f 268 a 
XXXVI. 
THE OCCURRENCE OF SEICHES IN LAKE DERRA-- 
VARAGH, CO. WESTMEATH, 1893, 1894. By J. R. H. 
MAC FARLANH, Staff-Commander R.N. (Retired). 
[COMMUNICATED BY PROFESSOR D. J. CUNNINGHAM, M.D., F.R.S., 
HON. SEC., B.D.S. | 
[Read NovemBrr 21; Received for Publication NovemBeER 23, 1894 ; 
Published Marcu 30, 1896. | 
Towarps the close of the last century, attention seems to have been 
first drawn to, and observations made of, phenomenal changes of 
the level of the water in many of the Swiss lakes; these alterations 
consisting of a series of rising and falling water occurring at in- 
definite periods, the whole movement occupying different spaces of 
time, varying from ten minutes to nearly an hour; the time so 
occupied being apparently dependent on the positions where the 
observations were taken on the several lakes. 
This complete movement of rise above, and fall below, mean 
level has been termed a seiche. 
The amplitude of a seiche is the extreme difference of level so 
produced. 
The duration of a seiche is the time in seconds from the moment 
when the water is at mean level, until it is again at mean level, 
after passing through one maximum and one minimum.* 
There would appear to be no records of observations made of 
seiches in any of the lakes of Great Britain or Ireland, but the 
non-existence of these alterations of level must not therefore be 
assumed, as they may be of such rare occurrence, or the rise and 
fall of so slight a character, that they have hitherto escaped 
notice. 
The observations made at Lake Derravaragh, although un- 
avoidably incomplete, leave no doubt as to the existence of seiches 
1See ‘‘ Lake,’”? by J. Y. Buchanan, Esq., F.R.S., m Encyclopedia Britannica, 
9th ed., vol. xiv., p. 220. 
