B6 NATURE AND PROPERTIES OF AIR. 



Steel into ribands, and impel loaded vessels against the fury 

 of the winds and waves. It has armed tlie feeble hand of 

 man, in short, with a power to which no limits can be as- 

 signed ; completed the dominion of mind over the most re- 

 fractory qualities of matter ; and laid a sure foundation for 

 all those future miracles of mechanic power which are to 

 aid and reward the labour of after generations. 



Questions. — 1. What is the principle of the diving bell ? 2. What 

 were the dimensions of Dr. IJalloy's diving bell .' 3. How was light 

 let in -^ 4. Fresh air? 5. How do divers make known tlieir wish to 

 be drawn up ? 6. Of what use is tliis invention ? 7. Describe the 

 steam-engine. 



LESSON 27. 



Nature and Properties of Air. 



Den'sity, the degree of closeness and compactness of the parti- 

 cles of a body, tlie property directly opposite to rarity. 

 Ab'solutely, completely, without restriction, positively. 

 Hem'isphere, lialf a globe, or sphere. 



The science which treats of the mechanical properties of 

 elastic or aeriform fluids, such as tlieir weight, density, com- 

 pressibility, and elasticity, is called Pneumatics. The air in 

 which we live surrounds the eartli to a considerable height, 

 revolves with it in its diurnal and annual motion, and, toge- 

 ther with the clouds and vapours that float in it, is called the 

 atmosphere. The height to which the atmosphere extends 

 has never been ascertained ; but at a greater height than 

 forty-five miles it ceases to reflect the rays of light from the 

 sun. The air is invisible because it is perfectly transparent ; 

 but it may be felt on moving the hand in it, or when it 

 moves and produces what we call wind. It is nearly nine 

 hundred times lighter than water, but the whole atmosphere 

 presses on all sides like other fluids, upon whatever is im- 

 mersed in it, and in proportion to the depths. Its pressure 

 upon a mountain is known to be less than in the plain or 

 valley beneath. If a alass tumbler be completely filled with 

 water, and covered wth a piece of writing paper, so as to 

 hold it tight, and accurately even, the water will not run out 

 although the glass be iuverteci and the hand removed. The 



