There were two main methods of making smoke. The first could only be done by ships with steam engines. This required the ship to introduce extra fuel to the boilers, or to reduce the flow of air. This reduced the fraction of the fuel that was burned, leaving the remainder to go up the funnel as a thick cloud of black smoke. Smaller ships like destroyers, and small craft that used petrol engines, also had chemical smoke generators. These used a chemical reaction to create smoke. In the Royal Navy during WWII, the smoke agent used was chlorosulphuric acid. This reacted with moisture in the air to produce clouds of white 'smoke' - actually hydrochloric and sulphuric acid vapour. These smoke generators were carried at the aft end of the ship, and deployed the smoke agent through vents pointing aft. Smoke shells and bombs, which used an incendiary or chemical agent to create smoke, could also be used, though these were rarely used in naval battles and were more common for amphibious assaults.