Wikipedia:Portal:Portal/Selected article/10

A porthole is a small, generally circular, window normally used on the hull of ships to admit light and air. Though the term is of obvious maritime origin, it is also used to describe round windows on armored vehicles, aircraft, automobiles (the Ford Thunderbird a notable example), and even spacecraft.

On a ship, the function of a porthole, when open, is to permit light and fresh air to enter the dark and often damp below-deck quarters of the vessel. It also affords below-deck occupants a limited, but often much needed view to the outside world. When closed, the porthole provides a strong water-tight barrier.

A porthole on a ship may also be called a sidescuttle or side scuttle (side hole). The use of the word sidescuttle is meant to be broad, including any covered or uncovered hole in the side of the vessel.

Portholes on spacecraft must be made from glass that can survive rapid temperature changes, without suffering the cracking that can result from thermal shock. Those on the International Space Station were made from quartz glass mounted on titanium frames, covered with enamel. Conversely, portholes on submarines are generally made of acrylic plastic, and can be several inches thick.