In the early 19th century, a process known as "airing" babies was thought to be a healthy way to get city infants some fresh air and sunlight inside of the home. Promoted by numerous parenting books, New Yorkers, and even Eleanor Roosevelt, the process involved a cage made of chicken wire and metal which attached to the outside of the window frame.
Essentially, the thinking was that this was part of a process to toughen up the babies, and make them better able to withstand common colds. It was believed that exposing infants to cold temperatures—both outside and through cold-water bathing—would grant them a certain immunity to catching minor illnesses.
Here is a short video showing similar cages in action, decades later in London:
Check out the full article on Mental Floss to learn more about these bizarre baby cages.
