SLIDE RULES FOR BATTERY COMMANDER’S CORRECTIONS, 205 
3° 29' on the scale of log sines, make a mark and number it 1000; 
against 3° 50’ put 1500 and so on; also filling in on the right end of 
the slide the nature of gun, muzzle velocity, and height for which the 
scale has been constructed. 
To use the rule. 
- Find by the depression range-finder how much the tide has risen or 
fallen from the mean ; set the mark, corresponding to the range of the 
target, on the upper edge of the slide against the number of feet rise 
or fall; under the black arrow will be found the correction required. 
- For instance if the tide has risen 15 feet, and the range is 2000 yards, 
bring 2000 on slide under 15 on the rule and under the arrow will be 
found 62, which is the required correction and must be added since the 
tide has risen. 
(d) Correction for travel. 
This correction is the allowance that has to be made for the altera- 
tion in range which takes place during the time that elapses between 
the moment of reading out the range and the arrival of the shot at the 
object. This time is made up of a constant “time of firing,” viz.: 
the time from reading the range to firing the guns, and a variable time 
of flight. 
The amount of correction necessary is ascertained in the following 
manner: the time in which the range alters by 50 yards is noted, then 
since we know the range and consequently the time of flight, and the 
time of firmg; we have the following proportion— 
Mia+t:: 50:2 
where 7 is the number of seconds the range takes to alter by 50 yards, 
a is the constant time of firing, ¢ the time of flight, and w the required 
correction, 
__ 50 (a + 2) 
Se 
and log w = log 50 + log (a + t) — log n. 
On the upper part of this rule in the centre.is a black arrow marked 
00, representing log 50 measured from a point on the left 3 below the 
slide is a scale of logs from 10 to 250, which scale starts from the 
same point, so that the two fifties are exactly opposite one another. 
On the top edge of the slide is a scale of logs from 2 to 25 marked 
“seconds ;” and on the lower edge is a scale of ranges which must be 
made out separately for each gun and for each time of firing. The 
method of constructing it is as follows :— 
Ascertain the constant “ time of firing” and add to it the times of 
flight for the different ranges ; for example— 
Gun 9-pr. R.M.L. M.V. 1440 fs. 
hence a 
Range. Time of flight. 
1000 yards 2 seconds 
1500 _,, By sy 
2000, 1 an 
&e, &e. 
es ERY ae ee er) oe ao Ld 
If the constant “time of firing ” has been found to be 3 seconds 
Use of rule, 
Correction for 
travel of 
object, 
Description 
of rule. 
