Barometer. 351 



to the depression of the point H, or the nearness of the plates to 

 each other. This affords an explanation of the second case 

 above stated. 



If one of the floating bodies, as A, for example, is susceptible of p 28 

 being wetted, while the other B is not, the fluid will rise around A 

 and be depressed around B. Accordingly, when the balls are near 

 to each other, the depression around B will not be symmetrical, 

 and the body being thus placed as it were upon an inclined 

 plane, its equilibrium will be destroyed, and it will move off from 

 the other body in the direction of the least pressure. 



These phenomena, of which we have given only a familiar 

 explanation, are all comprehended in Laplace's theory of ca- 

 pillary attraction ; and the attractive and repulsive forces are 

 capable, on that theory, of being subjected to a rigorous calcula- 

 tion. 



Of the Barometer. 



466. If we take a glass tube thirty-three or thirty-four inch- 

 es in length, closed at one extremity and open at the other, and 

 having filled it with mercury, place the finger over the open ex- 

 tremity and thus immerse it in a basin of the same liquid with- 

 out suffering the air to enter the tube, the mercury will settle 

 down in the tube, leaving a vacuum above it, till its weight is 

 exactly counterbalanced by the pressure of the atmosphere, ex- 

 erted upon the surface of the mercury in the basin. This in- 

 strument is called a barometer.] The perpendicular height at 

 which the mercury is ordinarily maintained at the level of the 

 sea, is very nearly thirty inches. 



From what has been said of the manner in which the pres- 

 sure of-fluids is propagated, it will be perceived that it is imma- 

 terial what be the extent of surface in the basin, or whether the 

 atmospheric pressure be applied at the top, or, by means of a 

 flexible bag containing the liquid, at the bottom and sides. If 



t See note on the construction and history of the barometer. 



