Trial-shots.—The gun is laid so that the shot should have a certain range, all corrections having been determined as described above, except of course that for travel.
The bar is set with the index at zero, and the trammel is set at the muzzle velocity used in the computation for the shot.
The gun is fired and the range of the shot is plotted. The range officer determines how much the shot has fallen short or gone beyond, and announces the result as plus or minus so many yards. The computer moves the bar plus or minus the number of yards announced, using the scale for this purpose.
The pointer now indicates the muzzle velocity to be used in computing the next shot. The velocity pointer is moved accordingly.
If a second trial shot is used, the corrections are computed as before, using, however, the new muzzle velocity as determined from the first shot.
In determining a second corrected muzzle velocity the bar should be moved for but half the longitudinal deviation of the shot from the expected range; the pointer then marks the velocity to be used for the next shot.
In case a third trial shot is used the process is the same except that the bar is moved for but one third of the longitudinal deviation.
The curves are given for every ten yards of range, for every ten per cent of weight of air, and for every ten miles of wind, etc.
For conditions in which the values lie between these readings, the trammel can readily be set by the eye sufficiently close for all practical purposes.