SEICHES AND OTHER OSCILLATIONS 
87 
wave-length increase no longer, unless the waves run into shallow 
water. This progressive surface wave motion may persist for a con- 
siderable time (in the ocean for days) after the wind has fallen, in the 
form of swell, and it may be propagated into regions where there has 
been no wind. In ordinary circumstances, owing to the continual 
variation in the strength of the wind, and in the case of lakes probably 
also to reflections from the shores, at any particular moment not one 
train of waves is generated, but many of slightly differing wave-length 
and differing phases. These trains interfere and cause a succession 
of wave maxima, commonly called wave groups."" 
Several observations of the periods of surface waves and wave 
groups on Loch Earn were made by coiniting the waves or wave maxima 
which passed a given point in a certain time. This is easy in the case 
of the wave maxima ; not so easy in the case of the single waves, 
which have a bewildering habit of losing themselves by running into 
and through each other and through the maxima. Still, the results 
were fairly concordant. The observations were made at the eastern 
binode and at Picnic Point during westerly winds of various kinds. 
The average of the periods for single waves was '035™, the smallest 
and greatest values being '024^" and '045™. The most usual value 
of the period for the groups was "5"^ to '66"^ the least and greatest 
values observed being '33™ and 1*17™. For the single waves ranges of 
6 in. to 12 in. were common ; but on one stormy day ranges of 2 ft. 
to 3 ft. were observed. 
From a set of observations made at my request by Mr James 
Murray on Loch Tay, the following data were calculated for that lake. 
T is the period, X the wave-length, and v the velocity of propaga- 
tion for the single waves ; T^, X^, Vg^ the corresponding magnitudes 
for the wave groups. The observations were made at Killin, when 
there was no wind, on swell coming in from the lake and running in 
water 13 ft. 6 in. to 12 ft. deep.^ 
For Single Waves 
T='017^, A=18 ft. to 25 ft., f=18 to 25 (ft./sec.) = 1 2 to 17 (mile/hour). 
For Larger Groups of ^ to Q Maximum Waves 
T^ = -5«^ to -75"^ A,/ = 252 ft. to 283 ft, y,^=8-4 to 6-3 (ft./sec.) - 5-7 to 4-3 
(mile/hour). 
For Smaller Groups of to S Maximum Waves 
T,^=-083ni to -17™ A^ = 42 ft. to 63 ft., y, = 8-4 to 6-3 (ft. /sec.') = 5-7 to 4-3 
(mile /horn"). 
^ The velocity of a "long wave" in vv^hich would be about 20 (ft./sec). 
