of surface per degree difference in temperature,
W is the weight in pounds of the gas flowing through the tube
per hour,
A is the area of the tube in square feet,
d is the diameter of the tube in feet,
c_{p} is the specific heat of the gas at constant pressure,
[lambda] is the conductivity of the gas at the mean temperature and
pressure in B. t. u. per hour per square foot of surface
per degree Fahrenheit drop in temperature per foot,
[lambda]_{w} is the conductivity of the steam at the temperature of the
wall of the tube.

The conductivities of air, carbonic acid gas and superheated steam, as
affected by the temperature, in English units, are:

Conductivity of air .0122 (1 + .00132 T)
Conductivity of carbonic acid gas .0076 (1 + .00229 T)
Conductivity of superheated steam .0119 (1 + .00261 T)

where T is the temperature in degrees Fahrenheit.

Nusselt's formulae can be taken as typical of the number of other
formulae proposed by German, French and English writers.[85] Physical
properties, in addition to the density, are introduced in the form of
coefficients from a consideration of the physical dimensions of the
various units and of the theoretical formulae that are supposed to
govern the flow of the gas and the transfer of heat. All assume that the
correct method of representing the heat transfer rate is by the use of
one term, which seems to be unwarranted and probably has been adopted on
account of the convenience in working up the results by plotting them
logarithmically. This was the method Professor Reynolds used in
determining his equation for the loss in head in fluids flowing through
cylindrical pipes and it is now known that the derived equation cannot
be considered as anything more than an empirical formula. It, therefore,
is well for anyone considering this subject to understand at the outset
that the formulae discussed are only of an empirical nature and
applicable to limited ranges of temperature under the conditions
approximately the same as those surrounding the experiments from which
the constants of the formula were determined.

It is not probable that the subject of heat transfer in boilers will
ever be on any other than an experimental basis until the mathematical
expression connecting the quantity of fluid which will flow through a
channel of any section under a given head h