PRINCIPLES OF GUNNERY. 
431 
Errors of fire are due principally to 
( 1 ) 
( 2 ) 
( 3 ) 
(4) 
( 5 ) 
( 6 ) 
Variations in muzzle velocity. 
Unsteadiness of the projectile as it leaves the bore, and during 
flight. 
Wind. 
Causes 
which 
affect 
accuracy 
of fire. 
Personal error in laying’. 
Inclination of the trunnions to the horizon. 
Unsteadiness of gun, carriage, and platform on firing. 
It is important, in order to obtain great accuracy of fire, to have the Variations 
muzzle velocity as uniform as possible. Variations in muzzle velocity veSy! 6 
result from want of uniformity in the explosion of the charge, which 
may result from the quality of the powder, the amount of moisture 
in the powder, or the variation in the cubic space occupied by the 
charge, and the shape of the cartridge. For instance, the degree of 
hardness in ramming home will cause a variation in muzzle velocity in 
those guns where there is not a special provision to stop the projectile 
at a fixed distance. Another cause of variations in muzzle velocity 
is the small difference in the weight of projectiles and charges. 
The temperature of the gun affects the muzzle velocity. Generally, 
the first round is short, owing to the muzzle velocity being below the 
average in a cool gun. 
The unsteadiness of a projectile as it leaves the bore is the most unsteadi- 
important source of errors in fire. It may result from accidental projectile 6 
causes—such as the shearing of the studs, the breaking up of the gas- ^iUeaves 
check, or from the contact of the body of the projectile with the lands and during 
of the bore. In B.L. guns it may result from the lead coat stripping; 
but in M.L. guns it more commonly arises from non-centring, or 
insufficient velocity of rotation, or eccentricity of the projectile, and in 
some cases on the system of rifling employed. This unsteadiness 
causes greater loss of velocity during flight, since the projectile then 
presents a greater surface for resistance than when the axis is steady, 
hence the range is diminished; also, since the drift is allowed for 
under normal conditions by the inclination of the tangent sights to 
the left, any unsteadiness of the projectile at the muzzle or during 
flight will cause a variation in the actual drift, which results in both 
lateral and vertical error. 
The variable velocity of wind causes errors in fire which must be Wind, 
allowed for in laying the gun. The deflection leaf on the tangent 
sight should always be used in making allowance for error due to 
wind, and the gun should be laid on the object. 
Didion gives a formula for calculating the error due to wind ;$ but 
as the variability of wind in a long range, both in velocity and direction, 
is often considerable, no reliable results can be obtained by theory.f 
* Vide Didion’s “ Traite de Balistique,” p. 392. 
f Colonel Maitland lias written a paper on this subject in “ Proceedings, R.A. Institution/* 
Vol. VIII. p. 343. 
