100 GREENHOUSE CONSTRUCTION AND HEATING. 
Now, although it is a very easy matter to run the pipes 
through a tank constructed of brick and cement (though 
they must’ of course be built in after being fitted), it is 
quite a different and more difficult thing to prevent the 
tank leaking at the points where the pipes enter and leave 
it, The work generally, as well as the cement, may be 
of the strongest and best, and most carefully put in, but 
as the pipes become heated and cool again they expand 
and -contract in length, with great force, if not very much, 
and the constant thrust and pull soon destroys the best 
cement and strongest work. I have, indeed, known not 
only several, but many instances where tanks, both of the 
kind now treated of as well as for bottom heat, have been 
rendered quite useless from this cause. There are only 
two ways out of the difficulty, one being to place the tank 
and arrange the pipes so that the latter may be fived where 
they pass through the former, and expand and contract in 
one or both directions from it. Thus, tanks fixed in corners, 
or the like, are much less liable to fail from this cause than 
those placed on a straight line of piping, fixed at one end. 
The other method of preventing leakage is to run the pipes 
through other and larger iron collars built in the sides, with 
an annular space between the two, filled up with an 
indiarubber ring; this allows the pipes plenty of play, 
while effectually preventing any leakage. 
I may mention that an extremely cheap and useful 
description of water tank for out-door use is an upright 
circular one, almost like a cask, but with straight sides, 
narrow at the top and widening towards the bottom, 
constructed of stout wooden staves, strongly hooped 
together, like the circular iron tanks mentioned on p. 96, 
