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).