140 
SHOUT NOTES ON PROFESSIONAL SUBJECTS. 
87. Directions for the use of Captain Tracey’s Range Scale. Com¬ 
municated by the inventor, Captain H. A. Tracey, R.A, 
1. Obtain the angle of depression to tbe target. 
The gun itself, with a spirit-level or plummet, is a 
good instrument to do it with, and always ready to 
hand. 
2. The height of the battery above the sea being 
known, make the angle of depression just obtained, 
as read on the sliding scale, correspond therewith, 
when the \[/ will point to the range. 
This scale is an adaptation of <c Gunter’s Logarith¬ 
mic Lines.” The lower course is simply a line of 
numbers (marked N on the Gunter lines on a sector), 
and the upper is the same, only arranged to read off 
feet instead of yards. 
The slider is a logarithmic line of tangents, and 
enables one to perform mechanically the processes 
necessary for the solution of right-angled triangles 
in which one side and the opposite angle are known. 
This mechanical application of logarithms has the 
following advantages:— 
1. A special scale is not wanted for every height. 
It is of universal application. 
2. Accurate correction can be made for rise or fall 
of tide. 
3. The smaller the angle of depression and the 
lower the height of the battery, the clearer, more dis¬ 
tinct, and therefore more accurate becomes the scale. 
In the scale as made by Elliott Brothers (not quite 
twice the size of the cut), every minute of angle up to 
one degree is shown without crowding; and up to 100, 
every other foot is marked. This allows corrections 
to be made to within one minute of angle, and our 
guns do not read to less. 
At the suggestion of the Committee of the R.A. 
Institution, this scale has been submitted to the 
Director of Artillery. Its manufacture (experimentally) 
has since been authorised, and it is now before the 
“Engineer Committee.” 
