20 



NATURAL PHILOSOPHY. 



[lesson VIII. 



/'. Suppose that I could remove 

 ".osphere from the surface of the 

 ^1 i; e, while the smoke, vapour, and balloons 

 were suspended, what would be the result? 

 P. They would fall to the ground, he- 

 cause their support would be withdrawn. 



!'J. T. How can you prove this ? 

 /'. -By an experiment. [Experiment Ik] 

 : :.ere a tall bell-glass (a), open at the 



a, bottom, which shall 



f, -J -, he placed on the plate 



A"ii~T7\ of an air-pump (/), 



K -L !i and the air it con- 



tains exhausted. You 

 observe that the top 

 is closed by a brass 

 cap (4). which has 

 two small stages (cc) 

 attached to its sides, 

 and a wire () pass- 

 ing through the cen- 

 tre, but lilted so that 

 /it will turn without 

 admitting the air. 

 The ends of the 

 ^ stages being made to 

 fry- 9- rest upon the lower 



part of the wire, a piece of lead (d) is placed 

 upon one, and a feather (e) upon the 

 other, or any two bodies differing in den- 

 sity. I will now exhaust the air, [performs 

 the experiment,] and as soon as the han- 

 dle of the wire (a) is turned, the stages will 

 fall, and the lead and feather also. Ob- 

 serve, you see the lead and feather have 

 loth reached the stage of the air-pump at 

 the same instant. I will now replace 

 them and admit the air, and you will see 

 that the lead reaches the stage long before 

 the feather. So it is with the air; being 

 heavier than the smoke, it descends, and 

 pushes the smoke, vapour, or balloon up- 



y 



wards, which may be easily proved by 



>f the air-pump [Kxperimei 

 You observe that 1 place a small piece of 

 candle with a long ignited wick, upon 

 -e of the air-pump, and cover it 

 with the bell-glass we used in our last 

 experiment. The smoke from the wick 

 ascends because the air in the glnss de- 

 scends and pushes it up, but when the air 

 is exhausted the effect will be very di. 

 and, to illustrate it better, I will place the 

 candle upon an inverted wine-gl'iss. [Per- 

 forms the experiment] Now you see that 

 the smoke falls to the bottom of the glass. 



93. T. Are there other kinds of at- 

 traction ? 



P. Yes ; there is the attraction of 

 cohesion, capillary attraction, chemical 

 attraction, magnetic attraction, and elec 

 trical attraction. 



GENERAL QUESTIONS ON LESSON VII. 



1. WHAT is meant by gravity ? 



2. Can you discover the direction of the 

 force of gravity by any means ? 



;>. What is the direction of the force of 

 gravitation, and how can you determine it ? 



4. Does the direction of the force of 

 gravity diiFer ? 



5. How is it that falling bodies do not 

 descend in parallel lines ? 



6. Are all bodies subject to the laws of 

 gravity ? 



7. What do you mean by the weight and 

 the mass of a body ? 



8. How can you account for smoke rising? 



9. What would be the result if the globe 

 was not surrounded by the atmosphere ''. 



10. How many kinds of attraction are 

 there ? 



LESSON VIII. 



RECAPITULATION, &c. The attraction of the earth's mass binds the atmosphere 

 around its surface, producing what is termed atmospheric pressure, a force equivalent 

 to about fifteen pounds on the square inch at ',' level of the sea, but less as we ascend ; 

 a fact we have considered before. If this pressure, or force, were not exercised, we 

 should not be able to remain upon the surface of the earth, but would be thrown off the 

 same as the dirt from the wheel of a carriage, or the water from a mop, when the 

 servant causes it to revolve rapidly. We have now to consider other forms of attraction, 

 of equal importance in their way with the attraction of gravitation. 



