This was a light automatic cannon that fired 1 pound shells. It was known as a Pom Pom gun because of the sound it made when firing. It is mounted on a field gun carriage for towing by horses and fast deployment.
The Pom Pm gun was designed by Hiram Maxim, inventor of the Maxim Machine Gun, in the late 1890s. It was essentially an enlarged version of the original Maxim gun and fired exploding shells. Because the shells were exploding, they had to be kept over a minimum weight so as not to violate treaties against the use of exploding bullets, and so that they could instead be classified as artillery.
Though the guns were produced under license by the British armaments firm of Vickers and Sons, the Boer Transvaal Republic bought several of these guns, and so during the war the British found themselves under fire from these very agile and deadly weapons. The British soon realized their mistake and began supplying their forces with the same weapons.