equivalent rate of heat loss; this may otherwise be expressed as
37 Fah. H.U, per sec.
or 15 Centigrade H.U, per sec.
or 6,600 (gram) calories per sec.
or 38 horse power.
Now, the actual form taken by the Cooling surface in the Lewis Gun is that of a number of radial gills of aluminium, whose aggregate surface is approximately 6 sq. ft., hence we have to dispose of the equivalent of 38⁄6 or, say, 6 h.p, per square foot.
Following the method laid down in the author's recent " James Forrest " Lecture, the above betokens both a very considerable temperature difference and a high velocity of air over the cooling gills; the product of these (deg. Fah.) being 84,000. Thus the temperature is currently given (under the conditions in question) as about 440 deg. Fahrenheit, which means a temperature difference of 400 deg. Fahrenheit; this corresponds to a velocity of the air through the jacket, say, 200 ft, per second, in order that the heat shall be disposed of with sufficient rapidity by the surface of the gills. This appears unexpectedly high, but there are two ways in which the figure may be checked. Firstly, it is clearly necessary that the total mass of air passing shall be adequate, and more than adequate, to carry off the waste heat; in other words, the air entering the jacket at the one end at atmospheric temperature must be able to absorb the whole of the waste heat before leaving the jacket, and this without its temperature being raised to such an extent as to prevent its being active as a cooling agent. Secondly, we may compare the calculated resistance of the jacket to the passage of air, with the known recoil as due to the powder gases; since the former is
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