One of the most interesting questions arising in a study of the heavenly bodies is whether or not any of them besides the earth are inhabited. Is there any good reason for supposing that our pigmy planet, so insignificant compared with many celestial bodies, is the only one containing life? On the other hand, life such as we know it could not exist on some of the other planets. Mercury would be too hot; Neptune too cold. Climatic conditions on Mars are most nearly like those of the earth. Within recent years the telescope has revealed on the surface of Mars a number of peculiar, regular lines. Many scientists hold that these are artificial canals or irrigation ditches, and that the planet must be inhabited. The theory does not seem at all unreasonable. But the most that can be safely said is that if any of the other planets are inhabited, the most likely one is Mars. CHAPTER IX THE COTTON-GIN Another great invention is the cotton-gin. It is great because of the commercial prosperity which it brought to the Southern states; because it cheapened and extended the use of an almost necessary article of life; and because of its effect on American history. The inventor was an American, Eli Whitney. The word _gin_ is an abbreviation of _engine_, and in former days was often used to denote a handy mechanical device of any kind. The cotton-gin is a machine for removing the seed from the fiber of the cotton-plant. Its essential parts are a number of saws which tear the fiber from the seeds, some stiff brushes used to remove the fiber from the saws, and a revolving fan which blows the lighter substance of the cotton away from the saws and brushes. The original cotton-gin has been little changed by improvement since its invention. It seems to be one of those inventions which have been perfected by the inventor himself. Eli Whitney was born in Westborough, Worcester County, Massachusetts, December 8, 1765. His father was a thrifty farmer. Nature bestowed upon the son marked ability in the use of tools. While he was yet a child, his inventive genius manifested itself. Before he was ten years old, he could use every tool in the farm workshop with the ease and skill of an old workman. He made a violin before he was twelve and later he came to be noted in the neighborhood as a skilful mender of fiddles. He also turned his attention to making nails, which in Revolutionary days were made by hand, and became the best nail-maker in Worcester County. When he was twenty-four years of age, a desire for a college education possessed him. His father agreed to furnish the money to pay for his schooling, with the stipulation that the son should pay it back. He entered Yale, where he was graduated in 1792.