11 
to the quantity of heat which has to pass. And again the 
heat will have to pass through the material of the tube, 
and the rate at which it will do this will depend on the 
difference of the temperature at its two surfaces. Hence if 
air be forced through a tube surrounded with boiling water, 
the temperature of the inner surface of the tube will not be 
constant but will diminish with the quantity of heat carried 
off by the air. It may be imagined that the difference will 
not be great : a variety of experiments lead me to suppose 
that it is much greater than is generally supposed. It is 
obvious that if the previous conclusions be correct this 
difference would be diminished by keeping the water in 
motion, and the more rapid the motion the less would be 
the difference. Taking these things into consideration the 
following experiments may, I think, be looked upon, if not 
as conclusive evidence of the truth of the above reasoning, 
yet as bearing directly upon it. 
One end of a brass tube was connected with a reservoir 
of compressed air, the tube itself was immersed in boiling 
water, and the other end was connected with a small non- 
conducting chamber formed of concentric cylinders of paper 
with intervals between them in which was inserted the 
bulb of a thermometer. The air was then allowed to pass 
through the tube and paper chamber, the pressure in the 
reservoir being maintained by bellows and measured by 
a mercury gauge ; the thermometer then indicated the tem- 
perature of the emerging air. One experiment gave the 
following results ; — With the smallest possible pressure the 
thermometer rose to 96° F., and as the pressure increased 
fell until with To inch it was 87°, with J inch it was 70°, 
with 1 inch it was 64°, with 2 inches 60°, beyond this point 
the bellows would not raise the pressure. 
It appears, therefore, (1) that the temperature of the air 
never rose to 212, the temperature of the tube, even when 
moving slowest ; but the difference was clearly accounted 
for by the loss of heat in the chamber from radiation, the 
small quantity of air passing through it not being sufficient 
to maintain the full temperature, an effect which must 
obviously vanish as the velocity of the air increased ; (2) as 
