32. There is another fact connected with this part of the
subject, which must have struck any one who has been at
any considerable pains in making observations, and that is,
that the heating surface near the fire is so much more effective
than those parts of the boiler which are beyond the occa-
sional reach of the flame, as hardly to admit of any com-
parison ; certain it is, that two or three square feet of addi-
tional surface over t&e hottest part of the fire, makes a con-
siderable improvement in the power as well as in the eco-
nomy of a boiler, while as many square yards of surface added
to the contrary end, or that next to the chimney, has scarcely
any effect at all, provided that the proportions of the boiler
in other respects remain the same. It has been found that
when boilers of twenty feet long have been lengthened by



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94 ON ENLARGING THE FURNACE.
ten feet, the alteration has made hardly any difference in the
evaporating power, while the area of the fire-grate remained the
same, and the saving of fuel, if any, has only been trifling.
Again, we have seen a boiler cut down from one hundred feet
in length to fifty without any sensible diminution in its eva-
porating power, in proportion to the area of its fire-grate. In
both these cases, both before and after the alterations, the
temperature of the air and smoke escaping into the flues of
the chimney was so considerable as to enable it to boil water
in an open vessel very rapidly.
