Mac Fartane—Seiches in Lake Derravaragh, Co. Westmeath. 289 
on that lake; later on I shall endeavour to show the difficulties 
in obtaining satisfactory results. 
Until the alterations of the level of the water in Lake Derra- 
varagh first came under my notice, seiches were quite unknown to 
me. I must therefore approach the subject with diffidence, giving, 
as fully as possible, the results of the observations made in the 
order in which the seiches occurred, and afterwards stating any 
ideas deduced from them. 
I may premise that the house occupied by me, during the time 
these observations were being taken, was situated about twenty 
yards from the edge of the lake, and therefore in a most favourable 
position for observing; also that, with a view to noting the rise 
and fall of the lake for fishing purposes, I had placed a batten, 
marked in feet and inches, on one of the piles of an old pier; this 
batten, and the gauge which afterwards replaced it, being situated 
on the weather side of the lake with respect to, and in perfect 
shelter from, the prevailing winds. 
1st Observation.— When going out in a boat on the lake, on 
the afternoon of the 38rd October, 1893, I noted that the level of 
the lake, then only slightly above its lowest during summer, was 
standing at half-an-inch above the lowest mark on the batten ; 
returning about two hours later the level was half-an-inch below 
the same mark. Puzzled by this change of level of 1 inch, in such 
a short period, I carefully watched the batten until dark, from 5” 
10™, to 6" 25", during which space of seventy-five minutes there 
occurred three complete movements; two falling and one rising, 
or about twenty-five minutes to each movement, the amplitude 
being two and a half inches.? 
The batten giving very rough results, I constructed a gauge 
which would rise and fall with the water, and a pencil attachment 
registering the amplitude; this enabled me, on the next day, to 
take a number of observations, but the change of level was then 
subsiding, the amplitude being only 1 inch, and the duration of 
the seiche thirty-six minutes. Gradually the movement became 
less, until about 4 p.m., when it entirely ceased. 
1 Although, in the definition of a seiche, already given, the time is said to be 
expressed in seconds, for want of delicate appliances I can give nothing less than half 
a minute, except when as a result of the mean of several observations. 
