T, from y , and if A be the ordinate of mean level, U and B 

 the amplitudes (semi-ranges) of the uninodal and binodal components, then A. = i(y + y 3 ) = y i , U + B=. , ,(v/ 3 -y 2 ), 

 U-B = V2(?/ 1 -A) = 1-41% 1 -A). 



