THE LAWS OF FLUID RESISTANCE. 89 



It is a crude but instinctive idea, that the resistance 

 experienced either by a ship or by a submarine body, such 

 as a fish, moving through water, is due to the necessity 

 of the body ploughing or forcing or cleaving a passage for 

 itself through the water; that it has to drive the water 

 out of its way and then to draw it in again after itself. 



When, however, an attempt was made to deal with the 

 matter in a scientific manner, it was seen that an explana- 

 tion was needed of how it was that water required force to 

 move it out of the way. For it may naturally be asked, 

 How can there be reaction or resistance in a perfectly 

 mobile material such as water seems to be 1 We can under- 

 stand earth, for instance, resisting a ploughshare dragged 

 through it, and we can understand that even a perfectly 

 thin flat plane would make resistance if dragged edgeways 

 through a sea of sand, or even through a sea of liquid mud, 

 owing to the friction against its sides. But water appears, 

 at first sight, altogether unlike this, and seems totally 

 indifferent to change of form of any kind. If we stir water, 

 the different currents seem to flow freely past one another, 

 as if they would go on flowing almost for ever without 

 stopping. But we find, that although to push a thin oar 

 blade through the water edgeways seems to require no 

 force, yet, if we push it flatways, as in rowing, it offers a 

 considerable reaction. The distinction, then, which sug- 

 gests itself is that the particles of water, although they 

 offer no resistance to anything merely sliding past them, 

 offer great resistance to anything pushing against them, 

 because the thing which is pushing against them sets them 

 in motion out of its way, and to set anything heavy in 

 motion requires the exertion of force to overcome what is 

 called its inertia. 



This, then, appears prima facie to be the characteristic 

 of water, that to set the particles in motion, or what is the 

 same thing, to divert them from a straight path, requires 

 force to overcome their inertia, although, when once set in 

 motion, they are able to glide freely past one another, 

 or past a smooth surface. This supposition embodies the 

 natural conception of a fluid, and if it were absolutely 

 exemplified in water, then water would be what we should 

 call a perfectly frictionless fluid. 



Now, though water is not absolutely frictionless, yet it 



