APPENDIX. 293 



LESSON 24 



Specific Gravity. Hydrostatic Balance, fig. 14. Engr. I. 

 If a body x, suspended under the scale, be first counterpois- 

 ed in air by weights in the opposite scale, and then im- 

 mersed in water, the equilibrium will be destroyed, then if 

 a weight be put into the scale from which the body hangs, 

 to restore the equilibrium, that weight will be equal to the 

 weight of water as large as the immersed body, — or it is 

 what the body loses of its weight in the fluid. 



The Hydrometer consists of a thin glass ball, with a gra- 

 duated tube : a smaller ball is attached to the instrument 

 below, containing a little mercury, for the purpose of making 

 it remain upright in the liquid under trial. The specific 

 gravity of the liquid is estimated by the depth to which the 

 instrument sinks. 



LESSON 25. 



The Common Pump, fig. 2L Engr. IL AB the barrel. 

 P the piston. R the rod. V the valve in the moveable pis- 

 ton, y a valve fixed in the body of the pump. S the spout. 

 The Forcing Pump, fig. 22. A B the barrel. P a solid 

 piston. D the pipe joined to the barrel. V a fixed valve. 

 When P descends it shuts the valve y, and forces the water 

 into D through V. When P is raised the valve y opens and 

 the valve V shuts, and the water ascends through y. The 

 forcing pump described in the Lesson differs a little from 

 this figure. We may suppose an air vessel to be placed 

 above V, and a pipe descending through it nearly to V, and 

 the elastic pressure of the air upon the surface of the water, 

 confined in the vessel, will force the water upwards through 

 the pipe. 



LESSON 27. 



The Air Pump, fig. 16. D E the base or wooden frame. 

 A A the two brass cylinders. B the head. C C the co- 

 lumns holding down the head. K the receiver. I a hole 

 in the brass plate, through which the air passes in a brass 

 tube to the cylinders. R R toothed rods. H handle or 

 winch. N a nut, on turning which the air may be excluded 

 25* 



