CORRECTIONS FOR QUICK TARGETS. 
11 
Fortunately, in many cases, guns are placed high enough to be un¬ 
influenced by tide to any great extent. Table III. has been compiled 
in order that it may be seen at a glance when a correction is necessary. 
When guns are fired with quadrant elevation, the angles of arrival 
should always be entered in the range table; these angles being ob¬ 
tained by taking the sum of the angle of depression, due to the site, 
and the angle of descent, due to the trajectory; when these are to 
hand, and the rise and fall of tide, r, is known, the table will at once 
give the necessary information. Thus, for example, let the tide be 
9 feet above mean, and, at a certain range and with a certain gun, let 
the angle of arrival be 1° 20', that is between 1° 32' and 1° 15', we see 
at once, from the table, that the correction must be 125 yards, and as 
the tide is high it is a plus correction. 
Table III. also shows that at times this correction may be of great 
importance, as in the example given in Table IV. 
The difficulty of tide can be got over by using tangent elevation, 
or having some sort of adjustable reader : the first is, of course, im¬ 
possible with the position-finder, and, at other times, would entail the 
loss of the many advantages of straight-edged sights and sighting 
steps; the second would appear feasible, but a satisfactory method is 
not forthcoming. If, then, we have to compete with a variable tide 
and a quick objective, we are face to face with a difficult problem, 
which cannot be solved by what is called the “ the light of nature ” : 
and any labour which can be carried out at leisure, which will help to 
its speedy solution at the critical time, cannot but be well spent. 
For a given velocity and a given height of tide, tables of corrections 
can be easily constructed, supposing the channel to be not very wide ; 
if the width exceeded 300 or 400 yards the tables would have to be 
made out for several values of G A (Fig. 1), say, for each 300 yards of 
difference. For example, suppose GA capable of variation, between 
800 and 2000 yards, tables for GA = 1100, GA = 1400, and GA = 1700 
would suffice. The information concerning each velocity might be 
placed on a card, or on a drum, so constructed that only the informa¬ 
tion needed would be visible. The drums or cards should be kept, 
properly arranged, at the fighting station of the fort, 
To show the form the information might be placed in, let the gun of 
Table IV. command a channel, the fairway of which is at a perpen¬ 
dicular distance of 1000 yards (G A = 1000), and let us suppose an 
objective of 24 knots (v = 12) : then, by the graphic method already 
described, we find the following corrections to hold good :— 
TABIall W. 
Range. 
Correction. 
5400—4950 
-150 
4950—4200 
-125 
4200—3250 
-100 
3250—2400 
- 75 
2400—1600 
- 60 
1600—1100 
- 25 
1100—1000 
- 0 
If, now, we correct these by Table IV. for differences from mean 
tide of 9 feet and 10 feet we get the following results :— 
