346 HISTORY OF ASTRONOMY. 



time all the transits, with trifling interruptions, have been 

 made by the same agency, except for the very slow 

 circumpolar stars, for which they use the same wires, but 

 by observation with eye and ear. The paper on which 

 the punctures are to be made is folded in a wet state 

 upon a brass cylinder covered with a single thickness 

 of woolen cloth, and has its edges united by glue. 



The punctures are produced by two systems of 

 prickers, which have nothing in common, except that 

 they are carried by the same traveling frame which 

 moves slowly in the direction of the barrel axis, while 

 the barrel revolves beneath it. One pricker is driven by 

 a galvanic magnet, whose galvanic circuit is completed 

 at every second of sidereal time. Professor Airy at first 

 intended that the completion of the circuit should be 

 effected by the same clock (regulated by a conical pen- 

 dulum) which drives the barrel. He found, however, 

 that he could not insure such a constancy in the arc of 

 the pendulum as would make its rate sufficiently uniform 

 to entitle it to be considered as the fundamental clock. 

 He therefore carried wires from the pricker magnet to 

 the transit clock, and connected them with springs whose 

 contact is made at every second by the transit clock. A 

 wheel of 60 teeth is fixed on the escape-wheel axis, and 

 the teeth of this wheel in succession make momentary 

 contacts of the galvanic springs. The position of the 

 springs is so adjusted that the effort of the wheel-tooth 

 upon them occurs only when one escape-tooth has passed 

 the sloping surface of the pallet, and the other escape- 



