weapon from the hopper, and would fire them one by one. A wedge shaped block would rise up and lock the round in place, a cam operated hammer would then strike the percussion cap, firing the round. The empty metal tubes would subsequently be collected in a pan located under the weapon. The metal tubes would then be reloaded and placed back in the hopper. This made quite a bit of work for the gun crews, since the tubes had to be reloaded quickly enough to keep up with the machine gun's relatively high rate of fire. The Agar machine gun had a single barrel. This design proved to be prone to overheating, especially during periods of sustained fire. The overheating problem was ameliorated somewhat through the use of replacement barrels which could be swapped when the barrel in use overheated. Two spare barrels were typically carried with each gun. Agar also added a cooling mechanism to the barrel, which consisted of a metal jacket through which air was forced to provide cooling. The air came from a turbine, which was powered by the same hand crank that was used to fire the weapon. This cooling air also tended to blow away any pieces of unburned paper from the cartridge that happened to be near the barrel. The rate of fire was limited to 120 rounds per minute, which also helped to prevent overheating. The Agar gun can be classified as a multiple-chamber, single-barrel design. In this classification scheme, it is in the same category as the much better known revolver cannons, still in use today. The Agar gun's hopper is in fact the only multiple-chamber configuration other than the ubiquitous revolver to have been used in practice. The Agar machine gun also had a built-in steel shield, which was a design feature not seen on other machine guns of the period. This gave the Agar a much more modern appearance, since single barrel machine guns with similar shields became common in World War I. Use In 1861, the Agar machine gun was demonstrated to President Abraham Lincoln at Washington Arsenal. He was so impressed that he ordered all ten available weapons to be purchased immediately, at a cost of $1,300 each, which was a very high price at the time. Later in the same year, General McClellan ordered an additional fifty weapons, at a reduced cost of $735 each.