ON A NEW VELOCITY EECORDER. 283 



Third, a tripping or disengagement action, whereby the pen is 

 periodically detached from the prime mover and returned to 

 zero, where it is again coupled to the prime mover ; the interval 

 of time between the uncoupling and recoupling being made as 

 small as possible with respect to the differentiating interval, but 

 it must be sufficient for the pen to travel back to zero, and to 

 settle at rest on the zero line after the little shock and vibration 

 consequent thereon. 



By making the motion of the paper intermittent (Figs. 1,3, and 4) 

 so that during the lifting of the pen it is at rest, and that during the 

 fall of the pen it makes a movement, the diagram becomes a series 

 of parallel lines. If, as in the writer's machines, a glass capillary 

 pen is used, the line from the up stroke (being drawn comparatively 

 slowly), is distinct and its intensity may be easily regulated ; but 

 the down stroke is made so fast that very little ink has time to 

 flow though the capillary point, and the lines are so faint that 

 they do not interfere with the up strokes. By using very care- 

 fully made pens, such diagrams have been drawn with as many as 

 one hundred and forty lines per inch, but the least dust or filament 

 on the pen will cause the lines to coalesce into a shading when 

 the lines are closer than about fifty per inch. 



If the paper moves continuously, (Fig. 2) then the up strokes are 

 irregularly curved according to the changes in the prime motion, 

 and if the differentiating interval is not less than about -^ inch on 

 the paper, the nature of such changes is to a considerable extent 

 visible in the up stroke. The down stroke is a very faint line 

 almost vertical and barely visible. 



If the instrument is to be a fixture in a factory or observatory, 

 the best clock for the purpose is a pendulum clock, with gravity 

 or remontoire escapement, driven by a weight, or with Mr. Russell's 

 double pendulum, so that ample power may be provided for the 

 tripping gear without affecting the rate. In the experimental 

 instruments constructed by the writer, exact time keeping has 

 been sacrificed to portability, and spring driven clocks with lever 

 escapements are used. Ordinary clock trains were adapted to 



