ON THE SEICHES OF LOCH EARN. 483 



The maximum depression (4 mm.) at the uninode and the maximum elevation (5 mm.) 

 at the binode were nearly simultaneous, the latter apparently following about l|- m after 

 the former. Unfortunately, owing to the irregularity of the clock at the uninode, 

 certainty on this point is not attainable. 



It seems clear that an abrupt elevation of the surface travelled along the eastern 

 part of the lake. The first rise began at the binode at 13 1 ' 55'31 m ; and at the Picnic 

 Point at 14 h 5 , 24 m , that is, 9*93 m later. The first maximum (5 mm.) is seen at the 

 binode at I4 h T05 m ; and at Picnic Point at 14 h 10-57" 1 , that is, 9-52 m later. The 

 velocity of propagation of the first rise would thus be 6'0 (mile/hour), and of the 

 first maximum 6*3 (mile/hour) ; and it is interesting to notice that by the time the 

 wave has reached Picnic Point a shallow minimum has developed in front of the 

 maximum. If the wave had travelled as a solitary long wave, it would have taken only 

 about 7 m to travel from the binode to Picnic Point. 



After the wave reached St Fillans it seems to have been reflected backwards and 

 forwards between the ends of the lake, at first with a good deal of irregularity ; but 

 gradually it developed the characteristics of a regular dicrote seiche. There are two 

 points (easily seen on the binodal limnogram), viz. 16 h and 17 h 20 m , where the range 

 of the seiche was suddenly increased, evidently by barometric disturbances which 

 occurred at these times. The increase at 17 h 20™ may have been partly due to the 

 heavy shower. 



At 17 h 20 m on the 23rd the dicrote is fully developed (2U = 11'5, 2B = 7'0). It 

 retains its character, with gradually decreasing range, until a little before 24 h on the 

 24th. About that time the microbarograph at Killin shows disturbances with periods 

 T = 10 m , T = 15'6 m ; and there is an alteration of the UB from 2U = 37 mm., 2B - 17 mm. 

 to 2U = 11 "4 mm., 2B = 1 '0 mm. The dicrote then remains steady until 22 h on the 25th, 

 when it undergoes a sudden disturbance, which rapidly destroys its configuration. This 

 sudden disturbance and the almost total destruction of the seiche about 5 hours later 

 are difficult to explain by the meteorological conditions, unless they were due to 

 variations of the wind. 



6. Effect of the Impact of Wind-Gusts. — Inasmuch as a wind velocity of 10 

 (mile/hour) is calculated to produce a pressure of about 1*5 mm. (Aq.) by direct impact 

 on a small area, it is reasonable to expect that the impact of wind-gusts, especially in 

 the case of lakes enclosed by high hills, may at times cause seiches. There are, however, 

 various difficulties in obtaining data on the subject. It is difficult to determine the angle 

 of impact of the wind-blasts. Then it is uncertain whether the wind ever falls at the same 

 angle and at the same time over large parts of the surface of a lake. The appearance of 

 the lake-surface on windy days very often suggests the contrary. What we frequently 

 see are patches of wind disturbance progressing over the lake- surface with varying 

 velocities. 



Then again it is difficult to separate the effect of wind impact from the disturbances 

 of the ordinary barometric pressure which always accompany high winds. 



