If it's heroes taking the stage of mid-19th century Romantic paintings, then the Industrial Revolution saw the entrance of a new kind of hero. With an increased labor force in England, the rise of the middle class was quickly underway. As industry grew, so did the number of factory and mill workers; and as their visibility in the public sector grew, so did their socioeconomic influence, culminating in Parliament's passage of the Reform Bill of 1832. This bill allowed for a wider range of suffrage to more middle-class towns and forever changed British government. But this was not simply a political movement; the revolution was also an ideological one. Behind the rise of the middle class we should see the influence of Romantic philosophy, established under the groundwork of earlier Enlightenment thought. If nature's holiness could be transmuted to the people who freely lived within it, then a higher view of the agrarian farmer, the middle-class worker out in the fields, certainly is in order. Separate from the corrosive influence of society, common workers like these Gleaners became a more highly regarded emblem of ideological and spiritual perfection.
The tranquility of nature is meant to be communicated here, very much like the landscape paintings of John Constable and Thomas Cole. And look how artist Jean-François Millet paints this scene: using soft colors and smooth brushstrokes. Never mind that the work of gleaning in harvesting fields is hard and exhaustive labor, the scene is affectionately painted because of its closeness with nature—that is its theme. This painting (produced in 1857) crosses the art history timeline also, much as I said David's work could be considered alternately Romantic and Neoclassical. For its depiction of the middle class, Millet's Gleaners is sometimes categorized under the Realist Period of art, but, again, with these crossovers it's hard to strictly define a work of art coming out of the 19th century.