SOLIDS, LIQUIDS, AtfD GASES. 295 



The theoretical or perfect fluid which is imagined by the mathema- 

 tician as the basis of certain abstract reasonings has no real existence. 

 He assumes (and the assumption is legitimate and desirable, pro- 

 vided its imaginary character is always remembered) that the sup- 

 posed particles move upon each other with perfect freedom, without 

 any friction or other impediment ; but, as a matter of fact, all liquids 

 exert some amount of resistance to their own flowing ; they are more 

 or less viscous, have more or less of that sluggishness in their obedience 

 to the law of finding their own level which we see so plainly displayed 

 by treacle or castor oil. 



This viscosity, added to the friction of the liquid against the solid 

 on which it rests, or in which it is enclosed, may become, even in the 

 case of water, a formidable obstacle to its flow. Thus, if w r e make a 

 hole in the side of a tank at a depth of 16 feet below the surface, the 

 water will spout from that hole at the rate of 32 feet per second, but 

 if we connect with this hole a long horizontal pipe of the same inter- 

 nal diameter as the hole, and then observe the flow from the outlet of 

 the pipe, we shall find its velocity visibly diminished, and we shall 

 be greatly deceived if we make arrangements for carrying swift-flow- 

 ing water thus to any great distances. 



Three or four years ago an attempt was made to supersede the water- 

 carts of London by laying down on each side of the road a horizontal 

 pipe, perforated with a row of holes opening towards the horse-way. 

 The water wus to be turned on, and from these holes it was to jet out 

 to the middle of the road from each side, and thus water it all. I 

 watched the experiment made near the Bank of England. 



Instead of spouting across the road from all these holes, as it would 

 have done from any one of them, it merely dribbled ; the reason being 

 that, in order to supply them all, the water must run through the 

 whole of the long pipe with considerable velocity, and the viscosity 

 and friction to be overcome in doing this nearly exhausted the whole 

 force of water-head pressure. Many other similar blunders have been 

 made by those who have sought to convey water-power to a distance 

 by means of a pipe of such diameter as should demand a rapid flow 

 through a long pipe. 



The resistance which water offers to the stroke of the swimmer or 

 the pull of the rower is partly due to its viscosity, and partly to the 

 uplifting or displacement of some of the water. If it were perfectly 

 fluid, our movements within it, and those of fishes, etc., would be 

 curiously different ; the whole face of this globe would be strangely 

 altered in many respects. 



I will not now follow up this idea, but leave it as a suggestion for 

 the reader to work out for himself, by considering what would remain 

 undone upon the earth if water flowed perfectly, without any inter- 

 nal resistance, or friction upon the earth's surface. 



The degrees of approach to perfect fluidity vary greatly with dif- 

 ferent liquids. 



Is there any such a thing as an absolute solid, or a body that has no 

 degree of fluidity, the particles or parts of which will admit of no 

 change of their' relative positions, no movement upon each other 

 without fracture of the mass ? This would constitute perfect rigidity, 

 or the opposite to fluidity. 



Take a piece of copper or soft iron-wire, about one eighth of an 

 inch in diameter, or thereabout, and bend it backward and forward 



