DETERMINATION OF THE LONGITUDINAL SEICHES OF LAKE GENEVA. 637 
made, and the values obtained from the eastern system were multiplied by appro- 
priate constants, these being the ratios of the values of £ at5c=l in the two 
systems. 
The calculation of the nodes was accomplished by taking a few intermediate 
values of I' n (0, x), and the positions of the zero values of £ obtained by interpolation 
from them. The following results were obtained : — 
Uninodal oscillations : node at x= '377. 
Binodal oscillations : western node at x = 'll 3. 
eastern node at X — 1'067. 
By interpolation in the table of latitude and longitude of the mid-points of the 
transversals (see Table I), we get the nodes defined by the transversals whose mid- 
points have the following latitude and longitude : — 
Uninodal oscillations : lat. = 46° 23 ,- 9, long. =4° 0 ,- 2 E. of Paris. 
Binodal oscillations : lat. = 46° 17 ,- 9, long. = 3° 52 ,- 4 E. of Paris. 
lat. = 46° 27'*3, long. = 4° 12'-8 E. of Paris. 
These sufficiently define the nodes, and reference to any map of Geneva will at once 
determine the exact positions. 
11. Comparison of Results with Observations. 
The zero of the chart used in the present calculations is given as “ l m 772 au 
dessous de repere de la pierre a Niton de Geneve.” 
Most of the observations on Lake Geneva are recorded by Forel in Le Leman. 
His zero plane is taken as “ un plan passant a 3'0 m. en contrebas du repere de la 
Pierre du Niton de Geneve” (vol. i, p. 455). His zero plane is therefore 1'228 m. 
below the zero plane of the chart, and as most of his observations ostensibly refer 
to this mean level of the lake, an exact comparison between calculations and obser- 
vations cannot be made. Forel’s mean value for the period of a uninodal oscillation 
is 73‘5 minutes ; our theoretical determination gives 74'45 minutes. On p. 121 of 
vol. ii of Le Leman is given a set of observations when the lake was 30 cm. 
above his zero level ; on this occasion the period was found to be 74 ‘0 minutes. 
Forel attempts to correct for the height of the lake, but his method is not very 
satisfactory ; indeed, it would be difficult to find an easy way of reducing observa- 
tions to standard height. The agreement between calculation and observation is, 
therefore, as good as can be expected when the conditions of observation are not 
identical with those of calculation. 
The value of the period of the binodal oscillations, according to Forel, is 
35 '5 minutes, but the mean level of the lake on the occasions of observation is not 
given. Our calculations give 35 7 minutes. No comparison is possible for trinodal 
oscillations. 
The positions of the nodes are not easily determined experimentally, and Forel 
TRANS. ROY. SOC. EDIN., YOL. LII, PART III (NO. 25).' 98 
