Where a1 is the angle of sight from O to G; a2 is the angle of sight from O to T.
Angles of elevation are read plus, angles of depression minus; T being the target, G the gun, and O the observer.
Before firing it is necessary to ascertain that the trajectory will clear the intervening crest—i.e. see that the angle of slope to the top of crest is less than the angle of Q.E. at which the gun is likely to be fired. If there is an obstacle some distance in front of the guns, to ascertain if the trajectory will clear it make a liberal estimate of the range to the obstacle and ascertain if the quadrant elevation to be fired at is greater than that which would be required to hit the obstacle.
If it is possible to place a mark to aim at near the crest with a given elevation which will ensure the fire striking the target, this will much simplify indirect fire, but the combination of circumstances required to enable this to be done will not often be found.
When all is ready to open fire the battery commander will take up a position from which to direct it, and will give the orders to open and cease fire and name the guns or sections to fire. The section commanders will watch the fire effect of their guns and give the orders necessary for correcting elevation or direction. The battery captain will assist the commanding officer by noting the fire effect.
It will be seen from the foregoing that considerable practice is required to enable indirect fire to be efficiently carried out; but the results obtained in covering the advance of infantry and thus enabling them to push up quickly to close range, without the delay and exhaustion caused by a prolonged fire fight, will more than repay the time spent in perfecting machine-gun detachments in this duty.
SUMMARY OF GENERAL PRINCIPLES
With Cavalry.—To enable it to retain its mobility and do away with the necessity of dismounting whole squadrons for fire action. To give it greater power both in attack and defence, and enable small detachments to hold important strategical or tactical points. To act as escort to artillery and to assist in the pursuit.
With Infantry.—To cover the first advance with fire. In attack they should be held back until the last reserve has been thrown in, when they must be used to bring an overwhelming fire to bear on the point selected for assault. Owing to their narrow beaten zone and great accuracy, they can safely fire over the heads of prone infantry within 100 yards of the position. A Japanese officer who commanded a machine-gun battery at the battle of Mukden said on one occasion he “continued this fire until their attacking infantry were within 30 metres of the enemy’s position.”[6]
They may also be used to reinforce threatened points, when their mobility will enable them to arrive at a distant part of the battlefield with the rapidity of cavalry. They should rarely be used in the firing line, where their fire, being dispersed, is less effective than an equal volume of rifle fire, and where they are at once the target for every rifle. Machine guns can never engage artillery, and should avoid engaging other machine guns or firing on a line of skirmishers.