526 bulletin: museum of comparative zoology. 



as are also the lower wheel and disc. By means of two pins the disc 

 drives the piece e, Plate 3, fig. 8, 9, which turns the film-spool. The 

 lower ratchet enables the operator to turn the film-spool forward by 

 means of the clock-key, so that when a new film has been put in place 

 and the front casting is replaced, he can roll up the black paper quickly 

 and "bring the film into position for the first exposure. The other 

 ratchet allows the winding arbor to be turned backward when the 

 spring is being wound. The rest of the lower train consists of five 

 wheels, the last of which carries a fan to govern the speed. 



The centre arbor of the upper train, which makes one revolution per 

 hour, projects through the upper plate of the clock and carries the 

 cam shown at B, Plate 4, fig. 13. The exposures are regulated by the 

 notches in the circumference of this cam. Several interchangeable 

 cams with intervals from two minutes to an hour are provided. 

 When the pawl C falls in a notch, the corresponding motion of the 

 lever D unlocks the lower train and allows it to roll up about ^ of an 

 inch of film. C also carries two screws, E and F, the functions of 

 which will appear presently. 



Since only a very short exposure is required to impress the film, it 

 is desirable that the length of exposure be accurately timed. The 

 cams which regulate the exposure should then be put on a rapidly 

 moving arbor. Two very small cams are used, and they are placed 

 one above the other on the arbor of the fifth wheel, which makes one 

 revolution per minute. Two long thin springs provided with contact- 

 points, one of platinum, the other of ivory, press against these cams. 

 Each therefore snaps forward when its cam reaches a definite position, 

 the one to make the electric contact, the other to break it. The time 

 of exposure thus depends on the relative angular position of the two 

 cams, one of which is adjustable. This device would make an ex- 

 posure every minute were it not for the screw E, which resists the 

 motions of the contact springs at all times save when the lever C falls 

 into a deep notch in its cam. One shallow notch is cut in each of these 

 cams and serves to double-space the records on the film at the end of 

 each hour. In addition to its two small cams the arbor of the fifth 

 wheel carries a radial spring which, as it revolves, passes very near the 

 screw F. If the crank G be thrown to the left, Plate 4, fig. 13, it 

 withdraws lever C to an abnormal position, so that screw F will engage 

 the radial spring as it comes around to a definite position and will thus 

 stop the clock. When the crank G is thrown right again the clock 

 will start on an exact minute. The operator can thus make the clock 

 register with his watch. 



