Conceptual Physics (12th Edition)

Published by Addison-Wesley
ISBN 10: 0321909100
ISBN 13: 978-0-32190-910-7

Chapter 13 - Think and Explain: 63

Answer

At such a depth, the water pressure is tremendous. A larger viewing window would have a correspondingly larger force on it, possibly cracking the window. A small circular viewing window makes sense - with no corners, there are no points of stress concentration. Also, such windows are often tapered, like a cone with the sharp end cut off. By placing the larger circle on the outside of the sub, the water pressure helps press the window into its frame, making a tight seal.
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