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The Lewis gun is a First World War era light machine gun, the most visible and distinctive part of the Lewis was a large finned aluminum tube that fitted over the gun barrel.The theory that became popular and was used to justify the design was that the barrel jacket created an injection or venturi effect.
The outer cylinder was narrower at the tip and had a larger clearance around the barrel at the rear, with each shot, the pressure of the propellant gases escaping the barrel would create a vacuum within the jacket. This vacuum would draw cool air through the front opening of the jacket, forcing it over the heat sink and out the rear opening.
Tests carried out later showed that the "injection" type cooling system was ineffective in practice, especially on land. The extra weight of the aluminum radiator and barrel jacket did not compensate for the marginal cooling the system provided in sustained fire.
Many later versions of the Lewis, especially those used in aircraft or modified in the field, eventually removed the heavy aluminum jacket and radiator, operating with the barrel exposed and relying solely on natural air cooling.