694 



the epochas of the limits. We must determine 

 at the same time the moment when the mer- 

 cury attains its minimum and no more changes 

 sensibly, and the moment when the mercury 

 begins again to mount. It happens, as in every 

 thing susceptible of a maximum and a minimum, 

 that the increase and diminution of the tides of 

 the atmosphere and the ocean, near the ex- 

 treme limits *, are in proportion to the square 

 of time elapsed since the epochas of the maxima 

 and the minima. The barometer consequently 

 remains stationary in appearance, before its 

 movement becomes retrograde. This stationary 

 state lasts a longer or shorter time, like the 

 state of the flux of the sea at low water. If, at 

 Calcutta, for instance, the heights observed were: 



inches 



at 2 h 0' 29,97 (angl. measure.) 



2 h 30' 29,97 



3 h 0' 29,96 



4 h 0' 29,96 



S h 0' . 29,96 



6 h 30' 29,96 



7 h 0'... 29,97 



7 30' 29,98 



It may be said, either that the barometer has 

 attained its minimum at 3 h , that it kept at the 

 same height till 6 h 30' and then began to re- 



* Laplace, Systeme du Monde, 1813, p. 84. 



