134 
SHOUT NOTES ON PROFESSIONAL SUBJECTS. 
81. Pocket Sextant. Communicated by Captain It. O’Hara, B.A. 
The value of the pocket sextant as an instrument for military sketching, seems 
to be very much overlooked; and yet, in comparatively open ground, it is far 
superior to the prismatic compass. With its assistance, a tolerably accurate sketch 
of country can be made from one high piece of ground, giving approximate 
differences of level; also, by means of empirical formula, it can be used to find 
ranges, &c., without the use of tables, fixed angles, or fixed bases (of course, with 
a fixed base, the work is a little simplified). One person can obtain the required 
result without assistance, which is an advantage of some importance. The prin¬ 
ciple of finding distances, &c., is the old one, of a right-angled triangle, as shewn 
in the “Handbook for Field Service,” 1854, p. 144. In a triangle ABC, the 
angle at B being the right angle, AB the distance to be found in yards, BC the 
known or measured base in feet. Then BAG — (90 — BOA) may be called the 
angle of depression, which it would be if EC' was a vertical height. 
Let 8 = 11,459, 
1 ) = BA = distance in yards to be found, 
II — BC = base (or height) in feet, 
M = number of minutes in angle of depression = (90 — BCA). 
Empirical Eormula. 
Then J) 
IIS 
im 
Mil 
30,000° 
HS 
10 M 
is, I think, sufficiently accurate for artillery practice. 
With pocket sextant, to find angle of depression from a height: — 
(1) By means of natural horizon. 
Measure the angle between the object and the horizon, and apply correction 
fdr dip. This correction may be obtained by empirical formula also, viz.:— 
•0 square of distance in miles = height of station in feet; 
■J distance in miles = correction for dip in minutes. 
Or a/ height of station in feet = correction for dip in minutes. 
This latter is not quite so correct. 
In the first of these, the height of station being known, distance in miles may 
be obtained, and from this the correction for dip. 
(2) Without horizon. 
Suspend a plummet by suture silk from screw fastening index arm. The angle 
between this and the object will be the complement of the angle of depression. 
N.B.—If there is wind, suspend the plummet in water. 
(3) Get a reflecting level adapted to fit the slot of telescope tube in the pocket 
sextant; make the reflection of object coincide with reflection of centre of bubble. 
Then angle read off on vernier = angle of depression. 
By these two last methods one can turn the pocket sextant into a good clino¬ 
meter. I have used these methods (except the last) for the last thirteen years, for 
finding ranges and sketching, and think they may prove of use to those who possess 
thfe “Handbook for Field Service.” 
