258 LECTUBE XXI. 



tical knowledge, to be perfectly convinced of the justice of all our conclusions, 

 without any reference to experimental proof. But here our abstract reason- 

 ings begin to fail; and whether from the imperfection of our modes of consi- 

 dering the mechanical actions of the particles of fluids on each other, or from 

 the deficiencies of our analytical calculations, or, as there is more reason to 

 suppose, from a combination of both these causes, all attempts to reduce the 

 affections of fluids to a perfect mechanical theory have been hitherto unsuc- 

 cessful. At the same time it will appear, that by a proper mixture of calcu- 

 lation with experiment, we may obtain sufficient foundations for all such de- 

 terminations as are likely to be of any practical utility. 



The whole of the subjects, which will be classed under the denomination 

 Hydrodynamics, may be divided into three general heads; Hydraulics, 

 Acustics, and Optics; terms which are sufficiently understood, as relating 

 to the common properties of fluids, to sound, and to light; but which do 

 not allow of a very strict definition, without a still further division. The 

 first subdivision which we shall consider, will relate to the laws of the 

 eqivilibrium of fluids, or of the opposition of forces acting on them, without 

 producing actual motion, comprehending hydrostatics, or the doctrine of 

 the equilibrium of liquids, either within themselves, or with moveable bodies ; 

 and pneumatostatics,or the equilibrium of elastic fluids. The actual motions of 

 fluids will be considered in the second subdivision: and the third will 

 relate to the instruments and machines in which the principles of hydrostatics, 

 hydraulics, and pneumatics, are applied to the purposes of the arts or of 

 domestic convenience. The science of hydraulics must be allowed to be of as 

 great importance to civil life, and especially to a maritime nation, as any de- 

 partment of practical mechanics. Let us only reflect for a moment to what 

 the metropolis of England would be reduced, if deprived of pipes for the con- 

 veyance of water, of pumps, and of fire engines; and how much the commerce 

 of the whole kingdom has been facilitated by the formation of navigable canals, 

 and we shall soon be convinced of the obligations that we owe to the art of 

 modifying the motion of water, and to the principles of hydraulics, on which 

 that art depends. 



The facts concerned in acustics and harmonics, or the doctrine of sound, and 

 the science of music, are not exclusively dependent on the characteristic pro- 



