556 
THE DEVELOPMENT OF ARMOUR. 
thickness of plate in centimetres. Turning this into English units and 
transposing 
wv 2 / (0-3937)3 X 
* 2 “ d ( 2-2046 x (3-2809)2 x 5800J 
which worked out by logarithms for a single constant becomes 
*2 = Zf. X __L_ 
d log” 1 6-3532 
or substituting d s into a constant, for w and cancelling, 
£2 = d 2 v 2 x constant 
or t = d v x constant 
which is the equation of the old “ Rule of Thumb,” the constant, how¬ 
ever, is nearly instead of that is to say, the thickness of best 
plate perforated is about one calibre for each 2,000 f.s. velocity. 
Speaking roughly then, the Rule of Thumb is thus advocated as the 
formula for hard steel for shot of the same form proportions, except 
that every thousand feet gives half a calibre perforation instead of one 
calibre as in wrought iron. 
As mentioned before, our 6-in. plates are called upon to defeat 6-in. 
shot with 1,960 f.s. velocity. For the rule just mentioned to be 
correct the shot with 40 f.s. more velocity should match the plate 
which probably they would not do, but the rule may appear to be 
fairly near to the truth. 
To save time and trouble to anyone working out the results of 
experiments the following example is given, being the first round fired 
at Vickers thick plate made on Krupp's process already mentioned. 
Dimensions of plate 10 ft. x 7 ft. x 11^ in. 
Calibre of gun or shots’ diameter = 12 in. and w = 721 • 5 lbs. 
Striking velocity, v = 1861. 
For perforation tests Tresidder’s 
formula is t = 
wv 2 
T~ 
x 
1 
log 18-8410 
where t = thickness of wrought 
iron in inches. 
w = weight of shot in lbs. 
v — striking velocity in f.s. 
d = calibre or diameter in Ins. 
C 3-2697 
3 log v (1861) = \ 3-2697 
(. 3-2697 
log w (721 *5) - 2-8582 
12-6673 
9-9202 
2 (2-7471 
t = 23.63 in. 1*3736 
log T thickness of plate in in. (lly^) 1*0675 
log d (12) 
constant factor in 
denominator 
1-0792 
8-8410 
9-9202 
L = 2.023 ... °-3061 
is the relation of the actual thickness of the plate to the 
calculated wrought iron that would be perforated, sometimes called the 
plate’s “ figure of merit,” when it is a match for the plate. 
