THE ACTION OF FROST IN WATER-PIPES. 303 



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 companies. 



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 of iron (about 

 fa of that of ordinary wrought iron), but it yields consider- 

 ably before breaking. It has, in fact, the property of vis- 

 cosity 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 will be easily understood that a pipe 

 which is perfectly cylindrical other conditions equal will 

 be less likely to burst than one whidh is of varying diameter, 

 as the sliding from a larger to a smaller portion of the pipe 

 must be attended with great resistance, or a certain degree 

 of block, beyond what would be due to the mere friction 

 along a pipe of uniform diameter. 



Let us now consider the relative merits of lead and iron 

 as material for Avater-pipes in places where exposure to frost 

 is inevitable. Lead yields more than iron, and so far has 

 an advantage; this, however is but limited. As lead is 

 practically inelastic, every stretch remains, and every stretch 

 diminishes the capacity for further stretching; the lead thus 

 stretched at one frost is less able to stretch again, and has 

 lost some of its original tenacity. Hence the superiority of 

 new leaden pipes. Iron is elastic within certain limits, and 

 thus the iron pipe may yield a little without permanent 

 strain or "distress," and if its power of elastic resistance 



