A series of 16th-century manuscripts that have been making waves on the Internet look like a Monty Python version of the Renaissance: They show cats outfitted with flaming backpacks, attacking castles and villages.

But the illustrations are legit. They're intended to show how cats and birds could in theory be used to set fire to a besieged city, according to a University of Pennsylvania scholar.

Mitch Fraas, scholar in residence at the University of Pennsylvania—the university digitized the manuscripts last year—says that the drawings are from artillery manuals and are accompanied by notes explaining how to use animals as incendiary devices.

View Images This 17th-century engraving shows that the idea of using cats as arsonists had staying power and was widespread. PHOTOGRAPH BY MATT ROURKE, AP

Fraas translated from the original German:

"Create a small sack like a fire-arrow ... if you would like to get at a town or castle, seek to obtain a cat from that place. And bind the sack to the back of the cat, ignite it, let it glow well and thereafter let the cat go, so it runs to the nearest castle or town, and out of fear it thinks to hide itself where it ends up in barn hay or straw, it will be ignited."

Fraas is skeptical that any army ever deployed what he calls a "pretty grisly" tactic: "It seems really hard to believe that would ever work."

The texts were likely expensive to create and were probably owned by nobility or others who were studying battle tactics and kept their books in a library, safe from conflict, Fraas said.

Surprisingly Common Idea

Fraas first heard about the cats when a friend alerted him to an Australian blog that had posted the peculiar images from Penn's digital collections last November.

Over the past three years Penn has digitized its collection of pre-1800 manuscripts and has shared them online for the public to browse.

Early modern and Renaissance manuscripts are rife with unusual doodles and unexpected marginalia, and Fraas said he "figured it was an idiosyncratic thing that a particular illustrator had drawn."

After getting the initial tip, Fraas turned to Twitter to see if he could find more explosive feline images. That unleashed more tips, which sent him hunting through more digital archives.

It turns out that pictures of explosive felines from the Renaissance are not all that uncommon.

View Images Another cat and bird maraud inside a city in this manuscript, "Book of Instruction for a Cannon Master" from around 1590. PHOTOGRAPH BY MATT ROURKE, AP

"It's a pretty stable form, and I think we've seen seven or eight instances of this illustration in manuscripts copied at various times over the 16th century," Fraas said.

Fiery attack cats and birds showed up in a number of hand-painted manuscript illustrations and also in etchings from volumes printed years later. "It's clearly something that had staying power," Fraas said.

And plans for deploying firebombs on animals were not limited to Europe.

"Over the past couple of days, I've gotten a lot of emails of people pulling examples out of history," Fraas said. "The folks in China and Japan have a long history of these manuals."

In the Chinese manuals, oxen and horses are the animal arsonists of choice.

The weaponized felines, for their part, have become known simply as "rocket cats" on the Internet.

"It sounds a little better than fire cat or cat with explosive sack," Fraas said.