368 SCIENCE IN SHORT CHAPTERS. 



exert some amount of resistance to their own flowing ; they 

 are more or less viscous, have more or less of that sluggish- 

 ness 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 we 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 

 internal 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-flowing water thus to any 

 great distances. 



Three or four years ago an attempt was made to super- 

 sede 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 was 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 vis- 

 cosity, 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 dif- 

 ferent ; the whole face of this globe would be strangely 

 altered in many respects. 



