ON THE SEICHES OF LOCH EARN. 



497 



those which are so marked in the lower limnogram in fig. 19, and noticed over and 

 over again that it would set one down in an interval of total or comparative calm. On 

 looking to windward when this happened, a black line would be seen on the water some 

 distance off, indicating a coming wind-squall ; then presently would be heard the rustle 

 of the wind in the trees overhead ; and the increased prattle of the waves among the 

 pebbles on the beach would show that the squall had reached the observer. In short, 

 the lake vibration had gone before, and the wind had followed after. The explanation 

 seems to be that the squall exerts a horizontal traction on the water and causes a drift 

 current. By and by this current becomes greater than the compensating return current 

 underneath. Thus a hump (or a group of waves) is raised on the surface, which is propa- 

 gated in the water with a speed usually exceeding the velocity of the wind in a moderate 

 breeze. We have here, in fact, in small a phenomenon with which sailors are familiar 



Fig. 20. 



on a large scale, when they point to the long swell which records or presages a distant 

 storm at sea. 



I obtained a striking confirmation of this view in the course of an observation 

 planned to test a totally different hypothesis. I had supposed that the vibrations might 

 be clue to some extent to simultaneous abrupt or periodic disturbances of the atmo- 

 spheric pressure. As explained in Part I. of this report, * the statolimnograph can be 

 used in rapid alternation as a limnograph and as a microbarograph. Fig. 20 shows the 

 result of an observation of this kind.t The limnogram is deeply embroidered; the 

 microbarogram is all but straight. Since the sensitiveness of the Richard statoscope is 

 fifteen to twenty times that of a mercury barometer, the ordinate of the microbarogram 

 represents the air-pressure on a larger scale than a water barometer. If we allow for 

 the damping effect of the well and access tube on the half-minute vibrations, we shall 



* Trans. R.S.E., vol. xlv., p. 368 (1906). 



t Another is given in Part I. of this report, Trans. R.S.E., vol. xlv., p. 370, fig. 12 (1906). 



