122 
SHORT NOTES ON PROFESSIONAL SUBJECTS. 
75. Description of a scale to be attached to instruments for 
FINDING THE DISTANCES OF SHIPS AT SEA FROM ELEVATED COAST BAT¬ 
TERIES.* —The problem of finding the distance of a ship at sea from an elevated 
coast battery, is one of easy solution; for (supposing the height of the battery above 
the sea level to be known), a single observation with a suitable instrument to find 
the depression of the water line of the ship, gives the necessary data from which the 
range can be found by a simple calculation.! 
This simple calculation (for obvious reasons) presents a difficulty, which has 
generally been overcome by resorting to one of two methods, viz.:— 
(1) The instrument for finding the depression is graduated for ranges for only 
one elevation above the sea. 
(2) A table is provided in which calculated ranges are placed opposite to various 
elevations and various angles of depression. 
<r 
Each of these methods has its disadvantages. In the first, the scale is only of 
use for one elevation above the sea, and is useless if taken to any other, greater or 
less. Even if it always remain in the same place, it cannot always give exactly true 
results, as the height of the battery above the sea level varies with the rise and fall 
of the tide. 
In the second method only an approximation can generally be obtained, as it is 
manifestly impossible to write down all angles of depression and all elevations above 
the sea. 
The difficulty caused by the calculation may probably be overcome by using some 
such scale attached to the instrument as is described below. By means of such a 
scale (adjusted for the height of the battery previous to making the observation), the 
range can be read off exactly and immediately, without any calculation or reference 
to tables. 
The scale described below, is one to be attached to the tangent scale of Bell’s 
* By Lieut. G. Mackinlay, E.A. *. 
f This is evident from the following considerations. (See Big. 1). Let AB represent the height 
Big. 1. 
of the battery (which is known), C the distant ship, floating on BC, the surface of the water. 
Join AC, through A draw AD horizontally. Then if we find the angle DAC by observation, 
we can solve the right-angled triangle ABC, since the angle DAC equals the angle ACB, and 
then we can find BC the range. 
