MINUTES OF PROCEEDINGS OF 
20 
the figure (p. 19), simply requires a + correction RGB , when it will be 
converted into the angle EGB, 
EGR = tan-i 4 .I. 
1 \ 
6' must be subtracted to reduce it to tangent elevation, because the zero 
of the tangent scale in all Armstrong's guns gives an elevation of 6' as referred 
to the axis. 
(2) The effect is the same if the gun is laid by tangent sight at an 
object the same height above the plane as its own axis; remembering how¬ 
ever that when the tangent sight is at zero the axis of the gun has an 
elevation of 6', as above stated, and therefore that the line of sight GR is 
not the same as when the gun is laid by spirit-level on the true horizon. 
(3) Next let us suppose the gun laid at the mark b, not the same 
height as itself, and the elevation given by tangent above b. This is the 
common case wdien the guns are fired from the permanent platforms at 
Shoeburyness, and the elevation is given above a mark 7 or 8 ft. high at 
1000 yds. or some other known distance. 
The angle 6 recorded is EGA of the figure, p. 19 : the true angle to 
which the ranges must be ascribed is EGB' or EGB, according as the 
range is less or greater than the distance m of GA produced. 
Knowing the height of the axis of the gun and of the mark A, we know 
the angle of depression of A, namely the angle whose natural tangent 
_ h — b T 
a 
In like manner, 
HGR=GRL — — .II. 
, R 
The correction to the elevation then is 
e = tan* 1 _ tan -1 \ ... III. 
a R 
and is additive when the second angle is greater than the first; this is the 
case when B falls within the distance a. 
In words. Pind the angle of depression OGA of the mark above which 
tangent elevations are given. 
Pind the angle subtended by the height of the gun at each mean range. 
Add the difference of these two angles to the given elevation as long as the 
second is the greater, which it will be for all ranges less than m : subtract 
the difference from the given elevation, when the second angle is the less, 
which it will be for ranges beyond m. 
When Gm coincides with GR 3 i.e. when the gun is laid by spirit-level or 
at a mark of the same height as its own axis, the correction is simply the 
angle subtended by the height of the gun at each distance B, and is always 
additive. 
5. Let us now examine Table I. after applying these corrections to the 
angles. 
