A Signal Gun or Noon Marker
"This is an ancient, muzzle-loading cannon, of 348 cm or 137 inches in its overall length, and 40 cm or 15.7 inches in the outer diameter and 20 cm or 7.85 inches in its bore, with base material probably made of a sort of bronze.
Manufactured in 1863, at the Tokugawa government's order, by Seijiro Kudara of the Tsuyama-han, Mimasaka (the present-day Tsuyama city, Okayama prefecture), it had been installed at Osaka's Tempozan battery until the Meiji Restoration, after which it had been transferred to the Osaka Castle grounds.
Beginning with the Meiji 3rd (1870) it had seen its use as a time-signalling gun, at first fired thrice a day, until the Meiji 7th (1874) in and after the year it had been employed as the noon marker, thus coming to be familiarized with the citizenry as the Castle's don (Bang!), or its Noontime don throughout the city.
To save gun powder and for other reasons, its use seems to have been terminated, sometime during the Taisho 12 or 13 (1923 or 1924) its exact date and reasons are not clear."