hat nine inches was the nearest to perfection.
And in these experiments two additional bridges were built in one
boiler; six feet behind the ordinary bridge was a concave bridge and six
feet behind that was a convex bridge. The concave bridge was built
close up to the bottom of the fire-tube, and resembled a small archway,
and extended down to within nine inches of the bottom or shell of the
fireplace; the convex bridge was built on the bottom of the shell and
reached to within nine inches of the fire-tube. When the flame from the
furnace shot over the ordinary bridge, it clashed down under the concave
bridge, then rose up and swept through the convex bridge and away to the
bottom flues; the object of these three bridges, in one tube and for one
fire, was to keep the flame and heat in the boiler as long as possible,
instead of the heat flying swiftly over the bridge and out of the
boiler. This experiment seemed to answer very well, but as there were
several other boilers connected with this one there was no opportunity
of testing it correctly, but the three bridges remained established, and
were frequently shown to engineers and others.

15. _Question._--What advantage is there in having the blow-off pipe of
a boiler entering it from the top instead of at the bottom?

_Answer._--I am not aware of any advantage in it, but I am aware of a
disadvantage in it, and it is this, that while the boiler is being blown
right out for the purpose of cleaning, or other reasons, the stoker will
often commence doing some other work, and in due course the boiler is
filled up with water, and the fire lighted, and by-and-by the stoker
comes to see what progress she is making; he looks at the water-gauge
but sees no water in it because it has syphoned out of the boiler; at
first the heated air pressed on the water and forced it through the
blow-off pipe, and then the pipe became a syphon, and the pressure
increasing as the water leaves the boiler, she is soon emptied, and if
the fire is not raked out, soon burnt. Such a mishap could not happen to
a boiler with the blow-off pipe at the bottom, for when the stoker blows
out his boiler he must shut the cock before he can fill her, and when
filled there is no chance of the water escaping out again.

16. _Question._--Is there not some disadvantage in having the blow-out
cock at the bottom of the boiler?

_Answer._--Yes; the cock and pipe are subject to corrosion on account of
water dripp