850 ON THE PERIODS AND NODES OF LOCHS EARN AND TREIG. 
§ 34. In October and November 1904, a series of observations on Treig were made 
under the superintendence of Mr E. Mactagan-WEDDERBURN. Only one limnograph 
was used, and it was placed at the northern end of the lake. The observations were 
brought to an untimely conclusion by the partial destruction of the instrument during 
a storm. The seiches observed were rarely pure for any considerable time; and the 
depth of the lake varied considerably during the observations, so that the periods do 
not all belong to the same surface-level. T, varied from 9°09’ to 9°45’, the mean of 
all the determinations attempted being 9°18’. T, varied from 5°11’ to 5°22’; mean, 5°15’, 
Nothing is known as yet regarding the actual position of the nodes of Treig. 
§ 35. In June 1905, observations were begun on Loch Harn by Mr James Murray 
under the superintendence of Professor CurystaL. ‘Two Sarasin limnographs were 
established—one at the uninode, the other at the binode, as determined by the above 
calculations. Unfortunately, these instruments proved insufficiently sensitive for the 
great majority of the delicate seiches which have occurred in the lake during June and 
July. Mr Murray has, however, acquired great skill in using the index limnograph 
of ENpR6s ; and a considerable number of his charts are already at our disposal. The 
results here given are merely preliminary, and must not be understood as anticipating 
the more accurate determinations which Mr Murray will doubtless make later on. 
Allowance being made for wind denivellations due to the shallow edges of the lake, 
the traces at the two supposed nodes are nearly pure sinusoids. The calculated positions 
of the uninode and binode cannot therefore be far out. The values obtained for T, vary 
from 14°35’ to 14°77’, the mean being 14°55’; for T,, 7°97’ to 8°36’, mean 8°10’. No 
good determination of T, has yet been made. 
§ 36. The close agreement between the observed and calculated periods may be 
partly fortuitous. We cannot regard this as finally established until we have additional 
observations, in which the essential data are more certainly determined. 
As regards the agreement between theory and observation to be expected in general, 
we may point out that more accurate calculation of the periods is to be expected than 
of the nodes ; and that least accuracy of all can be hoped for in the calculation of the 
amplitudes of the seiches at different parts of the lake. The periods obviously depend 
more on the whole configuration of the lake, and less upon local irregularities, than do 
the nodes or the amplitudes. The nodal line would be very seriously displaced by a 
sudden alteration in the depth or breadth of the lake which might affect the periods 
very little. Thus, for example, it is easy to see from the position of the dots in fig. 1, 
with reference to the parabola, that the Western Binode of Earn probably lies some 
distance west of the position calculated above. The amount of the displacement might 
be calculated by RayeicH’s method if the data from soundings were sutfticiently 
accurate. Also, a gently shelving shallow shore would cause flow across the lake, 
contrary to the hypothesis of the theory ; and the effect of this might be to deform the 
nodal line, and to alter very considerably the amplitude of the seiche near the shore. 
These points we propose to discuss in detail in a subsequent communication. 
