ASTRONOMICAL DETERMINATIONS. 19 



the telescope is 16 inches. For use as a zenith telescope, the telescope is 

 equipped with a vertical circle reading by vernier to 20 seconds, attached 

 to which is a delicate level. In the focus of the object-glass there is, besides 

 the ordinary reticule for use in transit work, a movable thread, which is 

 moved by means of a micrometer screw, by which measurements of 'differ- 

 ences of zenith distances are made. It is furnished with direct anil diagonal 

 eyepieces, the latter of which is commonly used in astronomical work. 



For use as a, transit instrument, the telescope is equipped with a deli- 

 cate striding level for measuring the inclination of the pivots, and a revers- 

 ing apparatus for turning the telescope in the wyes. The reticule, as the 

 stationary threads in the focus of the instrument are called, consists of five 

 threads for observing the transits of stars. The reticule is illuminated by 

 means of bull's-eye lamps, the light from which comes through the hollow 

 axis of the telescope and is reflected by a, mirror placed at the intersection 

 of the telescope with its axis. 



CHRONOGRAPH. 



The chronograph is used for the purpose of recording the time of 

 transits of stars as observed with the transit instrument. It may be popu- 

 larly characterized as an instrument for measuring time by the yard. It 

 consists essentially of a drum, upon which is wound a strip of paper, and 

 which is kept in revolution by .a train of clockwork controlled by an escape- 

 ment. A pen carried upon a small car, which is moved very slowly in a 

 direction parallel to the axis of the cylinder, traces a spiral line upon the 

 paper on the drum. This pen is held in place by a magnet, which is carried 

 upon the car, and as long as the current from the battery passes through 

 the coil and thus holds the armature the pen traces an unbroken spiral line. 

 If the current is suddenly broken and restored, the armature is set free for 

 an instant and a jog is made in the line traced. The battery commonly 

 used in connection with this outfit is the ordinary zinc, copper, and sulphate 

 of cooper battery, of which four cells are usually required. The ordi- 

 nary dry battery can also be used and is much more convenient. With this 

 apparatus break-circuit chronometers are used. These differ from ordinary 

 chronometers in the fact that they are arranged to break an electric circuit 



