10 
And various considerations lead to the supposition that A 
and B in (I.) are proportional to A^ and in (II.) 
The value of v which this gives is very small, and 
hence it follows that for considerable velocities the gas 
should emerge from the tube at a nearly constant tempera- 
ture whatever may be its velocity. 
This, as I am about to point out, is in accordance with 
what has been observed in tubular boilers as well as in more 
definite experiments. 
In the Locomotive the length of the boiler is limited by the 
length of tube necessa^ry to cool the air from the fire down 
to a certain temperature say 500°. Now there does not 
seem to be any general rule in practice for determining this 
length, the length varying from 16ft. to as little as 6, but 
whatever the proportions may be each engine furnishes a 
means of comparing the efficiency of the tubes for high and 
low velocities of the air through them. It has been a matter 
of surprise how completely the steam-producing power of a 
boiler appears to rise with the strength of blast or the work 
required from it. And as the boilers are as economical when 
working with a high blast as with a low, the air going up 
the chimney cannot have a much higher temperature in the 
one case than in the other. That it should be somewhat 
higher is strictly in accordance with the theory as stated 
above. 
It must, however, be noticed that the foregoing conclusion 
is based on the assumption that the surface of the tube is 
kept at the same constant temperature, a condition which 
it is easy to see can hardly be fulfilled in practice. 
The method by which this is usually attempted is by 
surrounding the tube on the outside with some fluid the 
temperature of which is kept constant by some natural 
means, such as boiling or freezing, for instance the tube is 
surrounded with boiling water. Now although it may be 
possible to keep the water at a constant temperature it does 
not at all follow that the tube will be kept at the same 
temperature ; but on the other hand, since heat has to pass 
from the water to the tube there must be a difference of tem- 
perature between them, and this difference will be proportional 
