201 
\ 
PROPOSED SLIDE RULES FOR CALCULATING | 
BATTERY COMMANDER’S CORRECTIONS e222. 
ag prt Oo F> 
Ye vie 
MAY 25 
MAJOR A. C. HANSARD, R.A. \ p 
ssssae KEE FATENT O%°- 
ee “ae 
Te corrections which have to be made by the Battery Commander to | 
the range of a moving object as given by the depression range-finder 
are i-— 
(a) ‘ Hrror of the day” or “ powder error.” 
(b) “Correction for length of target.” 
(c) “Correction for rise or fall of tide level’? (required when 
using Case II. for laying). 
(d) ‘Correction for travel of target.” 
ics I 
(a) As regards the “powder error” no method as yet proposed Powder error 
will enable us to calculate the correction ; the error is partly due to age 
and condition of the powder charge, which cause changes in the muzzle 
velocity ; and partly to atmospheric causes such as varying tempera- 
ture, barometric pressure, and direction and force of wind; which 
influence the resistance offered to the passage of the projectile through 
the air. 
All that can be said about it is, that it increases with the range (but 
not usually in any easily ascertained ratio) ; and if the force of wind is 
great it may vary with the direction of the target. 
(b) The correction for length is necessary from the fact that the corectiontor 
depression range-finder takes the range to the bow water-line of an “aun” 
advancing vessel, or stern water-line of a retiring one, and we usually 
wish to hit the vessel about the centre; therefore something must be 
added to the range, as given by depression range-finder, to carry the 
shot to the point we wish to hit; no calculation is however necessary 
for this. : ; 
(c)and (¢) ‘These errors can be calculated, and in order to eliminate riaat error 
as many causes of inaccurate shooting as possible it seems worth while for travel of 
to calculate them accurately. Bee 
At present tables are drawn up in which the corrections due to a present sys 
given change of level, or rate of travel, at certain ranges are given, bub °™ 
if the actual range, state of tide or rate of travel do not coincide with 
any given in the table, a guess has to be made; and it is impossible to 
give, as it were, a sufficient number of accurately calculated points, in 
a table of manageable size, to make this guess-work a matter of no 
importance. urther the corrections are tabulated to the nearest mul- 
tiple of 25 yards which ‘introduces another source of error; for if, for 
5. Vou, XXIt. 28 
