55 
A METHOD OF CALCULATING THE PROBABILITY 
OF COAST DEFENCE FIRE. 
BY 
MAJOR R. M. B. F. KELLY, R.A. 
In the Rivista di Artigleria e Genio for June 1894 there is an 
interesting article on the method of calculating the probability of coast 
defence fire against moving objectives entitled, ‘A contribution to the 
study of the probability of fire of Coast Artillery.’ 
I began a translation of this but found when I came to the calcula- 
tions and tables that the results for Italian guns with their method of 
observation, ranging &c. would be of little interest or use to our service, 
and besides I could not quite agree with all the bases of calculation in 
the Italian article, factors of probability for instance having been 
omitted. I have therefore taken my idea from the article above quoted 
and worked it out so as to correspond as far as possible with English 
systems. 
The problem is as follows. Granted a Range Finder’ and system of 
working it and the guns, by means of which predictive firing is possible, 
and assuming that although the course of the objective will neither be 
in an absolutely straight line nor traversed at a constant speed, yet, 
that during short intervals of time the course will be practically a 
straight line and constant in speed, that is to say, the changes in speed 
and direction will not be so frequent as to make predictive firing 
impossible. What should the percentage of strikes be at various ranges 
for various speeds of objective ? 
The result arrived at will be the maximum under the most favour- 
able circumstances, when no errors beyond those inherent in the 
instruments, guns and ammunition occur. 
The article I have referred to commences with a long discussion on 
the possibility of predictive firing, but as that forms the basis of our 
system of fire control, I do not think I need reproduce it in this article. 
I think we are prepared to admit, that if we have observed the path of 
a ship for a certain time we can fairly predict her range or her position 
at the end of another short space of time. 
1 Or Position Finder, 
2. VOL. XXII. 8 
