24 CHAP. IV. AUTOMATIC RECORD OF EVOLUTION OF OXYGEN 



of different drums have therefore to be available for records 

 of various duration. 



(2) The speed is liable to be slowed down by the un- 

 winding of the coiled spring of the clock-work. 



For our present purpose we require a single drum, the 

 speed of which can be varied so as to enable us to obtain 

 record for as short a time as ten minutes, or as long a period 

 as ten hours. It is also necessary that the speed should be 

 maintained constant for a long time. These requirements 

 have been secured by the particular governor which will 

 now be described. 



The Governor. — I have been able to devise a perfect type 

 of frictional governor which consists of a pair of hinged 

 levers, carrying small spherical weights at the lower ends ; the 

 upper end of each lever is hook-shaped. By a contrivance 

 to be presently described, the speed of the drum is regu- 

 lated by causing increasing divergence between the levers 

 which carry the weights. The speed is at its highest when 

 the hinged arms are spread out nearly horizontally ; it is 

 slowest when the arms are parallel and vertical. The 

 angle of divergence can be gradually increased or decreased 

 by means of a plate of metal coated with plumbago, which 

 presses against the hooked inner arms of the levers, h. 

 The revolving contact of the governor against the ad- 

 justing plate takes place at practically two points. The 

 plate itself can be moved up or down by a micrometer screw : 

 hence the angle of divergence of the arms of the governor 

 may be adjusted by turning the micrometer screw in one 

 direction or the opposite. For every reading of the micro- 

 meter screw (which has a circular scale) the speed of rota- 

 tion of the drum has a definite value, which remains un- 

 changed. It is thus easy to obtain a variation in the speed 

 of from one to ten times, which is continuous. 



Readings were taken of the successive strokes of a light 

 hammer actuated by one of the revolving wheels in the clock- 

 work. The constancy of the speed of rotation for long 

 periods is shown by the fact that at a certain adjustment 



