IRON AND WOOD ROOFS FOR STOVES, &c. 
b9 
it by its warmth along the pipe to the situation where he finds the metal cold, and 
then in the next bend or angle should he not find an air tap, I should recommend 
him to procure a blacksmith's drill, and to have a hole made in the pipe, when he 
will find the air to pass off rapidly, and the water to follow instantly ; then should 
he not be prepared with an air pipe, a small wooden plug would suffice until an 
opportunity offered to fix one properly, as in all probability it might be many 
months, and perhaps years, if the cistern be carefully and continually attended to, 
before such an accident would occur again at that particular point or bend. I would 
however strongly recommend that in every apparatus ample provision be made for 
the escape of the air at every bend where it is likely to collect or lodge, for I have 
witnessed during the time I had the management of six systems of hot water, that 
from some unknown cause, an apparatus which had worked properly for one or 
two years would suddenly get out of order, when on tracing the pipes, as before 
described, as far as I found them warm, I have then, on drilling a hole at the 
first turn or bend where the pipe began to feel cold, found an accumulation of 
carbonic acid gas, the heaviest of all the gases, lodged in the angle, and as soon as 
this was allowed to escape, the apparatus worked as regular as usual. To remove 
this difficulty, which to persons unacquainted with the cause of the obstruction 
would appear formidable, nay almost insurmountable, not more than ten minutes 
were sacrificed ; and on interrogating the man who had the management, as to 
whether he had allowed the water in the cistern to fall below its proper level, I 
discovered that the derangement had been caused through his negligence and inat- 
tention in having suffered the water to sink below the level of the top of the pipes, 
which of course left a vacuum for this foul air to collect. I would therefore advise 
all persons when not using the apparatus, either to draw the whole of the water 
off, or to keep the cistern as full as when in use; this precaution will prevent the 
air from collecting ; but when the boiler is filled again, they should be careful to 
have the air taps open until the water begins to flow out, or till the boiler is full, 
to prevent a repetition of the inconvenience. There is also another highly 
important arrangement connected with hot-water apparatus, a property of the 
metals which should be attended to with great care, — I mean the allowing a 
sufficient longitudinal expansion for the pipes on their becoming hot, as it should 
be borne in mind, that iron pipes when heated to 200° will expand nearly two 
inches in a length of 100 feet ; and as a proof of the necessity of attending to this 
fact, I may relate a circumstance which fell under my own observation. A few 
years ago, a nobleman's conservatory in Hampshire was heated with hot water at 
an expense of between 300 and 400 pounds, and the pipes from the boiler were 
introduced through the stone that formed the footpath, in which holes were cut 
just large enough to admit a four-inch pipe, but not of sufficient diameter to allow 
for the expansion of the metal, and I well remember that in consequence of this 
oversight, in about fourteen or fifteen different situations where the pipes had to 
pass through the stone, the joints burst. In each of the eighty feet lengths of pipe, 
