The agricultural revolution in the nineteenth century involved two things: the invention of labor-saving machinery and the development of scientific agriculture. Labor-saving machinery naturally appeared first where labor was scarce. 'In Europe', said Thomas Jefferson,' the object is to make the most of their land, labor being abundant here it is to make the most of our labor, land being abundant.' It was in America, therefore, that the great advances in nineteenth-century agricultural machinery first came. At the opening of the century, with the exception of a crude plow, farmers could have carried practically all of the existing agricultural implements (农具) on their backs by 1860,most of the machinery in use today had been designed in an early form. The most important of the early inventions was the iron plow. As early as 1790 Charles New-bold of New Jersey had been working on the idea of a cast-iron plow and spent his entire fortune in introducing his invention. The farmers, however, would have none of it, claiming that the iron poisoned the soil and made the weeds grow. Nevertheless, many people devoted their attention to the plow, until in 1869 James Oliver of South Bend, Indiana, turned out the first chilled-steel (冷淬钢) plow. What is the main topic of the passage? ( )