ARTILLERY IN COAST DEFENCE. 
471 
is a very different thing'from the actual range, or distance from the 
gun to that object. .. j 
The first point to be considered by the Fire Commander is whether 
his guns are laid by tangent sights for elevation, in the drill-book 
called case I.; or by index plates, or some other means of giving 
quadrant elevation, in the drill-book case II. If the latter is used he 
will have to make a correction to the range due to the state of the 
tide. The reason for this is shown by the diagram. 
The quadrant elevation is calculated for mean tide level, or rather 
for seven feet above that level (seven feet being about half the height 
of side of a battleship). Now, if the gun is elevated to a certain 
range on the index plate, successive shots similarly laid will follow the 
same trajectory [E AC in diagram) cutting the mean tide level A B in 
A ; if the tide rises to the level E F, the trajectory cuts it at E\ and 
in order to reach an object at the same distance from the gun as 
before, as A! 3 more elevation must be given. If the tide falls to CB, 
the trajectory cuts the water-line at a greater distance from the gun, 
and elevation must be taken off. The error due to tide also varies 
with the range; at long ranges, where the trajectory is very curved, 
the errors are small; but at short ranges, where the trajectory is flat, 
as at M NO, the errors are large. Also the errors will not be the 
same when using R.M.L. guns with low velocities and consequently 
curved trajectories, as with B.L. guns having high velocities and, 
therefore, flat trajectories, especially at short ranges. In practice it is 
found sufficient if a table is made showing the correction necessary at 
each 1000 yards in range, and for each quarter-tide, the corrections 
being given in multiples of 25 yards. This correction is not necessary 
when laying the guns for elevation by tangent scale, as the elevation 
is then given with reference to the line of sight and not from a hori¬ 
zontal plane. 
The next error to be considered is that due to the fact that different 
brands of powder do not give a constant muzzle velocity to the pro¬ 
jectile; and that atmospheric influences, such as increase or decrease 
of density, due to the presence of more or less moisture, increase 
or decrease of temperature, barometric pressure, or of wind blowing 
up or down the range, give variations in the amount of resistance to 
the passage of the projectile through the air. The effect of these 
different influences is known as the “ powder error,” or the “ error 
63 
