67 



The variable pressures, which the surface of 

 the sea undergoes by the changes in the weight 

 of the air, are another cause of motion which 

 deserves particular attention. It is well known, 

 that the barometric variations do not in general 

 take place at the same moment on two distant 

 points, which are on the same level. If in one 

 of these points the barometer stands a few lines 

 lower than in the other, the water will rise where 

 it finds the least pressure of the air, and this lo- 

 cal intumescence will continue, till, from the 

 effect of the wind, the equilibrium of the air is 

 restored. Mr. Vaucher thinks that the tides in 

 the Lake of Geneva, known by the name of the 

 seiches, arise from the same cause. Under the 

 torrid zone, the horary variations of the barome- 

 ter may produce small oscillations at the surface 

 of the seas, the meridian of 4 h , which corresponds 

 to the minimum of the pressure of the air, being 

 situate between the meridian of 21 h and ll h up- 

 on which the height of the mercury is the great- 

 est ; but these oscillations, if even they were 

 perceptible, will be accompanied by no change 

 of place *. 



When this last movement is produced by the 

 inequality of the specific weight of the particles, 

 a double current is formed, the upper of which 

 has a contrary direction to the lower. Thus in 

 the greatest part of the straits, as in the seas of 



* Mouvement de translation. 

 F 2 



