SEJCHES OF THE LAKE OF GENEVA. £01 



out the fame thing happening over the reft of the furface; 

 or ftill more if the weight fhould be augmented upon that 

 remaining furface, the water will be forced to rile in that 

 lafi place, and will again defcend when the atmolphere fliatl 

 have returned its equilibrium. It is known, in fact, that thefe 

 variations of the barometer are fo frequent, that it can never 

 be faid to be exactly ftationary : it is known, that they can 

 be produced by changes of temperature, and De Sauflure has 

 calculated that a diminution of three degrees in the column of 

 air will account for a variation of 0,85 of a line in the ba- 

 rometer. It is known, that thefe variations are mod frequent in 

 mountainous countries in autumn and in fpring, and ptevious 

 to frorms, circumftances which coincide with the greater 

 frequency of feiches at thofe times. This general caufe tends 

 to explain the flight variations of level which are common to 

 all the lakes; it is even of fuch a nature as to be applicable 

 to all extended furfaces, and it is therefore probable, that 

 thefe variations of level likewife take place in the fea, inde- 

 pendant of the flux and reflux, which may have hitherto pre- 

 vented their being obferved. The variations in the weight of 

 the atmofphere may perhaps contribute to thofe fudden and 

 local elevations of the waters of the fea, which have all been 

 indiftinctly confidered as of the nature of water-fpouts. The 

 fame caufe ought likewife to act or. rivers, but inftead of railing 

 or diminilhing their level, it ought, according to Mr. Vaucher, 

 to produce a momentary acceleration or retardation of their 

 courfe; an obfervation difficult to be made, and not hitherto 

 attempted. 



As to the fecond part of the explanation, namely, that which —and he fup- 



(liould account for the great intenfity of the phenomenon at pf)fes the S reat€ * 



i ,*ix ii ^> „„»,- rife in the lake 



the extremity ot the Leman lake, near Geneva, Mr. Vaucher of Geneva to be 



recurs to two circumftances peculiar to that lake, and which cau / ed b y' ,ts P s - 



are found in a lets degree in thofe of Zurich and Confiance, CU Uf g " re * 



where the feiches are moft remarkable after thofe of the lake 



of Geneva; namely, the contraction of a lake in a given 



place, and the defcent of its waters towards the place of their 



dilcharge. With regard to the fill} of thefe circumftances, it 



will be fufficient, if attention be paid to a chart of the Leman 



lake, to (hew that it is very remarkably contracted at its weft- 



em extremity, fo that at half a league diftance from Geneva, 



it has not one third of the breadth of that before Thonon. Now 



we 



