The Caldwell machine gun is a hand-cranked water-cooled machine gun of Australian origin. 
Designed by Thomas Frederic Caldwell of Melbourne, Australia, who moved to the United Kingdom to bring his invention to the attention of the British Government. It was lighter than its contemporary, the Maxim gun, and was also said to be cheaper to build and produce and was even said to be "foolproof". It is unclear whether any were produced, or if it advanced past prototype stages.
This two-barreled crank-operated machine gun looks similar to the Maxim gun, but includes a steam valve. The two barrels can fire either simultaneously or separately, with a variable rate of fire dependent on how fast the crank is operated, à la gatling guns of the time.
↑CALDWELL MACHINE GUN CO. The Argus, Saturday 20 March 1915, Page 21
↑CALDWELL MACHINE GUN. Poverty Bay Herald, Volume XLII, Issue 13645, 27 March 1915, Page 4