32 SCIENCE IN SHORT CHAPTERS. 



I have constructed an apparatus which illustrates this very 

 strikingly. It is an iron syringe with a cylindrical interior of 

 about half an inch in diameter, and a terminal orifice of less 

 than T^-g. of an inch in diameter. Its piston of metal is driven 

 down by a screw. Into this syringe I place small fragments 

 of ice, or a cylinder of ice fitted to the syringe, and then 

 screw down the piston. Presently a thin wire of ice is 

 squirted forth like vermicelli when the dough from which it is 

 made is similarly treated, showing that the ice is plastic like 

 the dough, provided it is squeezed with sufficient force. 



The viscosity of ice is displayed on a grand scale in glaciers, 

 the ice of which actually flows like a river down the glacier 

 valley, contracting as the valley narrows and spreading out as 

 it widens, just as a river would ; but moving only a few inches 

 daily according to the steepness of the slope and the season, 

 slower in winter than in summer. 



Upon this, and the slowness of the act of freezing, depends 

 the possibility of water freezing in iron pipes without bursting 

 them. Even iron yields a little before bursting, but ordinary 

 qualities not sufficiently to bear the expansion of -fa of their 

 contents. What happens, then ? The cylinder of ice con- 

 tained in the tube elongates as it freezes, provided always the 

 pipe is open at one or both ends. But there is a limit to this, 

 seeing that the friction of such a tight-fitting core, even of 

 slippery ice, is considerable, and if the pipe be too long, the 

 resistance of this friction may exceed the resistance of tenacity 

 of the pipe. I am unable to give any figures for such length ; 

 the subject does not appear to have been investigated as it 

 should be, and as it might well be by our wealthy water com- 

 panies. 



We all know that lead pipes frequently succumb, but a little 

 observation shows that they do so only after a struggle. The 

 tenacity of lead is much less than that iron (about ^ of that 

 of ordinary wrought iron), but it yields considerably before 

 breaking. It has, in fact, the property of viscosity similar to 

 that of ice. At Woolwich the lead used for elongated rifle 

 bullets is squirted like the ice in my syringe above described, 

 powerful hydraulic pressure being used. 



This yielding saves many pipes. It would save all new pipes 

 if the lead were pure and uniform ; but as this is not the case, 

 they may burst at a weak place, the yielding being shown by 

 the bulge that commonly appears at the broken part. 



From the above it may bo easily understood that a pipe 



