“About five seconds before third contact a black ligament connected the planet’s disk with the trembling edge of the Sun.”
―Winslow Upton, “Transit of Mercury at Ladd Observatory.” Popular Astronomy, 1907.
Prof. Winslow Upton tapped a telegraph key as he watched the planet Mercury transit the Sun. The signal was sent to a chart recorder where an electromagnet moved a pen on a slowing turning drum of paper to record the observation. The instrument is called a chronograph. This one was made by the Warner and Swasey Company of Cleveland in 1890. Primarily it was used for calibrating the astronomical regulators (precision pendulum clocks) to set the time accurately. It was also used for timing events such as lunar occultations of stars.