306 



THE POPULAR SCIENCE MONTHLY. 



ries a kymographic pen, suitable for draAving a line upon a sheet of 

 paper drawn past the point of the pen by a clock-work constructed for 

 that purpose. 



The test-tube M is so hung that its axis is traversed by the verti- 

 cal descending limb of the glass tube G, and the test-tube can move 

 up or down without striking the tube occupying its center. 



Fig. 1. Mosso's Plethysmograpii. 



If, now, the glass cylinder containing the arm and the tubes F and 

 G are filled with water, and the arm begins to expand, then the water 

 will be forced out into the test-tube M, which will therefore descend 

 into the fluid in the beaker, P, raising, as it descends, the counter- 

 weight and pen, N. If, on the contrary, the arm contracts, the water 

 will be sucked back, the test-tube M will be lighter, the counter-weight 

 will pull it back, and the pen will descend. Let us suppose that the 

 point of the pen rests against a sheet of paper which is being drawn 

 along horizontally with a constant velocity. As long as the pen is 

 stationary it will draw a horizontal line ; if it ascends, it will draw an 

 ascending curve ; when it falls, a descending curve. Such a line is 

 technically termed a tracing. It shows the extent and duration of the 

 motion, and exemplifies one of the many applications of the graphic 



