British Vickers machine gun during the Battle of Menin Road Ridge in 1917.

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The first practical self-powered machine gun was invented in 1884 by American inventor Sir Hiram Maxim. Heavy guns based on the Maxim such as the Vickers machine gun were joined by many other machine weapons and  saw their first major use in World War 1. The biggest single cause of casualties in World War I was actually artillery, but combined with wire entanglements, machine guns earned a fearsome reputation.

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The Maxim machine gun used the recoil power of the previously fired bullet to reload rather than being hand-powered, enabling a much higher rate of fire than was possible using earlier designs. Maxim also introduced the use of water cooling, via a water jacket around the barrel, to reduce overheating. Maxim's gun was widely adopted, and derivative designs were used on all sides during the First World War. The design required fewer crew and was lighter and more usable. First World War combat experience demonstrated the military importance of the machine gun. The United States Army issued four machine guns per regiment in 1912, but that allowance increased to 336 machine guns per regiment by 1919.

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Many machine guns were water cooled. If they ran low of coolant they would overheat and eventually seize if the water wasn't topped up. As a last resort gunners would use urine!

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"During the attack of the 24th, 250 rounds short of one million were fired by ten guns: at least four petrol tins of water besides all the water bottles of the Company and the urine tins from the neighbourhood were emptied into the guns for cooling purposes".

Source: Wikipedia/vickersmg.blog