he proportions of a flue, in the
case of a marine boiler?

_A._--The Nile steamer, with engines of 110 horse power by Boulton and
Watt, is supplied with steam by two boilers, which are, therefore, of 55
horses power each. The height of the flue winding within the boiler is 60
inches, and its mean width 16-1/2 inches, making a sectional area or
calorimeter of 990 square inches, or 18 square inches per horse power of
the boiler. The length of the flue is 39 ft., making the vent 25, which is
the vent proper for large boilers. In the Dee and Solway steamers, by Scott
and Sinclair, the calorimeter is only 9.72 square inches per horse power;
in the Eagle, by Caird, 11.9; in the Thames and Medway, by Maudslay, 11.34,
and in a great number of other cases it does not rise above 12 square
inches per horse power; but the engines of most of these vessels are
intended to operate to a certain extent expansively, and the boilers are
less powerful in evaporating efficacy on that account.

263. _Q._--Then the chief difference in the proportions established by
Boulton and Watt, and those followed by the other manufacturers you have
mentioned is, that Boulton and Watt set a more powerful boiler to do the
same work?

_A._--That is the main difference. The proportion which one part of the
boiler bears to another part is very similar in the cases cited, but the
proportion of boiler relatively to the size of the engine varies very
materially. Thus the calorimeter _of each boiler_ of the Dee and Solway is
1296 square inches; of the Eagle, 1548 square inches; and of the Thames and
Medway, 1134 square inches; and the length of flue is 57, 60, and 52 ft. in
the boilers respectively, which makes the respective vents 22-1/2, 25, and
21. Taking then the boiler of the Eagle for comparison with the boiler of
the Nile, as it has the same vent, it will be seen that the proportions of
the two are almost identical, for 990 is to 1548 as 39 is to 60, nearly;
but Messrs. Boulton and Watt would not have set a boiler like that of the
Eagle to do so much work.

264. _Q._--Then the evaporating power of the boiler varies as the sectional
area of the flue?

_A._--The evaporating power varies as the square root of the area of the
flue, if the length of the flue remain the same; but it varies as the area
simply, if the length of the flue be increased in the same proportion as
its other dimensions. The evaporating power of a boiler is referable to the
amount of