City apartments, as many people know, can be small and stuffy. And while fresh air is a wonderful, healthful thing for people of all ages, in the late 19th century, the idea of actively "airing" your baby to promote health started cropping up in parenting books.

The concept was introduced by Dr. Luther Emmett Holt who wrote about "airing" in his 1894 book The Care and Feeding of Children.

"Fresh air is required to renew and purify the blood, and this is just as necessary for health and growth as proper food," he wrote. "The appetite is improved, the digestion is better, the cheeks become red, and all signs of health are seen."

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. Holt responded to a series of hypothetical questions about his proposed methods, saying:

Is there not great danger of a young baby's taking cold when aired in this manner?

Not if the period is at first short and the baby accustomed to it gradually. Instead of rendering the child liable to take cold, it is the best means of preventing colds.

How should such an airing be given?

The child should be dressed with bonnet and light coat as if for the street and placed in its crib or carriage, which should stand a few feet from the window. All the windows are then thrown wide open, but the doors closed to prevent draughts. Screens are unnecessary.

What are the objections to an infant's sleeping out of doors?

There are no real objections. It is not true that infants take cold more easily when asleep than awake, while it is almost invariably the case that those who sleep out of doors are stronger children and less prone to take cold than others.

What can be done for children who take cold upon the slightest provocation?

They should be kept in cool rooms, especially when asleep. They should not wear such heavy clothing that they are in a perspiration much of the time. Every morning the body, particularly the chest and back, should be sponged with cold water (50° to 60° F).

Dr. Holt himself does not seem to have specifically suggested an outdoor baby cage, but the airing he advocated soon led parents without garden space to improvise.

In 1922, Emma Read of Spokane, Wash., applied for a patent [PDF] on a "portable baby cage." (Long before there were commercial options, Eleanor Roosevelt bought a chicken-wire cage in 1906 to hang out the window of her New York City townhouse on East 36th Street for her first child, Anna, to nap in—a practice for which her neighbors threatened to call the authorities on her.) In Read's application, which was granted the following year, she cited that "it is well known that a great many difficulties rise in raising and properly housing babies and small children in crowded cities, that is to say from the health viewpoint." The solution she proposed was as follows:

With these facts in view it is the purpose of the present invention to provide an article of manufacture for babies and young children, to be suspended upon the exterior of a building adjacent an open window, wherein the baby or young child may be placed. This article of manufacture comprises a housing or cage, wherein the baby or young child together with proper toys may be placed.

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A similar product makes an appearance in Louis Fischer's 1920 book The Health-Care of the Baby, which describes "a convenient outdoor sleeping compartment readily attached to any window," called the Boggins’ Window Crib. The wire device, 36" x 24" x 27", is described as being "admirably adapted for city apartments," with an insulated roof that will keep the baby cool enough to build up a cold-weather tolerance even in summer.

The Health-Care of the Baby via Google Books

Although patented in the early 1920s in the States, it wasn’t till the '30s that these cages took off—and in smog-filled London, of all places. The wire cages were attached to tenant buildings and distributed to members of the Chelsea Baby Club who lived in high buildings without garden space in which to air their babies.

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The fad proved to have some staying power, and in 1953, British Pathé produced this pun-filled promotional video espousing the benefits of baby cages.

It's not entirely clear when exactly the baby cage's popularity began to wane, but it likely had something to do with growing concerns for child safety in the second half of the 20th century. But if it still seems like a great idea to you, these days there are similar options for cats.

[h/t Apartment Therapy and Gothamist]