ON THE SEICHES OF LOCH EARN. 481 



lasted nearly two hours, and caused a maximum depression of 14 mm. (Aq.) ; it came 

 from W. 62° S. to W. 67° S., and travelled with a velocity of 17 to 22 (mile/hour), 

 i.e. with a velocity of 33 to 51 along the lake. The sections of the disturbance at 

 Killin and Lochearnhead on the one hand and at Ardtrostan on the other were very 

 different. The minimum was rounded and pretty flat at the two former places, but 

 cuspidal at the latter. Again, at Killin and Lochearnhead the minimum was followed 

 by a sharp-pointed maximum, with an almost perpendicular rise ; while at Ardtrostan 

 the recovery after the minimum is very gradual, and there is only a little wart corre- 

 sponding to the peaks at the other two stations. 



It is interesting to notice that the minimum of the disturbance, although it destroys 

 the configuration of the UB, and generates one of the best-marked BT-dicrotes that we 

 observed, yet produces no great change in the total range of the seiche. It does produce 

 a small rise of level at Picnic Point of 7 mm. to 1 1 mm. This is confirmed by the 

 limnogram taken at the binode, where at that time the Sarasin was running at high 

 speed (160 mm. per hour). This shows a rise of level of 5 mm., and a diminution in 

 the range of the uninodal seiche of about 8 mm. 



At 14 h 13 m there is a sudden rise of level of about 14 mm., evidently due to the 

 intense action of the maximum of pressure developed towards the western end of the 

 lake, which there is nothing to counterbalance on the eastern part. It is after this 

 point that the new BT configuration becomes conspicuous. As will be seen from the 

 anemogram, the barometric and seiche disturbances at 14 h were associated with a very 

 sudden rise in the average wind velocity from 5 to 25 (mile/hour). 



The Glen Ogle Storm. — It seems unnecessary further to multiply instances of the 

 connection between abrupt barometric and seiche disturbances. I shall therefore con- 

 clude this part of my report by describing one of the most remarkable observations we 

 made on Loch Earn. 



On the 23rd August, after a dead calm during the night and heavy rain in the 

 early morning, at 8 h 20 m there was a light breeze, W., 5 (mile/hour). There was low 

 cumulus on the hills to E. and N.E. ; but there was bright sunshine, and the clouds (3) 

 in general were high. The main drift was from S.E. ; but there was a mackerel forma- 

 tion apparently moving in a different direction ; also a mare's-tail showed to S.W. 



The waves were running from W. — a slight swell diversified by oil bands, which 

 were seen at intervals throughout the day. 



From 8 h 20 m to 12 h 30 m the wind was light, fluctuating with a rough period of l h . 

 At 12 h 30 m there was a sudden gust of 15 (mile/hour). After that the wind rose 

 somewhat, and fluctuated for about 5 hours between and 13 (mile/hour) mean 

 velocity. It was unusually gusty, and at 14 h 5 m an extreme velocity of 25 (mile/hour) 

 was registered. At this moment a black rain-cloud came down Glen Ogle, and reached 

 over the western part of the lake as far as Ardvoirlich, where it stopped. 



At 14 h 50 m there came on a sudden rain-shower, the wind being then W. by S. 

 After this there was rain at intervals till 20 h 18 m , an especially heavy shower at 17 h 20 m . 



