1825.] 



Barometrical Measurement of Heights. 



435 



another, is equal to the difference of their distances from the 

 centre of the earth. 



Of the Pressure of Fluids. 



4. Fluids gravitate in lines directed to the centre of the earth 

 (also that of gravity), and are so constituted that their particles 

 yield to the action of the slightest pressure in any direction. 

 6. Every point of the surface of a fluid when at rest is equally 

 distant from the centre of the earth. 



6. The pressure downivards on 

 the horizontal plane A (or upwards 

 against the similar plane B) in 

 contact with the uniformly dense - 

 fluid contained in the vessel V 

 (placed in a vacuum), will vary 

 directly as the vertical height of 

 the surface of the fluid above the 

 plane A (or the plane B), without 

 regard to the figure or volume of 

 the fluid ; for the pressures are as 



the weights, and the weights are as the heights of the incumbent 

 columns of the fluid. 



7. The pressure depending solely on the vertical height of the 

 fluid above the planes, without regard to its horizontal extent, 

 deptli beloiv the planes, &c. the surface of the mercury (or other 

 uniformly dense fluid) contained in an inverted syphon, will be 

 level (or stand at the same height) in both branches, however 

 different or irregular their diameters, and without regard to the 

 degree of inclination of either. (The tubes are supposed to be 

 sufficiently wide to render the eflfect of capillary attraction insen- 

 sible.) 



8. The pressure dowtiwards 

 within the shorter branch of the 

 inverted syphon S, exerted on the 

 surface A of the mercury (or other 

 uniformly dense fluid) therein, in 

 contact with the fluid contained 

 in the vessel W (placed in a 

 vacuum), will be equal to the 

 weight of a cylinder 0, having its 

 base of the same area as the hori- 

 zontal surface A ; of the same vertical height as that of the sur- 

 face of tlie fluid above A, and uniformly of the mean specific 

 gravity of that intercepted portion of the fluid. 



9. The tfjuiponderaiit uniformly dense cylinder D, representing 

 the weight of the counterpressure upwards of the mercury in the 

 syphon S, will have a base equal to that of the cylinder C ; its 



•Jf2 



