LE ROY C. COOLEY. 



125 



ing pressures both above and below the pressure of an 

 atmosphere. 



Fourth, To provide an apparatus for projection 

 wherewith the mathematical relation of volume to pres- 

 sure may be accurately exhibited to large classes. 



A vertical standard made of wood is 

 provided with a groove which extends 

 throughout its length. In this groove 

 a wooden block, a, (fig. 1), moves, with 

 little friction, and carries a glass reser- 

 voir of mercury counterpoised by a 

 weight acting over a pulley on the top 

 of the standard. A rubber tube con- 

 nects the bottom of the reservoir with 

 the lower end of an accurately gradu- 

 ated glass tube, t, containing a conven- 

 ient volume of air. This air- tube is 

 placed at a distance of about six 

 inches from the standard, to which it is 

 attached by means of a metallic arm. 



The zero of the graduation of this air 

 tube is at the top, and the scale reads 

 downward. The scale reading at the 

 top of the mercury always shows the 

 volume of. the air. And since it is the 

 exact measurement of the volume and 

 not any particular quantity that is 

 necessary, this simple artifice obviates 

 the necessity of adjusting the volume 

 *"*§• 1 * for experiment. 



The lifting and lowering of the reservoir along the 

 standard, which is easily done by means of the counter- 

 poise, increases and diminishes the pressure upon the 

 air at pleasure, the transition from one degree to another 

 being quick, easy and precise. 



Moreover, since the air tube is placed about the mid- 



