RECAPITULATION. 455 



7. This relative weight is generally compared 

 with an equal bulk of water as a standard. 



8. The instrument for comparing these weights 

 of solids is called the hydrostatic balance. 



9. That for comparing the specific gravities of 

 liquids is called the hydrometer. 



10. Air-balloons rise in the atmosphere, because 

 they are specifically lighter, or lighter than an 

 equal bulk, of air. 



11. They are of two kinds — jire-balloons, and 

 injlammable-air balloons. 



12. The diving-bell is an empty vessel inverted, 

 and made so heavy as to sink in water. 



ABSTRACT OF PNEUMATICS. 



1. Pneumatics treat of the mechanical properties 

 of air. 



2. Air is a solid and material substance, as Well 

 as water and other fluids. 



3. Its invisibility is owing to its transparency. 



4. It possesses weight, compressibility, and elas- 

 ticity. 



5. Its weight is demonstrated by experiment. 



6. Its density and elasticity are in proportion to 

 the force that compresses it. 



7. The density of the atmosphere diminishes 

 upwards: and if altitudes in the air be taken in 

 arithmetical proportion, the variety of the air will 

 be in geometrical progression. 



8. Air-pumps are machines for exhausting the 

 air from vessels. 



9. A vacuum is a space emptied of air. 



g g 4 



