This chronograph is a weight driven instrument, and was used in conjunction with a transit and a specialized precision regulator clock. It has a drum with graph paper on it, and a long threaded rod, both of which are driven by the weight drive. A special clock would have sent signals to the recorder with a "click" every second. A pen would have recorded a spiral line around the drum as it turned. As the clock sent a signal, a "blip" would have shown up on the ink. The observer would look through a transit telescope equipped with crosshairs in the eyepiece, or more likely it would have had a filar micrometer. When a certain star passed through the crosshairs, the observer would click a switch. The pen (or possibly a different pen) would have made a different "blip". Tables were published which told the time when the star was at a certain point in the sky. By interpreting the marks, you could tell what time it was according to the clock, and reset it if need be.