348 Hydrostatics. 



pressions will, like the elevations, be inversely as the diameters 

 of tubes, and the whole theory above given, with this single mod- 

 ification, is strictly applicable. Between plates also, and between 

 concentric tubes, a depression will take place corresponding to 

 the elevation in the case where the upper surface is concave. 



It is to be observed, however, that the deficiency of attrac- 

 tion in the case of mercury used in connection with glass, taken 

 in its ordinary state, is to be ascribed to a want of contact be- 

 tween the fluid and the substance of the tube or plate, arising 

 from a film of moisture which ordinarily attaches itself to glass, 

 and which being completely removed, mercury is found to pre- 

 sent a concave surface like water, and consequently to rise in a 

 tube and between plates above its natural level, t Indeed water 

 may be made to exhibit the same apparent anomaly, by having 

 the surface of the glass, whether tube or plate, smeared with a 

 thin coat of tallow or wax. 



462. The peculiar character of this theory consists in this, 

 that it makes every thing depend upon the form of the surface. 

 The nature of the solid body and tha' of the fluid determine simp- 

 ly the direction of the first elements, where the fluid touches the 

 solid, for it is at this point only that their mutual attraction is 

 sensibly exerted. These directions being given, they become 

 the same always for the same fluid and the same solid substance, 

 whatever be the figure of the body itself which is composed of 

 this substance. But beyond the first elements and beyond the 

 sphere of action of the solid, the direction of the elements and 

 the form of the surface are determined simply by the action of 

 the fluid upon itself. 



We have seen that the elevation of a liquid between parallel 

 plates, is half of that which takes place in tubes whose diameter is 



t Barometer tubes properly cleansed and freed from humidity, 

 by having the mercury repeatedly boiled in them, will exemplify 

 the truth of this remark. Moreover, with the knowledge of this 

 fact, we can readily satisfy ourselves by simple inspection, whether 

 the requisite attention was paid to this particular in the construc- 

 tion of the instrument. 



