PRINCIPLES OP GUNNERY. 
429 
The rectangle, ABCD, enclosed by the intersection of these zones, 
represents the area in which 50 per cent, of 50 per cent.— i.e., 25 per 
cent.—of the shot will probably strike. A vertical rectangle may also 
be obtained in the same way, if the mean vertical and lateral errors 
are known. This is the method of calculating the probable rectangle 
usually adopted on the Continent. 
A comparison of rectangles made out from the results obtained by 
firing different guns, affords a good criterion of their respective accu¬ 
racy of fire. 
These rectangles are called probable rectangles. 
The dimensions of the British probable rectangle hitherto adopted 
are 1*56 times larger than those of the former, but it is proposed in 
future to adopt the probable rectangle in use on the Continent. 
The British probable rectangle represents the area in which 50 per 
cent, of the shot will probably strike— i.e. } twice the probable number 
of shots contained in the Continental rectangle. 
The sides of the British 50 per cent, probable rectangle may be 
directly obtained by multiplying the mean errors by 1*69 x 1*56 = 2*64. 
As the Continental method is simpler for calculation, an example of 
its employment will be given in the case of the 40-pr. M.L. (long) 
gun at 5° elevation. ( Vide p. 427.) 
The length of the 25 per cent, probable rectangle = 17*9 X T69 = 30‘25 yds. 
u breadth « « « « = 1*23 X 1*69 — 2*08 « 
And at 10° elevation 
The length of the 25 per cent, probable rectangle = 43 , 6xl*69 = 73*69 yds. 
„ breadth „ « „ u = 3‘44 X 1‘69 = 5’81 « 
2‘08 
£ 
CS 
These rectangles represent the area (horizontal) in which 25 per 
cent, of the shot will probably strike. 
When the mean errors of a gun at a given range are known, the 
probability of hitting a target of given dimensions can be calculated 
by means of a Table of Probability Factors. 
The following table gives factors by which the dimensions of the 
zones containing 50 per cent, of the shot must be multiplied in order 
to determine the per-centage of hits on a target of given dimensions :— 
40PSML 
5ELEV- 
5'8i 
40 P&.M.L. 
1O^ELEVH 
Probable 
rectangles, 
Table of 
Probability- 
Factors, 
