fittings. In European practice, until recently, cast
iron was used with apparently satisfactory results. The claim of
European engineers was to the effect that their cast iron was of better
quality than that found in this country and thus explained the results
secured. Recently, however, certain difficulties have been encountered
with such fittings and European engineers are leaning toward the use of
steel for this work.

The degree of superheat produced by a superheater placed within the
boiler setting will vary according to the class of fuel used, the form
of furnace, the condition of the fire and the rate at which the boiler
is being operated. This is necessarily true of any superheater swept by
the main body of the products of combustion and is a fact that should be
appreciated by the prospective user of superheated steam. With a
properly designed superheater, however, such fluctuations would not be
excessive, provided the boilers are properly operated. As a matter of
fact the point to be guarded against in the use of superheated steam is
that a maximum should not be exceeded. While, as stated, there may be a
considerable fluctuation in the temperature of the steam as delivered
from individual superheaters, where there are a number of boilers on a
line the temperature of the combined flow of steam in the main will be
found to be practically a constant, resulting from the offsetting of
various furnace conditions of one boiler by another.

[Illustration: 8400 Horse-power Installation of Babcock & Wilcox Boilers
and Superheaters at the Butler Street Plant of the Georgia Railway and
Power Co., Atlanta, Ga. This Company Operates a Total of 15,200 Horse
Power of Babcock & Wilcox Boilers]

PROPERTIES OF AIR

Pure air is a mechanical mixture of oxygen and nitrogen. While different
authorities give slightly varying values for the proportion of oxygen
and nitrogen contained, the generally accepted values are:

By volume, oxygen 20.91 per cent, nitrogen 79.09 per cent.
By weight, oxygen 23.15 per cent, nitrogen 76.85 per cent.

Air in nature always contains other constituents in varying amounts,
such as dust, carbon dioxide, ozone and water vapor.

Being perfectly elastic, the density or weight per unit of volume
decreases in geometric progression with the altitude. This fact has a
direct bearing in the proportioning of furnaces, flues and stacks at
high altitudes, as will be shown later in the discussio