He began to work out his idea of the cotton gin, but met with many obstacles. There were no wire manufactories in the South and he could not obtain wire even in Savannah. Therefore he had to make his wire himself. Still further, he was obliged to manufacture his own iron tools. Step by step he overcame all obstacles, until he had a machine that he thought would answer the purpose. [Illustration: A COTTON BALL.] Accordingly, one day, he entered the room where Mrs. Greene was conversing with friends and exclaimed, "The victory is mine!" All the guests, as well as the hostess, went with the inventor to examine the machine. He set the model in motion. It consisted of a cylinder four feet in length and five inches in diameter. Upon this was a series of circular saws half an inch apart and projecting two inches above the surface of the revolving cylinder. The saws passed through narrow slits between bars; these bars might be called the ribs of the hopper. At once the saw teeth caught the cotton which had been placed in the hopper and carried it over between the bars. The seed was left behind, as it was too large to pass through. The saws revolved smoothly and the cotton was thoroughly separated from the seed. But after a few minutes the saws became clogged with the cotton and the wheels stopped. Poor Whitney was in despair. Victory was not yet his. [Illustration: THE COTTON GIN.] Mrs. Greene came to the rescue. Her housewifely instincts saw the difficulty at once and the remedy as well. "Here's what you want!" she exclaimed. She took a clothes brush hanging near by and held it firmly against the teeth of the saws. The cylinder began again to revolve, for the saws were quickly cleaned of the lint, which no longer clogged the teeth. "Madam," said the grateful Whitney, "you have perfected my invention." The inventor added a second, larger cylinder, near the first. On this he placed a set of stiff brushes. As the two cylinders revolved, the brushes freed the saw teeth from the cotton and left it in the receiving pan. Thus the cotton gin was invented by the Yankee schoolmaster, Eli Whitney. Though improved in its workmanship and construction, it is still in use wherever cotton is raised. One man with a Whitney cotton gin can clean a thousand pounds of cotton in place of the five pounds formerly cleaned by hand. When a safety lamp was needed, Davy invented it. When faster water travel was demanded, Fulton constructed the steamboat. When the world needed vast wheat fields, McCormick devised his reaper. When the time had come for the telegraph, Morse studied it out. In the fullness of time, Bell, Edison, and others invented the telephone. When a cotton gin was needed, Eli Whitney made it. Here again the law holds that "necessity is the mother of invention."