180 
SHOUT NOTES ON PROFESSIONAL SUBJECTS. 
78. The line of sight employed in laying field guns. Communicated 
by Captain J. It. Oliver, E.A. 
It lias been repeatedly noticed as a curious fact that field guns give greater or 
less range for the same apparent elevation according as they are laid with the eye 
at some little distance from the tangent sight, or close up to it; and it has been 
found by actual experiment at Aldershot with the breech-loading 9-pr., that a 
difference of 3 ft. in the position of the eye makes a difference of about 150 yds., 
more or less, in the range of the projectile. Again, if we lay a gun carefully 
on some object in the ordinary manner—that is, with the eye close to the notch 
in the tangent sight—and then step back two or three feet, we shall always find 
that from the new position the gun appears to have been laid low—the straight 
line that (as we think) joins the eye, the notch, and the trunnion sight, passing 
below the object; and the farther we step back, the greater will be the apparent 
difference. That this is really the case, any one may easily satisfy himself by 
making the experiment. The explanation is as follows :— 
Let a represent a section through the tangent sight, b the same through the 
trunnion sight, c the object—the top of a flagstaff for instance —e and e' the two 
positions of the eye. The respective lines of sight are indicated by the dotted lines, 
the figure having been purposely distorted to show the effect of change of position 
more clearly. 
When the eye is close to the notch, the object can be seen through it, and there 
is little difficulty in bringing the point of the trunnion sight into the straight line 
joining the eye and the object. But when the eye is withdrawn a few feet, the 
notch looks very small; in fact, it becomes impossible to see through it, and we are 
compelled—perhaps without being aware of it—to view the object over the top of 
the tangent sight. At this distance the apparent size of the trunnion sight is also 
much diminished, and we are obliged to see more of it to see its tip at all. In other 
words, we now use a higher point of the tangent sight and a lower point of the 
trunnion sight for our line of sight, which, as a necessary result, will pass below 
the object, as shewn in the figure. It follows that, if we lay a gun with the eye 
some little distance from the tangent sight, we must give less elevation than when 
using the ordinary method. 
79. Abstract of a report of an experiment carried out at Colombo, 
by Colonel Cox, Commanding Boyal Artillery, Ceylon, to determine 
the penetration of round shot. 
The fort of Colombo, originally constructed by the Portuguese in 1520, was 
rebuilt by the Dutch about 1660, who surrendered it to the English in 1796, 
without firing a shot. 
The works were in a very good state of preservation. It has lately however 
been decided that the ramparts should be pulled down. 
Colonel Cox considered that the occasion ought not to be lost, as a means of 
instruction and gaining experience for both officers and men, and obtained authority 
to establish a breaching battery opposite the face of one of the bastions. 
