180 OF THE PRESSURE OF UNMIX ABLE FLUIDS OF DIFFERENT DENSITIES 



185. EXAMPLE. The height of a column of mercury is 30 inches, 

 or 2 1 feet, and its specific gravity 13598 ounces per cubic foot; what 

 is the height of the equilibrating column of water, its specific gravity 

 being 1000 ounces per cubic foot? 



The operation performed according to the rule, is as below. 



186. A column of mercury 30 inches or 2| feet in perpendicular 

 height, is found to equiponderate with the atmosphere in a medium 

 state of temperature; consequently, a column of water 33.995 or 34 

 feet nearly in perpendicular height, will produce the same effect ; it 

 is therefore manifest, that water will ascend in a vacuum tube, to the 

 height of about 34 feet, by means of the pressure of the atmosphere ; 

 and on this principle depends the operation of the sucking pump, 

 to which we shall have occasion to advert in another place. 



187. If in the equation numbered (139), the specific gravities are 

 equal to one another, that is, if szr s', then / o, or the perpendi- 

 cular altitudes of two fluids whose specific gravities are equal, are also 

 equal, the fluids being supposed to communicate with one another in 

 the arms of a bent tube, whatever may be the shape or position of the 

 arms through which the communication takes place. 



188. This explains the reason why the surfaces of small pools 

 or collections of water near rivers, are always on a level with the 

 surfaces of the rivers, when there is any subterraneous communication 

 between them. 



189. It is on this principle also, that water may be conveyed from 

 any one place, to any other place of the same or a less elevation ; for 

 by means of pipes, a communication can be opened between the places, 

 arid whatever may be the number of elevations and depressions, or 

 deviations from the same vertical plane, and whatever may be the 

 distance from the source to the point of discharge, the water will 

 continue to flow along the communicating vessels, provided always, 

 that none of the intervening elevations exceeds the level of the stag- 

 nant fluid, or the source from which the water flows. 



190. When the point to which the water is conveyed, is of the 

 same altitude as that from which it proceeds, the surface will be in a 

 state of quiescence ; but if the point of discharge be lower than the 

 point of supply, the fluid, by endeavouring to rise to the same level, 

 will cause a stream to flow. 



It is by this property of fluids endeavouring to rise to the same 



