IN NATURAL PHILOSOPHY. 31 



difference is, that when we lift the long arm we make it in 

 effect the short arm, and the other arm the long one. 



8. When the mercury stands at 29^ in. in the barometer, 

 how high above the surface of the water can we place the lower 

 pump-valve ? 



In theory, 29^ in. X 13^ = 398^ in.; in practice, the distance 

 is much less than this. 



9. Why cannot we raise water, by means of a siphon, to 

 a higher level? 



There is no power in a siphon ; it is only a way of guiding 

 the flow of water to a lower level. 



10. If the air in the chamber of a fire-engine be condensed 

 to fe its former bulk, what will be the pressure due to the ex- 

 pansive force of the air on every square inch ? Ans. 240 Ibs. 



1 1 . What causes the bubbles to rise to the surface, when we 

 put a lump of loaf-sugar in hot tea ? 



The bubbles of air contained in the pores of the sugar rise 

 because they are lighter than the water. 



12. To what height can a balloon ascend? 



Until its specific gravity is the same as that of the air in 

 which it floats. 



What weight can it lift? 



A weight equal to the difference between its own weight and 

 that of the air it displaces. 



13. Why is the air lighter in foul and heavier in fair 

 weather ? 



This question is answered in the Philosophy. Another 

 reason may be, that the upward currents of air partly remove 

 the pressure in foul weather. 



14. When smoke ascends in a straight line, is it a proof of 

 the rarity or density of the air? 



Of its density, because it shows that the smoke is much 

 lighter than the air, and so rises immediately to the top. 



it,. Why do we not feel the heavy pressure of the air on out 

 bodies? 



