298 On the Seiches of the Lake of Geneva. 



the gentle and periGdic movement of the seiches ; and it assured- 

 ly would not be correct reasoning to deny the action of electri- 

 city, in explanation of the seiches which occur during serene 

 weather, and then to admit this same cause in explanation of 

 those exhibitions of them which are observed when clouds are 

 scattered over the skies. 



In reflecting upon the different agents which might produce 

 the seiches, according as we have described them, I have been 

 able to discover only one which explains their different appear- 

 ances in a satisfactory manner, and this is the agency of the at- 

 mosphere. Theory and observation alike agree in teaching us, 

 that many causes with which we are acquainted, and others of 

 which we are ignorant, contribute to change, almost continually, 

 the weight of the different columns which compose it. Let us 

 only suppose that clouds are unequally spread over the skies, and 

 that some of these intercept the solar rays from the lake, it will 

 result from this simple supposition, that there will be irregular 

 refrigeration among the different columns, and consequently an 

 inequality of density, so that they will press unequally upon the 

 surface of the lake ; but the liquid thus unequally pressed, and 

 ever having a tendency to maintain its equilibrium, will fall in 

 one place, and rise in another ; there will thus be alternating ris- 

 ings and fallings, which in all accuracy being effected on the 

 water of the lake, independent of all agitation of the air, can 

 thus be scarcely ever on an exact level. And if, instead of sup- 

 posing a simple refrigeration occasioned by the interception of 

 the solar rays, we suppose such a state of the atmosphere that it 

 rains at one place whilst it is fair at another ; and if, moreover, 

 we recognise the sudden and local variations which so often hap- 

 pen in the air at the approach of storms, and which are so vio- 

 lent that they may produce hail in the higher regions of the at- 

 mosphere, we may easily conceive how the waters of the lake 

 should be so unequally pressed upon, as to rise and fall. 



These suppositions are far from being gratuitous. Saussure, 

 in his Hygrometrie, and principally in the chapter entitled Des 

 Variations du Barometre, after having discussed the causes of 

 general variations, then proceeds to more particular ones ; and 

 he mentions, that whenever a partial shower refreshes the air, it 

 is found that, on the spot where it fell, the barometer imme- 



