Page:Aircraft in Warfare (1916).djvu/260

App. I. greater in the older recoil actuated gun than in the Lewis type.

The importance of the self-contained magazine in a weapon to be handled from aircraft is obvious, and is so great as to make this feature almost a sine qua non. In the Lewis Gun each magazine contains 47 rounds, and can be replaced with but a few seconds pause in the discharge of the weapon. The resulting "breaks" in the continuity of discharge do not seriously affect the value of the arm in general usage, and in aeroplane fighting they count for nothing. The advantage, on the other hand, of a gun with no "appendages," which can be directed upward or downward or to any point of the compass at will, is one of real and decisive value.

''The Problem of Direct Air Cooling. A Study of the Lewis System. Some Approximate Figures.'' The cooling of the barrel of a machine gun by air in place of the more usual water jacket is a problem of no mean difficulty. It is not ordinarily realised how great is the output of a machine gun in continuous firing expressed in horse power. Thus in the case of the M.VI service ammunition the muzzle energy is 2,000 ft. lbs., and the power represented by the energy of the stream of projectiles is approximately 0.06 horse power per shot per minute. At a maximum rate of fire of 800 per minute this gives 48 h.p, or at a normal speed of fire—say 480 per minute—29nbh.p. The problem of air cooling a machine gun then, is comparable to that of air cooling an internal combustion engine, of roughly 50 b.h.p., approximately the power of an aeronautical motor such as until recently in general usage.

The comparison with an ordinary petrol motor is closer than might be supposed, since the energy and heat account, in the rifle barrel and the motor cylinder