Bats.

People The Londonist on "The Day Batman Crashed Into Chelsea":

"June 1874, and a peculiar sight could be spied over Chelsea. A hot-air balloon hovered a kilometre above the ground with the most curious of payloads dangling beneath: a gigantic bat with a human at its controls.

"This Victorian Batman was M Vincent de Groof, otherwise known as “The Flying Man” or L’homme Volant. Newspaper accounts of this ambitious individual contradict in almost every conceivable way. He was either Belgian or French, sometimes Dutch. He was aged either 35 or 36. He’d had success with his flying contraption on the Continent, or else he’d rarely left the ground. Some accounts suggest he’d made a successful flight over Epping Forest a week before, yet seemingly nobody witnessed the feat. Whatever the details of his biography, he was now set on piloting his fragile wings through the skies of Chelsea."

The silhouette in the accompanying drawing does have some resemblance to Bill Finger's original artwork from Detective Comics.