It's air-cooled, belt-fed and gas-powered – not the bike, that is, but the machine gun this US policeman has mounted to his 990cc Indian v-twin. 1918 was a good time to be a bike cop in New York, Indian supplying the force with a number of its then-advanced sidevalve, chain-driven Powerplus machines, equipped with 450 rounds-per-minute Colt M1895s.
Such a dramatic tooling-up of the police stemmed from fears immigrants could foment a Bolshevik-like revolution on US streets. Indians at the time had a left-hand throttle, so the rider could carry on firing even if his passenger caught a bullet. Well, that's not why Indians had left-hand throttles, but it was surely a bonus.