This brings us to the second factor--_Visibility_. It is absolutely necessary for the successful tactical employment of machine guns that they should be as inconspicuous as possible when in action; the gun itself is a very small object when close to the ground, and its visibility will depend almost entirely on the nature of mounting and its adaptability for use behind cover of varying heights. All our infantry carriages are so conspicuous as to be quite unconcealable except in defence, the wheels being 4 ft. 8 in. in diameter and the gun axis 3 ft. 6 in. above the ground. The Mark IV. tripod is the handiest and least conspicuous of the mountings at present in use in our service, and although it weighs 48 lb. it can be carried into almost any position and easily concealed. It can be adjusted to fire at any desired height between 14½ and 30 in. above the ground level, and consequently can be used from behind any suitable cover. _Vulnerability._--The question of vulnerability would appear at first to depend entirely on visibility; or, in other words, on the target presented to the enemy’s fire, but this is only true to a certain extent. To obtain the minimum vulnerability it is of course necessary to have the gun as low and inconspicuous as possible, because the less it can be seen and the better cover it can obtain, the more difficult it will be to locate and hit. But the true vulnerability of the gun in comparison with infantry lies in the amount of front they occupy respectively; or in other words, the breadth of the target exposed to the enemy and the percentage of loss they can each sustain _without their fire effect being reduced_. Infantry will never again fight in two ranks in civilised warfare, and the closest formation possible for a firing line is one pace per man; 50 men will therefore occupy a front of, roughly, 50 yards; in other words, the target presented to the enemy is 50 yards in breadth, and, provided the elevation is correct, shots striking anywhere within this 50 yards will be effective. The machine gun, however, only occupies a front of from 4 ft. to 5 ft. 2 in., or 1/25th _the front offered by infantry having equal fire effect_. It is on this point that the wonderful tactical possibilities of the machine gun rests: _the maximum of rifle fire from the minimum of front_. It is obvious that 10 per cent. of casualties in the infantry firing line reduce the fire effect by just that amount, while from 30 to 40 per cent. will probably silence its fire altogether or render it ineffective. The machine gun, on the other hand, is unaffected by even 50 per cent. of loss, while it can suffer 80 per cent. of loss without diminishing its fire effect, though such a loss would of course cause it to lose its mobility and seriously affect the morale of the gunners. A machine-gun detachment consists of from 16 to 24 men, but only two of them actually work the gun, and one man alone can fire the gun