SLIDE RULES FOR BATTERY COMMANDER’S CORRECTIONS. 2038. 
correction, which formula is suitable to the construction of a Slide 
Rule. 
Fre. 1. 
In the figure AB is the mean height, BC the change in tide level, 
AD the range to the object and FDH is a tangent to the trajectory 
at D. 
The trajectory which passes through D may be taken as reaching 
the mean level BH at H, since for the small distance DH it will not 
vary sensibly from its tangent; therefore the problem is to ascertain 
the range AH; for it is evident that a gun at A, laid by an index plate 
graduated for the height AB, must be given the elevation for the range 
A# if the trajectory is to pass through D. 
Let AB=H, BC=h, AD=R, and AH=R +7; all these quantities 
being measured in yards; 7 will then be the correction required. 
Now the angle DEB= FDC (angle of arrival!) FDC=FDA (angle 
of descent) + ADC (angle of depression). 
Let angle of descent = w, and the angle of depression = B,, then 
iDIgUs) sah 15 (8, esata 
Now it is obvious that, practically, the error will be very small if we 
take AH = AD + DH; since CB is usually very small compared to CD. 
Therefore correction will equal DH. 
Now DH = BC cosec DEB 
v.e. 7 = h cosec (RB, + w) 
also if the tide level has fallen to D'C’ 
r = — h cosec (By + w) 
Peer 
R 
where 2 = sin 
By taking a mean value for 6 
Rag et Les 
vine (2 = aa = 
=: R 
we obtain a mean value for either rise or fall of tide; and unless h is 
1 The angle of arrival (a term first used I believe by Lieut.-Colonel Jocelyn, R.A.) is the angle 
made by a tangent to the trajectory and the surface of the water; and is made up of the angle of 
descent as given in the range table, and the angle of depression. This is the same thing as the 
‘angle of descent”’ as defined in the drill-book, but there being no convenient term for the range 
table angle of descent, I have preferred to use the term ‘ angle of arrival’’ as above, keeping angle 
of descent to mean, range table angle of descent, i eta: 
