This instrument, which has been invented in America, consists of an electro-magnetic machine, which, being placed in connection with an ordinary astronomical clock, does not interfere with the regularity of its going, while it marks the instant of each vibration of the pendulum on a revolving cylinder, whose circumference moves at the rate of one inch per second. Two wires being then taken to an observer at any distance, if he, when he observes a star crossing the meridianwire of his telescope, makes contact with the wires, that moment is immediately marked on the same moving cylinder where the even seconds are registered. The fraction of a second may be then obtained with as much accuracy as the space of an inch may be subdivided by ordinary mechanical means: say to the hundredth of a second. This method is further available in many cases where the present mode of noting transits is not, and admits of a great multiplication of observations during the short space of time that the star occupies in crossing the field of view.