Page:Aircraft in Warfare (1916).djvu/264

App. I. {|
 * Cr.
 * colspan=2|
 * Dr.
 * colspan=2|
 * Recoil due to powder gases
 * 5.6||lbs.
 * colspan=3|
 * Resistance of jacket
 * 1.8||lbs.
 * colspan=3|
 * Efflux momentum of air||3.35||lbs.
 * colspan=3|
 * Total||5.15||lbs.
 * colspan=3|
 * Credit Balance|| .45||lbs.
 * |||5.6|||lbs.
 * |||5.6|||lbs.
 * }
 * colspan=3|
 * Credit Balance|| .45||lbs.
 * |||5.6|||lbs.
 * |||5.6|||lbs.
 * }
 * |||5.6|||lbs.
 * }

There is, consequently, nothing inconsistent in the velocity of 200 ft. per second, though the margin of driving force as due to the momentum of the powder gases seems narrow. It is to be remembered, however, that the figure taken for the gas momentum is based on experiments made with the ordinary service rifle; it is without doubt considerably augmented by the special nozzle with which the barrel of the Lewis Gun is fitted, so that in reality there should be an ample margin.

It is worthy of note that the work done by the blast represents approximately 2 horse power, and the actual work done against the skin-friction on the gilled surface is about two-thirds of a horse power; an appreciable and valuable return for so simple an expedient as the muzzle ejector.

Summarising the foregoing, we have, under conditions firing, at 600 rounds per minute:—

 The mean velocity of the induced air current in the cooling jacket is approximately 200 feet per second. The temperature difference has a mean value of about 420 degrees Fah., that is to say the ordinary temperature of the gills is in the region of 500 Fah. The momentum represented by the exhaust powder gases is a measure of the force available for  210