ON THE SEICHES OF LOCH EARN. 387 



The curves A — B and A — B — T show the result of residuating out successively the 

 binodal and trinodal seiches, by means of the approximate periods T' 2 = 8*02, T' 3 = 6 '02. 

 The curve A — B — T thus purified still shows departure from pure sinusoidality, some, 

 but orobably not all, of the disturbance being accidental, due, say, to slight puffs of 

 wind. As it stands, it gives T" 1 = 14'45 from e to f\ and T" 1 = 14 , 51 from g to h. 

 Assuming the watch error to be negligible, we have thus set comparatively close limits 

 to the uninodal period of Loch Earn; and we now see that T 2 = 8"08 / and T 3 = 6'02' 

 must be very close to the true values of the Binodal and Trinodal periods corresponding 

 to the depth of the lake on 10th July 1904. 



PART II. 



In the early summer of 1905 the observation of the seiches of Loch Earn was under- 

 taken, under the direction of Professor Chrystal. The apparatus employed consisted 

 at first of two Sarasin limnographs, and two small portable limnographs on the model 

 of Endros. 



The permanent recording limnographs were intended to be set up at the Uninode 

 and Binode, the positions of which had been calculated by Professor Chrystal. The 

 positions of the nodes were to be checked experimentally by the index limnometers ; 

 but the weather conditions at the time were unfavourable, and we adhered to stations 

 near the calculated positions, which proved to be accurate enough for the object in 

 view, viz., the elimination of one of the principal seiches at each node. 



The Sarasin Limnographs. 



Situation. — At both of the nodes selected as the stations for the permanent 

 limnographs, viz., the Uninode and the East Binode, the shore of the loch presented a 

 very gently sloping beach, some 60 feet broad, terminating landward against rock a 

 few feet from the edge of the water. In the lake, beyond the storm beach, the slope 

 was very rapid into deep water. These conditions rendered impracticable two of the 

 approved methods of setting up limnographs. The first method, afterwards adopted 

 with Professor Chrystal's limnograph near St Fillans, is to place the cylinder 

 directly in fairly deep water ; the second method (recommended by Endros) is 

 to dig a well inland, at some distance from the loch, and to connect it with the loch 

 by a tube. 



Here a compromise had to be made, and a well about 2 feet deep was dug in the 

 loch itself. 



The Staging. — To support the instrument a wooden framework was built, the 

 essential structure of which, and the disposition of the various parts of the instrument, 

 can be seen from the accompanying figure. 



