Proposed 
system. 
Tide correc: 
tion. 
902 SLIDE RULES FOR BATTERY COMMANDER’S CORRECTIONS. 
instance, in a given case each of the corrections ought to be 18 yards, 
they would each be found in the tables given as 25 yards (13 being 
nearer to 25 than to zero) ; and if they are both in the same direction, 
using the tables a correction of + 50 yards would be given, the true 
correction required being + 26 yards, an avoidable error of 24 yards. 
By the application of the principle of the Slide Rule these corrections 
can be made absolutely accurately, or with a margin of error so small 
as to be practically negligeable, and the Slide Rule can be made of a 
manageable size to give the same amount of information as a very vol- 
uminous table. A-Slide Rule has already been introduced for Siege 
Artillery in connection with the Hickman Plotter, and it is believed no 
difficulty has been experienced in teaching its application to non-com- 
missioned officers. 
(c) The correction for alteration of tide level is due to the fact that 
the index plates, multipliers, and hydro-clinometers, are graduated in 
yards; and when elevation is given by their means to the gun, such 
elevation refers to the mean tide level; so that if a gun is elevated to 
a given range by their means the trajectory will pass through a plane 
at mean tide level at the given range from the gun. (The trajectory 
will actually pass through a point 7 feet above this level, the index 
plates &c. being graduated to allow for this; but this does not affect 
the question of corrections). If the sea level has risen or fallen from 
the mean, the trajectory, with the same elevation as before, will cut 
the new plane at a less or greater distance from the gun; and if it is 
desired to hit an object at the same distance as before, it is obvious 
that the trajectory and consequently the elevation must be raised or 
lowered. 
The angular difference in elevation is easily calculated but is of no 
practical use, since the index plates &c. are graduated in yards. What 
ig required is the elevation in yards on the index plate at which the 
gun must be laid in order that the trajectory may cut the new plane at 
the required range. This can be directly ascertained as follows :— 
A table of (angular) quadrant elevations, corresponding to given 
ranges is made out for the mean height; and a similar table for the 
height at extreme change of tide level; then comparing the two the 
range in yards at the mean height, which has the same angular eleva- 
tion as the required range at the new height, can be ascertained. 
For instance a 9-in. R.M.L. gun is at a mean height of 300 feet, the 
tide has risen 20 feet, range to object 2000 yards. 
The elevation (quadrant) fora range of 2000 yards at a height of 
280 feet is 22°5 minutes and this elevation would give a range of 
2059°5 yards at a height of 800 feet. ‘Therefore the gun must be ele- 
vated to 2059°5 yards on the index plate to hit an object 2000 yards off 
when the tide has risen 20 feet, in other words a correction of + 59°5 
yards must be given. 
For less changes of level the corrections are very nearly proportional, 
so that for a rise of 10 feet the correction would be 29°5 yards. This 
method however can only be applied with accuracy to voluminous 
tables, probably quite unsuited to practical purposes. ‘There is how- 
ever a simpler method, by which a formula can be used to Be the 
