SEICHES AND OTHER OSCILLATIONS 
69 
half hours before the wind began to rise, the liinnograni begins to 
show serious disturbance. This disturbance becomes strongly marked 
at 5*^, when the total range of the seiche reaches 60 mm., and there 
is a strong development of seiches of higher nodality, in particular 
of one having a period of about 2*9™. 
At when the wind suddenly rises into a gale, there is no very 
marked change in the seiche. But between 8^^ 30"^ and 9^^ there is 
an increase in the total range from 56 mm. to 78 mn]., due no doubt 
to the simultaneous microbaric disturbance, which has a period of 
about 13'6"\ After this the seiche tends to settle down into a LTB- 
dicrote, strongly embroidered with higher components while the gale 
lasts. It is worthy of note that at i.e. seven hoars after the 
gale commenced, the mean level of the lake at Picnic Point near 
St Fillans has only risen about 6 mm. About 16^^ there is a decrease 
in the total range of the seiche from 64 mm. to 51 mm. This may be 
due partly to the drop in the wind, but much more probably to the 
simultaneous microbaric disturbance, which has a period of about 
17™, and would strongly affect the uninodal component of the seiche. 
The rano;e of the disturbance on the microbaroo;ram was a little 
under 2 mm., and the data from the triangle of microbarographs 
showed that it travelled along the lake with a velocity of 53 miles an 
hour. For rough purposes and for convenient calculation we may 
take 48 instead of 53, and suppose the period of the pressure dis- 
turbance and also the uninodal period to be 15™, and the circumstances 
as to phase to be the most favourable possible. The mathematical 
theory ^ then gives 3 mm. for the addition to the amplitude of the 
uninodal in 15™. The effect after two undulations will therefore be 
6 mm., that is, an alteration of 12 mm. in the range of the seiche, 
which, as it happens, is within a millimetre of the value observed. 
3. Case in which a Seiche was probably caused by a Flood. — 
Fig. 24 shows the limnogram, taken near the binode, of a seiche 
disturbance beginning at 16^^ 9'6™ on 4th August 1905. The upward 
slope is due to a sudden rise in the lake caused by heavy rain. On 
the 3rd there had been "96 inch of rain, and on the 4th 2*03 inches, 
the greatest rainfall observed during August and September. The 
limnograms taken at the uninode and Picnic Point are similar, except 
that the former shows merely a feeble binodal seiche, while the latter 
has a well-marked trinodal superposed on the uninodal seiche. 
The wind on the 4th was light and easterly, but a well-marked 
barometric depression, travelling with a velocity of about 18 
(mile/hour), passed in a direction towards N. 15° E., probably a 
little to the west of Loch Earn, the centre being nearest about 
Oh 52™ on the 5th. 
1 Trans. Roy. Soc. Edin., vol. xlvi. p. 516, 1908. 
