This image was acquired from
wikimedia. It was marked as Public Domain or CC0 and is free to use. To verify, go to the source and check the information there.
Keywords from Image Description:
PSM Counting oscillations of pendulum at seconds interval. Counting oscillations of pendulum at seconds intervals archive org Popular Science Monthly Volume author PDold Extracted image from book to grayscale cropped whitespace ineuw archive org Popular Science Monthly Volume Popular Science Monthly illustrations Automatograph