Page:Aircraft in Warfare (1916).djvu/265

Rh  impelling the induced air current, and is consistent with the above.

In any actual measurements the jacket temperature recorded will depend very much upon where the bulb of the thermometer is placed, for there is a very steep temperature gradient from the surface of the bore of the barrel outward. Rough calculation shows that there must be a difference of about 200 degrees Fah. between the inner and outer surfaces of the barrel alone, apart from the difference in the gills themselves; thus it is improbable that the temperature of the rifled surface of the barrel when the gun is in continuous usage can be less than 700 Fah., but this does not seem to have any marked effect on its durability. Even in the case of a water cooled gun, when the water is on the boil, the internal temperature of the barrel at a high rate of fire is probably not less than 400 Fah.

The aggregate sectional area of the aluminium gills taken normal to the radius is approximately 250 sq. c. m., and the quantity of heat being 6,600 gram calories, we have the mean temperature gradient about 52 deg. C. per centimetre, or 240 Fah. per inch (measured radially): this means a difference between the root of the gills and the outer casing of roughly 300 Fah. Hence the total difference of temperature between the outer casing and the rifling, under conditions of continuous fire, will be about 500 Fah. This is quite sufficient to give rise to uncertainty when jacket temperature is under discussion; the portion of the jacket must be specified. 211