THE EOYAL ARTILLERY INSTITUTION. 
373 
In making the allowance for drift, either the vertical plane through the 
axis of the piece is brought into the vertical plane through the object fired 
at, when the allowance is nil; or the vertical plane of the axis of the piece 
is set in a diverging position with reference to the object; this is generally 
speaking the case with rifled guns in consequence of the drift of their 
projectiles. 
The two operations above alluded to are performed in one with mountain 
guns; the sight being taken either by line-of-metal or on the tangent 
scale. 
Laying by line-of-metal . 
17. In laying by line-of-metal the notched line on the breech and the 
notch of the muzzle sight are brought into one straight line with the object 
aimed at. In this position the axis of the piece is only elevated or depressed 
to the amount of the ground angle; and when the gun stands on horizontal 
ground and is laid as above described, the axis of the piece is in the same 
vertical plane as the object. 
In laying by line-of-metal no allowance for drift is necessary, as at the 
short distances at which line-of-metal laying is used, the drift of the 
projectile is inconsiderable. 
Laying by tangent scale . 
18. We have seen in par. 6 that the tangent scale is so constructed as 
to afford the means of giving the gun the required elevation in conjunction 
with the requisite allowance for drift. As at most ranges at which the 
tangent scale is made use of, the drift of the projectile is so considerable 
that it cannot be neglected, even in firing at objects of considerable width; 
to hit a given point, the gun must not be laid directly on it, but just so 
much to the left as the drift is found to be to the right. As however this 
allowance for drift is at times too large to allow of the line-of-sight being 
taken on any part of the object; and as further it is extremely difficult to 
estimate with anything like accuracy on the object the amount of this allowance 
at long distances, the point to be hit is invariably the point aimed at, but the 
rear sight-notch is carried to the left proportionately to the amount of allow¬ 
ance to be made; this is effected by cutting the sight-notch in the traversing 
bar. When the rear sight-notch, thus carried to the left of the vertical 
plane through the axis of the piece, the muzzle or centre sight according to 
circumstances, and the point to be hit are brought into one straight line 
by moving the trail to the right ; under these circumstances the requisite 
allowance for drift has been made, as may be seen by Eig. 23 : a is the rear 
sight-notch, traversed to the left a distance ac, proportional to the drift fg ; 
the point a is no longer then in the plane of the axis of the piece; b is the 
muzzle sight-notch,/“the point to be struck, abf the line-of-sight, eg the 
axis of the piece produced. When the centre-sight is used and the foot 
of the tangent scale entered into the groove at the breech, the method of 
making the allowance for drift is similar. 
In laying the gun by tangent scale, No. 3 selects the scale of divisions 
on the tangent sight in accordance with the nature of projectile and 
[vol. vi.] 49 
