OF THE PRESSURE OF FLUIDS ON DYKES AND EMBANKMENTS. 211 



232. We are now arrived at that particular division of our subject, 

 which comprehends some of the most interesting and important 

 departments of hydrodynamical science ; it unfolds the principles of 

 floatation, explains the method of weighing solid bodies in fluids, 

 determines the relations of their specific gravities ; and moreover, it 

 investigates the laws of equilibrium, and assigns the conditions neces- 

 sary for a state of perfect or imperfect stability. Every term in this 

 enumeration conveys the idea of mechanical action. 



Floating bodies, those which swim on the surface of a fluid, which is bulk for 

 bulk heavier than the body afloat, are pressed downward by their own weight in a 

 vertical line passing through their centre of gravity : and they are supported by 

 the upward pressure of the fluid, which acts in a vertical line passing through the 

 centre of gravity of the part which is under the water. When these lines are coin- 

 cident, the equilibrium of floatation will be permanent. In the present instance we 

 have merely to consider the principles of floatation as fluids exhibit the properties 

 of the mechanical powers, as the lever or balance, the screw, &c. The pulley, in 

 lowering a great weight or in lifting it up again, does no more than the ocean tide 

 when it silently recedes and leaves dry, or majestically advances and without effort 

 floats a stupendous ship. The lever or balance does no more than a canal lock 

 effects, when it transfers from one level to another a heavy barge or vessel laden 

 with ponderous commodities. And we behold too the ocean, like a vast screw or 

 press, forcing down to its dark recesses vast masses, which in shipwrecks are sub- 

 merged in its bosom, and which yet might be fashioned to be bulk for bulk much 

 lighter than the devouring flood which has swallowed them up in its insatiable 

 womb. The eternal and immutable laws of Nature, in all these cases, are most 

 satisfactorily accounted for in the doctrine of fluid pressure and support j but this 

 doctrine, like all the rudiments of human skill applied to natural phenomena, must 

 depend on matters of fact, which can only be learned from observation and experi- 

 ment, and which can generally and successfully be applied by the help of mathema- 

 tical and philosophical investigations. This is the only scientific view we ought 

 to take of all those truths that are denominated the phenomena of fluids, whose 

 affections, from a series of concurring experiments, we undertake to expound j or 

 assuming these as established pi-inciples that operate generally in the pressure and 

 elasticity of fluids, we demonstrate them to be adequate to the production, not only 

 of the particular effects adduced to prove their existence and power, but of all 

 similar phenomena. This is the only method by which to make the results of 

 practical men available in scientific discussions, and on the other hand render these 

 discussions the handmaids of genius in constructive mechanics. This is the province 

 of the mathematician; and we shall in the sequel follow it very closely, in expound- 

 ing the doctrine of floatation and the specific gravities of bodies, the laws of equili- 

 brium, and the conditions necessary for a state of perfect or imperfect stability, &c. 



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