Tanks are moving fortresses containing light artillery, machine guns, and rifles, and while capable of inflicting heavy losses by fire they can also destroy obstacles, weapons, and personnel. Their garrisons are protected against the fire of small arms and from shrapnel bullets, but they are very vulnerable to other forms of artillery fire. Their mobility and radius of action are governed by the amount of petrol carried and by the physical endurance of the crew, but except over deep cuttings, {175} broad streams, swamps, very heavily shelled ground, rocky and mountainous country, or in thick woods they can move without difficulty. "The power of delivering successful surprise attacks against almost any type of defences is one of the most important advantages of the use of Tanks in large numbers" ("Field Service Regulations," vol. ii. (1921)). During the _First Battle of the Somme_ (September 1-November 18, 1916) "Our new heavily armoured cars, known as 'Tanks,' now brought into action for the first time, successfully co-operated with the infantry, and coming as a surprise to the enemy rank and file, gave valuable help in breaking down their resistance. . . . These cars proved of great value on various occasions, and the personnel in charge of them performed many deeds of remarkable valour" (Sir D. Haig's Dispatches). AIRCRAFT Two classes of Aircraft are used in the field. Aeroplanes, which are self-propelled and have an almost unlimited radius of action; and Kite Balloons, which, in favourable weather, can be towed by a lorry and can be moved frequently without loss of efficiency. AEROPLANES are of the greatest value for reconnaissance and inter-communication, and not only obtain, and return to their base with, information of the highest value, but facilitate personal reconnaissance of the battlefield by commanders and staff officers. Their offensive and defensive action is also very great and the moral effect of their offensive action is of the highest value. Although aeroplane squadrons are mobile units they lose efficiency if the units are moved too frequently. The action of aircraft in various phases of fighting is dealt with throughout the Lectures. KITE BALLOONS carry two observers, who can remain in telephonic communication with the ground up to a {176} height of 5,000 feet. Inflated balloons can be moved in favourable weather at a maximum speed of 8 miles an hour while at a height of about 500 feet. Their extreme vulnerability to artillery fire prevents their use close to the battle front. GAS