FROM SHORE BATTERIES OH ELEVATED POSITIONS, TO FIND THE DISTANCES 
OF OBJECTS AT SEA, BY THE AID OF THE ARMSTRONG GUN. 
By Captain A. W. DRAYSON, R.A. 
There are several instruments by the aid of wliich the distances of objects 
may be obtained, but to supply every coast battery with any one of these, 
would be a very expensive proceeding. As in almost every coast battery an 
Armstrong gun is, or will be, placed, it may be of great practical use to show 
how distances may be obtained by its aid. 
Tor all practical purposes the Armstrong gun would be as available, as a 
theodolite or any other instrument for measuring angles, and this is all that 
is necessary. 
The method of proceeding is as follows : — 
(1) Obtain by levelling, the accurate height in yards of the gun above low 
water mark. This has most probably been already done at all stations, so 
that we may assume that the height of the battery is known. 
(2) By the aid of a common level placed in the bore of the gun, bring the 
axis of the piece exactly horizontal, and directed as nearly as possible towards 
the object whose distance is required. 
(3) By the aid of the tangent scale and vernier, find the angle of depres¬ 
sion of the object. 
(4) Multiply the cosecant of the angle of depression by the number of 
yards representing the height of the gun in the battery, and the product will 
be the range in yards. 
The principle brought to bear in this problem will be evident from the 
following diagram. 
[voi. IV.] 
1 
