CORRECTIONS FOR QUICK TARGETS. 
comes into view, and further, until the proper point on the horizontal 
scale 1 under the column is made to abut against the vertical scale on 
the outside cylinder. This proper point is, of course, that, correspond¬ 
ing to the value oi G A [see Fig. 5). When set, the drum can be 
clamped. 
Again, suppose the fort to be at the mouth of a channel leading 
into the open sea, the method will still hold good as long as the ship 
employs a straight run in, and there is an obligatory point of passage. 
For if Fig. 6 represent a supposed case, the value of G A can only 
Fig. 6. 
vary within small limits; thus, the triangle GAP may be supposed to 
have a sort of movement of rotation about G, so that while G A does 
not alter materially, the point P may have a large arc of travel. 
In general, it may be claimed that the method is applicable as long 
as the distance G A can be approximately fixed, but the triangle GP A 
may be anywhere in the horizontal plane. 
Now, when the conditions on which the method depend disappear, 
the necessity for extreme rapidity in evaluating corrections disappears 
also ; when they exist, an officer need only ask from his range-finding 
detachment one operation, which can generally be gone through before 
the guns can bear, even if it is not rendered unnecessary by local 
knowledge of tides and speeds, or expert naval information; and, once 
having set and clamped his drum when any range is called or exhibited, 
1 e.g the scale A' A' of Fig. 3. 
