VIII. Set _b_ at 400 on scale _dd_ and slide _z_ till _b_ cuts 300 (normal) on scale _dd_.
The range correction is now found on scale _mm_ opposite pointer _a_. This number is now set on the gun-arm of the plotting-board and each next plotted position will read on the range-scale of the gun-arm just that many yards more or less than the true range, i.e., the corrected range.
_Platen._--The rectangular sliding frame.
_Wind-arm._--The arm pivoted to the board on the left of the platen.
_Wind-component Scale._--The scale above the movable end of the wind-arm.
_Drift-curve._--The curved edge of the metal plate attached to the left end of the platen.
_Travel-arm._--The arm pivoted on the platen.
_Azimuth Correction-scale._--The sliding scale below the platen.
_Deflection-scale._--The fixed scale immediately above the azimuth correction-scale.
[Illustration: THE DEFLECTION-BOARD FIG. 66.]
_Travel-scale._--A scale for making corrections for angular travel of the target; there are two, one below the azimuth correction-scale and one on the platen.
_"T" Square._--The sliding "T" square having the time graduations at one edge, corresponding to given ranges.
Place the travel-scale on the platen in the lower or upper position according as the observing interval is 10 or 20 seconds.
As soon as the wind-component device is set note the deflection reference-number indicated, and set the wind-arm to the corresponding reading on the wind-component scale.
Set the platen so that the point of the drift-curve corresponding to the given range will be accurately over the right-hand edge of the wind-arm.
As soon as the reference-number indicating the _angular travel_ of the target during the observing interval is announced, set the travel-arm (right edge) for that travel by the travel-scale on the platen and set the azimuth correction-scale for the same travel by means of the travel-scale below it.
Set the "T" square so that the point of its scale corresponding to the given range will be accurately over the right edge of the travel-arm.
The azimuth correction to be applied to the gun-arm in all cases is then read from the azimuth correction scale at the bevel edge of the "T" square.
When Case I or II is being used the deflection to be sent to the guns is read from the deflection-scale at the bevel edge of the "T" square.