ON THE SEICHES OF LOCH EARN. 473 



The seiche of 3rd September 1905 (fig. 5) is interesting because it was accompanied 

 by the strongest gale experienced during the two months of observation. 



For some hours before midnight the wind had been very light, and at 2 h it was 

 practically calm. About 2 h 37 m the wind began to rise ; and in an hour it had reached 

 a mean velocity of about 15 (mile/hour). The velocity fluctuated between 6 and 15 

 till 7 h , when a very sudden rise began. By 7 h 30 m the average velocity had risen to 35, 

 with extremes of 45 to 50. About 8 h 30 m there was a sudden drop to about 25, then a 

 more gradual drop to 10 at 9 h 20 m . After that the gale rose again to a mean velocity 

 of 35 to 40, with extremes occasionally reaching 53. After lasting four hours, the gale 

 began to abate about 15 h ; and then fell more or less uniformly to calm about 20 h , there 

 being two rather sudden lulls at 17 h and 19 h 20 m . 



Throughout the whole of this time the microbarogram is much disturbed. During 

 the strongest parts of the gale it shows the characteristic wind blurring, and throughout 

 there are fluctuations of various periods : e.g. 7 '2' at 2 h , 5-6' at 4 h 30 m , 13 '6' at 8 h 30 m , 

 17' at 16 h . 



Till about midnight there had been a fairly regular UB-seiche with a small trinodal 

 component, the total range of the whole being about 31 mm. Soon after midnight, 

 that is, more than 2\ hours before the wind began to rise, the limnogram begins to show 

 serious disturbance. This disturbance becomes strongly marked at 5 h , when the total 

 range of the seiche reaches 60 mm. ; and there is a strong development of seiches of 

 higher nodality, in particular of one having a period of about 2'9 m . 



At 7 h , when the wind suddenly rises into a gale, there is no very marked change in 

 the seiche. But between 8 h 30 m and 9 h there is an increase in the total range from 

 56 mm. to 78 mm., due no doubt to the simultaneous microbaric disturbance, which has a 

 period of about 13'6 m . After this the seiche tends to settle down into a UB-dicrote, 

 strongly embroidered with higher components while the gale lasts. It is worthy of note 

 that at 14 h , i.e. 7 hours after the gale commenced, the mean level of the lake at Picnic 

 Point has only risen about 6 mm. About 16 h there is a decrease in the total range of 

 the seiche from 64 mm. to 51 mm. This may be due partly to the drop in the wind, 

 but much more probably to the simultaneous microbaric disturbance, which has a period 

 of about 17 m , and would strongly affect the uninodal component of the seiche. 



The range of the disturbance on the microbarogram was a little under 2 mm. ; and 

 our data from the triangle of microbarographs showed that it travelled along the lake 

 with a velocity of 53 miles an hour. For rough purposes and for convenient calculation 

 we may take 48 instead of 53 ; and suppose the period of the pressure disturbance and 

 also of the uninodal period to be 15 ,n , and the circumstances as to phase to be the most 

 favourable possible. The formula (57) of the mathematical Appendix to this memoir 

 then gives for the addition to the amplitude of the uninodal in 1 5 m dk x = f dp = 3 mm. 

 The effect after two undulations will therefore be 6 mm., that is, an alteration of 

 12 mm. in the range of the seiche, which, as it happens, is within a millimetre of 

 the value observed. 



