MOTION AND RESISTANCE OP FLUIDS. 387 



the deck, tlie greater its stability ; but if it be too low, the ship 

 returns to the vertical position with such celerity as to endanger the 



masts '*. 



SECTION II. 



Motion and Resistance of Fluids. 



HOWEVER satisfactory the general principles of motion may be, 

 when applied to the action of bodies on each other, in all those cir- 

 cumstances which are usually included in that branch of natural 

 philosophy called mechanics, yet the application of the same prin- 

 ciples in tlie investigation of the motions of fluids, and their actions 

 on other bodies, is subject to great uncertainty. That the different 

 kim!s of airs are constituted of particles endued with repulsive 

 powers, is manifest from their expansion when the force with which 

 they are compressed is removed. The particles being kept at a 

 distance by their mutual repulsion, it is easy to conceive that they 

 may move very freely among each other, and that this motion may 

 take place in all directions, each particle exerting its repulsive power 

 equally on all sides. Thus far we are acquainted with the consti- 

 tution of these fluids 3 but with what absolute degree of facility the 

 particles move, and how this may be effected under different de- 

 grees of compression, are circumstances] of which we are totally 



ignorant. 



In respect to those fluids which are denominated liquids, we are 

 still less acquainted with their nature. If we suppose their par- 

 ticles to be in contact, it is extremely difficult to conceive how they 

 can move among each other with such extreme facility, and produce 

 effects in directions opposite to the impressed force without any sen- 

 sible loss of motion. - To account for this, the particles are sup 

 posed to be perfectly smooth and spherical. If we were to admit 

 this supposition, it would yet remain to be proved how this would 

 solve all the phenomena, for it is by no means self-evident that it 

 would. If the particles be not in contact, they must be kept at a 

 distance by some repulsive power. 1' >1 it is manifest that these 

 part ides attract euch other, from the drops of all perfect liquids 



* There are two excellent papers in the Transactions on the stability of 

 ships, by Mr. Atwood. Phil. Trans. 1796, Vol. Ixxxvi. p. 46; and 1798, vol. 

 Uxxvii'. p. 201. 



2 C2 



