Description 
of the rule. 
204 SLIDE RULES FOR BATTERY COMMANDER’S CORRECTIONS. 
very large in comparison with H this will not introduce any appreciable 
error. 
The formula for calculating the error then will be 
y = + h cosec (8 + w) 
This may be written 
eae h 
a (B + w) 
and log r= + {log h — log sin (8 + w)} 
and this is a form suitable for solution by means of a Slide Rule. (It 
should be noted that there is a case where this formula is not suitable, 
viz: where the mean height is small the rise of tide large and the angle 
of descent great for under these conditions the assumption on which it is 
founded viz: that AH = AD + DH is no longer approximately true). 
On the left of the rule, above the slide, is a scale of logs of numbers 
from 1°67 to 16°67, the figures however are multiplied by 3, so that 
they read from 6 to 50, this being necessary as heights are always given 
in feet. Below the slide is a similar scale of logarithms but numbered 
naturally, as it is intended to read yards. Both these scales are mea- 
sured from the same point so that 10 on the lower will be found directly 
under 30 on the upper. On the centre of the slide is a scale of log 
sines, commencing from the black arrow, but as the angle of arrival 
will hardly ever fall outside the limits of 1° to 15°, only the divisions 
between those limits are shown. This scale is not used for finding the 
corrections, but is put on to enable the scale on the top edge of the 
slide to be completed for each particular case, as of course it will vary 
according to the nature of gun and mean height above sea level. ‘'T'o 
complete this scale then, for a particular case, we have to ascertain the 
angles of arrival for different ranges (at intervals of 500 yards would 
usually be sufficient) ; this is done by taking the angles of descent from 
the range table and adding to them tke angles of depression. 
For example :— 
Gun 9" R.M.L. Height above mean sea 
M.Y. 1440 fis. level 100 feet. 
Range Angle of Angle of Angle of - 
yards. descent.* | depression. arrival. 
1000 1° 35’ 1° 64/ 38° 29/ 
1500 2° 34/ 1° 16’ 3° 50’ 
2000 8° 45’ 57’ 4° 42/ 
2500 & 46! 5° 63/ 
3000 6° 35’ 38’ 7° 13! 
3500 8° 10’ 33’ 8° 43’ 
4000 9° 50’ 29’ 10° 19’ 
4500 11° 40’ 26/ 12° 6’ 
5000 13° 41’ 23/ 14° 4/ 
* From range table. 
Now to complete the scale, on the upper edge of the slide, opposite 
