Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/233

Rh They are greater at the extremities than at the middle of lakes, at the head of long gulfs whose sides converge gently than at stations in the middle of a long straight coast, and in shallow as compared with deep lakes or parts of a lake. They also appear to increase with the size of the lake. The duration of the seiches is found to vary considerably, but the mean deduced from a sufficient number of observations is fairly constant at the same locality. Thus, for, Dr Forel has found it to be for the half seiche 315±9 s. At different stations, however, on the same lake and on different lakes it varies considerably. Thus on the it is, for the complete seiche, 630 s at, and 1783 s at ; on  it is 2840 s at , and 264 at. The curves traced by the at have been subjected to a preliminary harmonic analysis by Professor Soret, and he has decomposed them into two undulations, the one with a period, from crest to crest, of seventy-two, and the other with a period of thirty-five, or a little less than half the larger period. As the amplitudes of the composing curves vary much, there is great variety in the resultant curves. Besides these two principal components, there are others which have not yet been investigated. With regard to the cause of the phenomenon, Dr Forel attributes the ordinary seiches to local variations of, giving an impulse the effect of which would be apparent for a long time as a series of oscillations. The greater seiches, such as those of 1·5, he attributed to shocks; but, as a very sensible  passed over  quite recently without leaving the slightest trace on the , he has abandoned this explanation, and is inclined to attribute them to pulsation set agoing by violent downward gusts of , especially at the upper end of the lake. M. Plantamour, who has devoted much attention to the same subject, assured the writer, in the of, that he was completely at a loss for a satisfactory explanation of them. Seiches have not been observed on the lakes, though there is little doubt that they would be found if sought for. There are, however, records of disturbances of some of the lakes, especially in, of which the following may be cited as an instance.

A violent disturbance of the level of is reported in the Statistical Account of Scotland, xvii. p. 458, to have occurred at on 12th, continuing in a modified degree for four , and again on 13th 1794. lies at the north-eastern end of the lake, where the  issues from it. It lies at the end of a shallow. “At the extremity of this bay the was observed to retire about 5  within its ordinary boundary, and in four or five  to flow out again. In this manner it ebbed and flowed successively three or four times during the space of a quarter of an, when all at once the rushed from the east and west in opposite s,.... rose in the form of a great , to the height of 5  above the ordinary level, leaving the bottom of the  dry to the distance of between 90 and 100  from its natural boundary. When the opposite s met they made a clashing noise and foamed; and, the stronger impulse being from the east, the after rising to its greatest height, rolled westward, but slowly diminishing as it went, for the space of five, when it wholly disappeared. As the subsided it flowed back with some force, and exceeded its original boundary 4 or 5 ; then it ebbed again about 10, and again returned, and continued to ebb and flow in this manner for the space of two , the ebbings succeeding each other, at the distance of about seven , and gradually lessening, till the  settled into its ordinary level. During the whole time that this phenomenon was observed the was calm. On the next and four succeeding an ebbing and flowing was observed nearly about the same  and for the same length of, but not at all in the same degree as on the first .” The above is the account given by the Rev. Thomas Fleming, at the time of, who was an eye witness. It resembles in all essential particulars the descriptions of which accompany actual, yet in his account he goes on to say—“I have not heard (although I have made particular inquiry) that any motion of the was felt in this neighbourhood, or that the agitation of the  was observed anywhere but about the  of .” It is well known that there were great  observed in  at  of the , and there is a tradition in the neighbourhood that  near  was largely increased in extent by the dislocations which took place.

In all lakes there are changes of level corresponding with periods of and of. They are the more considerable the greater the extent of draining into them, and the more constrained the outflow. In the great n lakes, which occupy nearly one-third of their area, the fluctuations of level are quite insignificant; in  the   give as the maximum and minimum ly range 1·64 and 0·65. In the the mean  oscillation is 5, and the difference between the highest and the lowest s of  is 9·3. The most rapid rise has been 3·23 es (82 ) in twenty-four s. A very remarkable exception to the rule that large lakes are subject to small variations of level is furnished by  in. Since its discovery travellers have been much perplexed by the evidence and reports of considerable oscillations of level of uncertain period, and also by the apparent absence of visible outlet, while the freshness of its s was of itself convincing evidence of the existence of an outlet. By the careful observations of successive explorers the nature of this phenomenon has been fully explained, and is very instructive. It has recently been visited by Captain Hore of the, and it appears from his reports that the peculiar phenomena observed depend on the fact that the area of  into the lake is very limited, so that in the dry  the  running into it dry up altogether, and its outlet gets choked by the rapid growth of  in an ial. A or  is thus formed which is not broken down until the s of the lake have risen to a considerable height. A catastrophe of this kind happened whilst Captain Hore was in the neighbourhood, and he noted the height of the at different s near his station at, and observed it fall 2  in two s It continued to fall until in seventeen s it had fallen over 10. Taking the length of the lake at 330 s, and the mean breadth at 30 s, its surface is 9900 s. If this surface be reduced 2 in sixty s, the  will have to escape at the rate of 137,500  per. The mean rate of discharge of the is 207,000  per. Hence, without taking into account which would be brought into the lake by  during the two s, we require for cutlet a  at least two-thirds of the size of the, and in the  such a  is found. When visited it the  was quite stopped up with dense growth, and no  was issuing; the lake was then rising; when Captain Hore visited it the lake was falling rapidly, and the  was a rapid  of great. One of the chief to the lake was found to be discharging at the rate of 18,750  of  per ; a few s later it was dry and the  closed with. During the dry too the lake, with its 10,000 s of surface, is exposed to the  action of the south-east trade, and when the supply is so insignificant this must be sufficient of itself to sensibly lower the level. Ordinarily then we might expect the lake to be subject to a ly ebb and flow corresponding to the periods of and s; and, from what we learn of the great fluctuations of  one  with another, we should expect that during a series of dry s the obstructions to the outflow would gain such a head 