what sight becomes a melancholy object for swift and others who walk through the town and cities?

Swift complains about feeling sad at the sight of so many beggars and poverty stricken people around the town.

It is a melancholy object to walk through this great town, or travel in the country, when they see the streets, the roads, and cabin doors crowded with beggars of the female sex, followed by three, four, or six children, all in rags and all importuning every passenger for an alms.