PROCEEDINGS OF THE CANADIAN INSTITUTE. 57 



Harbour of Morges, upon Lake Geneva, and obtained a mean of 4 min. 24 sec. lor 

 their total duration. At Geneva Vaucher's observations give a mean duration of 

 26 1-2 minutes. De Saussure and Duillier refer to seiches of i 1-2 metres in ampli- 

 tude, and M. Venie mentions one at Geneva of 2.14 metres, but usually they vary 

 from about four inches to one foot. He (Forel) suggests that these are not true 

 waves or progressive undulations of the water surface, but a movement of " oscilla- 

 tion of balance," or fixed oscillation, which may be both longitudinal and trans- 

 verse. (Archives des Sciences Naturelles. Geneva, 1874.) 



In 1876 Forel set up an automatic instrument to register these movements, and 

 from records extending over four months, he deduced the existence of three vari- 

 utes; intermediate, duration 25 minutes, 

 eties of seiche, viz: transverse, duration 10 minutes; longitudinal, duration 70 min- 



Upon this instrument he also observed movements of what he terms " vibration." 

 caused (i) By steamers. The interval between these is from 9 to 60 times greater 

 than that between ordinary waves, and they preceded the approach of a vessel by 

 about 25 minutes, or when it was 9 1-2 kilometres distant, continuing for two or 

 three hours afterwards. 



(2) By wind, having no regular time or rhythm, and varying in amplitude from 

 nothing to 10 millimetres, and in duration from 45 seconds to three or four minute-. 

 He remarks that " sometimes there are little or none with a strong wind." 



It may be mentioned that Guthrie has experimented upon this movement of 

 oscillation, or balance in water, using, however, vessels in which the depth exceeded 

 half the length, thereby eliminating the influence of depth altogether. (Proceedings 

 Phys. Soc, Vol. I., 1875). 



Lord Kelvin gives a theoretic law for the duration of these seiches in any lake, 

 viz.: the semi-period of an oscillation is equal to the time that a body, travelling at 

 the rate which it would acquire in falling from a height equal to half the mean 

 depth of the lake would take to traverse the length of the lake. Thus, the 

 duration of a seiche is proportional to the length of the lake, and in- 

 versely proportional to the square root of its mean depth. (Archives "des 

 Sciences Naturelles, Geneva, 1876). Applying this to Lake Ontario, and assuming 

 the mean depth to be 300 feet, we obtain a theoretical duration for a longitudinal 

 seiche, of over five hours. As will be shown later, the mean interval between the 

 longest undulations, as taken from the Humber traces, is about 4 hours and 49 

 minutes. 



In t88o Professor Forel, in a letter, states that the smaller and more nnid 

 oscillations may be accounted for by dividing the lake surface into more than one 

 nodal point. (Archives des Sciences Naturelles, Geneva, 1880). 



That you may more fully understand the following illustrations, it is necessary 

 to become somewhat familiar with the movements of the upper atmosphere, where 

 the chief cause of these lake oscillations is to be found. Permit me to quote a few 

 lines from the late Professor Helmholtz. of Berlin (although previously cited in 

 an earlier paper), who made a special study of atmospheric waves from theory, 

 and analogy with ocean waves : " As soon as a lighter fluid lies above a denser 

 one, with well-defined boundary, then, evidently, the conditions exist at this bound- 

 ary for the origin and regular propagation of waves, such as we are familiar with 

 on the surface of water. This case of waves, as ordinarily observed on the boundary 

 surfaces between water and air, is only to be distinguished from the system that may 

 exist between dififerent strata of air, in that in the former the difference of densitv of 

 the two fluids is much greater than in the latter case. Since the moderate winds 

 that occur on the surface of the earth often cause water waves of a metre in length, 

 therefore the same winds, acting upon a stratum of air of 10 degrees diflfcrence in 

 temperature, maintain waves of from two to five kilometres in length. Larger ocean 

 waves, from five to ten metres Ions:, would correspond to atmospheric waves of from 

 thirteen to thirty kilometres, such as would cover the whole sky of the observier, 

 and would have the ground at a depth below them less than that of one wave- 



