Page "Christiaan Huygens" Paragraph 21

Huygens also observed that two of his pendulum clocks mounted next to each other on the same support often become synchronized, swinging in opposite directions.

He reported the results by letter to the Royal Society of London and it is referred to as " an odd kind of sympathy " in the Society's minutes.

This may be the first published observation of what is now called coupled oscillations.