BEFORE MARGARET Knight, most paper bags were simply tubes pinched together at one end and glued; they didn’t have flat folded bases, so they couldn’t stand up or open square. Knight invented a machine that gave them that shape.

She was apparently marked to be an inventor early. Born to a working-class family in Maine in 1838, she liked to bring lunch to her brothers, who worked in a cotton mill when she was a child, so she could watch the machines. As she later said, “Dolls never possessed any charms for me. I couldn’t see the sense in coddling bits of porcelain with senseless faces; the only things I wanted were a jackknife, a gimlet, and pieces of wood.” She “sighed sometimes because I was not like other girls, but wisely concluded that I couldn’t help it, and sought further consolation from my tools.”

One day at the mill the steel tip of a shuttle flew off and severely injured a worker; to prevent such injuries in the future, she designed a simple device to automatically shut down the machine when something malfunctioned. She was 12. Soon she, too, was working long hours in the mills, and she studied every machine she operated.

In 1867, not yet 30, she was working in a paper-bag factory in Springfield, Massachusetts, when she had her idea for the machine to make square-bottomed bags. She drew a sketch and a month later constructed a model out of paper. Like existing bag machines, hers would bring around the sides of a sheet of paper to make a tube, but rather than simply cut one end with one ply longer than the other so it could be lapped over and glued across, her machine formed the end into a “diamond fold.” It did this by inserting what she called a “finger” into the bag, to fold a section of it back on itself, and then using a transverse blade to align that fold and push up the sides to their proper fold.

In February 1868 she attached several pieces to a bag machine at the factory where she worked. They did the job. She next made a complete model, a “rickety wood thing, all shaky,” which she followed with an iron version. Then she took her iron model to a machine shop in Boston for further minor improvements.

She hired an assistant named Abbot to help her with it, and Abbot showed the device to a friend of his named Charles F. Annan. On April 3, 1869, Annan, in an act of sheer perfidy, filed for a patent on the bag-folding machine as his own invention. Knight learned about this after she filed her own patent application the next February. The commissioner of patents then had to declare an interference, a proceeding to determine who had true priority.

Annan’s patent application, it turned out, gave no history of how he had arrived at the invention and provided no witnesses. Knight had plenty of both. But had she ruined her chances by acting slowly? The commissioner relied on an earlier Supreme Court decision that in such cases the first inventor should get the patent if that inventor’s device “will … accomplish the end practically and usefully in the way pointed out.” Margaret Knight’s 1868 wooden model did so, the commissioner found; Annan’s very modest enhancements showed only “the skill of the mechanic, not the genius of the inventor.” The commissioner added that “her success in overcoming the many difficulties encountered is a matter of great surprise.” She got the patent.

Her paper-bag machine was soon superseded by others; her design is not that of the typical modern bag. But she went on to other inventions anyway. She retained a Washington patent attorney to look after her interests and was pleased to find at one point that he had arranged to have her photograph hung on a wall at the U.S. Patent Office. She devised a protective lining for dresses and skirts, a clothing clasp, shoe-cutting machinery, a window frame and sash. Her final inventions, after the turn of the century, involved steam-engine improvements.

In the end, Knight had more than 20 patents and altogether four times that many inventions. But they hardly made her wealthy, partly because she tended to sell them for cash. She never married and lived her last quartercentury in Framingham, Massachusetts. When she died, in 1914, she left an estate valued at $275.05.