THE EOYAL ARTILLERY INSTITUTION. 
427 
necessary to raise a special tangent sight working on the top of turret so 
that it should be in line with the object, and with a fore sight which is 
fixed in the centre of the roof. 
The direction of the object and the heel of the vessel are still obtained by 
the sights on the top of the turret, but the elevation is given (see List of 
Changes, 1st January, 1869), by means of a pointer, screwed to one of the 
trunnions, and working on an arc in rear attached to the cheek of the 
carriage. This arc is graduated with scale of degrees, yards, and fuze. 
Breech. ^HHH^---Muzzle. 
In order to compensate for the deflection of the projectile to the right, 
caused by the right-handed rifling, the tangent sight must be inclined to 
the left. This inclination, which is called "the permanent angle of 
deflection,” happens to be the same for all the B.L. series, but it varies 
for the M.L. natures according to the systems on which they are rifled. 
When a new nature of gun is about to be introduced, the permanent 
angle is calculated from actual practice at Shoeburyness with the specimen 
gun sent there on trial for range and accuracy. The gun is not sighted, 
but elevation is given by temporary means. A fine day being chosen for 
the practice, several rounds are fired with various elevations, and the angle 
is calculated for each round. The mean of the angles so obtained is 
adopted as the permanent angle of deflection. 
The formula used for determining the angle for each round is 
, ^ deflection , . 
tan 6 = —-- x cot elevation, 
range 
which is proved as follows:—- 
Let It represent the line of sight, the gun being laid straight on the target ^ 
and let ts be the deflection of the shot; now, the line drawn from s* 
c 
through fore-sight f 9 will give us the point a at which the head of the 
* This figure is drawn out of actual proportion, that it may be a convenient size. 
