May 23, 1878] 



NATURE 



lOI 



'■ 'Itis to ehicidate this question that the experiments of 

 M. Forel tend. They were referred to in Nature, vol. 

 xii, p. 134, and a paper on them was read at the Physical 

 ■-Society, May 27, 1876 (Nature, vol. xiv. p. 164). Working 

 with an artificial basin, then on several Swiss lakes, he 

 has proved the constancy of the duration of the wave of 

 libration ; the increase of this duration with increase of 

 length of the basin ; its diminution with increase of 

 <3epth. The character of the wave of libration is, that 

 the water rises at one extremity of the basin while it 

 sinks at the other, and vice versa. As to the intensity of 

 the wave it varies much, either under the influence of the 

 mysterious cause which produces it, or according to the 

 region of the basin where it extends. The seiches are 

 much more pronounced at Geneva than in the broader 

 part of the lake, either on account of the contraction of 

 the inass of the water on the south-west shore, or of the 

 considerable decrease in its depth. 



If we examine the seiches at Morges, their theory 

 becomes infinitely more complicated, because, besides the 

 phenomenon of the longitudinal seiche, mainly visible at 

 the two extremities of the lake, the changes of level pre- 

 sent other periods in relation -vvith the transverse libration 

 of the liquid mass. They are also modified by oblique or 

 cross reflections of waves in motion, which are connected 

 with the form of the basin. We may only expect a real 

 regularity in the form of the waves of libration if that 

 form of the basin is regular. 



In order to be able to base his conclusions on more 

 varied observations, M. Forel has measured the duration 

 of the seiches of seven other Swiss lakes, besides that of 

 Geneva, and he has been able to draw the following in- 

 ference : — The duration of the wave increases with the 

 length of the basin, and diminishes in proportion as the 

 depth increases. In other words, the rhythm of the longi- 

 tudinal seiches is a direct function of the length of the 

 different lakes and an inverse function of their depth. 

 From the figures which have permitted the inference of 

 this general law, we may, by neglecting the influence of 

 the depth, approximately conclude the duration of a 

 seiche corresponding to a lake of a given length ; and 

 reciprocally infer the probable kilometric length of a 

 seiche whose duration is known. It will then be possible 

 .to infer the direction of any particular wave, among those 

 which show themselves on the shore of the Lake of 

 Geneva, for example. The above enunciation is then 

 generalised, and is thus modified : — The duration of 

 seiches is a direct function of the length and an inverse 

 function of the depth of the section of the lake along 

 which they oscillate. 



M. Forel has found a mathematical formula relative to 

 the movement of liquids in basins in the works of a Bale 

 savant. Dr. J. R. Merian (1828), completing the formula 

 given by Prof. Guthrie, and applicable to the movement 

 of seiches (Nature, vol. xv. p. 91). It may be stated 

 thus : — The duration of seiches is proportional to the 

 length of lakes and inversely proportional to the square 

 root of their mean depth. 



By experiments made in the first place on the Lake of 

 Neuchatel, it was found that the movements of libration 

 of the water were alternate and simultaneous. The water 

 rises at oiie of the extremities, while it falls at the other. 

 The amplitude of these movements is very variable, but 

 their constancy is proved, preserving the same rhythm ; 

 their cessation or their absence would be the abnormal 

 fact. 



These conclusions have been extended and confirmed 

 in a remarkable manner since the establishment of auto- 

 matic instruments, intended to take a graphic tracing of 

 these phenomena. A pencil in constant connection with the 

 level of the lake draws a line on an endless paper, which 

 is unrolled by a clockwork movement. Following the 

 forms of this line, we discern in a very exact manner the 

 influence of the various waves of oscillation, whose am- 



plitude may vary from a few millimetres to a metre and 

 more, if it has to do with the extremity of the lake, near 

 Geneva. At Morges the first model of this apparatus 

 was set to work, named by its inventor " registering 

 limnimetre ; " the curves obtained are compounded of the 

 actions of various categories of seiches. They are more 

 or less difficult to discern, but ordinarily recognisable 

 and in connection with the rhythm corresponding to each. 

 At Secheron, near Geneva, where M. Ph. Plantamour has 

 had an apparatus of the same kind constructed, the longi- 

 tudinal seiche shows itself in a much more sensible 

 manner, corresponding to a duration of about seventy- 

 three minutes. The vibrations which affect the level of 

 the lake under the influence of the wind or the passing of 

 steamers disturb here much less than at Morges the 

 study of the rhythm of libration, and above all, the 

 measurement of the amplitude of this libration, which is, 

 as we have said, much more considerable than at Morges. 



This second registering limnimetre has only been 

 working regularly since June, 1S77. It has already served 

 to confirm the fact of the alternation of the movement of 

 the water, which rises at Geneva, while it falls at Morges, 

 and vice vetsd, following the period of seventy-three 

 minutes for the great longitudinal seiche of the lake, thus 

 confirming the presumption of an oscillation of the liquid 

 mass around a median line, normal to its length. 

 Morges, being situated at a short distance to the east of 

 this line, offers a movement of the water of very inferior 

 extent to that of the terminal part of the lake near 

 Geneva. It is desirable that a similar apparatus be set 

 up at the eastern extremity of the lake, between Vevey 

 and Villeneuve, with a barometer comparable to that at 

 Geneva, and giving constant indications. 



It is generally presumed, in fact, that it is to changes of 

 pressure on various parts of the lake that the variations in 

 the intensity of the oscillatory movement of the water are 

 due. These variations are nearly always marked during 

 stormy weather. Before the storm comes on, before even 

 the barometric column is disturbed, the libration increases 

 in amplitude. But concordant observations at the two ends 

 of the surface which is agitated have not yet been made 

 to furnish data to determine if an increase of pressure at 

 one of the extremities of the lake coincides with a 

 depression at the other. The point remains, meantime, 

 very doubtful, and very worthy of being investigated. 



As to the oscillations of exceptional amplitude, such as 

 those of October 3, 1841, when the difference of level 

 observed at Geneva exceeded 2*14 metres, the presump- 

 tion was that we ought to seek for the cause in some 

 movement of the earth's crust, of the basin of the mass of 

 oscillating fluid. Great, then, was the anxiety of Swiss 

 observers, when, on October 8, 1877, they were awakened 

 at 5*i6 A.M. by a strong shock of earthquake, on running 

 to their limnimetre, which they found working very 

 regularly. But no trace of the action of that violent com- 

 motion was shown by the registering pencil. Not only 

 must that absence of eff'ect make us seek elsewhere for the 

 cause of these mysterious accidents, but we have reason 

 to be astonished at the insensibility of a liquid surface 

 which remains calm though balanced on ground which 

 was so agitated as to crack the woodwork of houses, ring 

 beUs, and displace furniture in all the neighbouring 

 region. This disappointment is a new motive to continue 

 this interesting research, and to enlist the physicists of 

 every nation, all the more that wherever there are lakes 

 there ought to be seiches. E. G. 



EXAMINATION OF THE PHONOGRAPH 

 RECORD UNDER THE MICROSCOPE^ 



MR. FRAZER referred to previous results obtained by 

 him of some examinations of the tin foil which had 

 been indented by the stylus, or needle point, of the pho- 



* Abstract of paper presented at the meeting of the Franklin Institute 

 April 17, 1878, by Persifor Frazer, Jun , A.M. 



