100 MECHANICAL ACTION. 



Q. What is the use of GREASING CART 

 WHEELS ? 



A. The grease lessens the friction \ 

 and (by diminishing the friction) the 

 latent heat is less disturbed. 



Q. Why is the TOP of a MOUNTAIN COLDER 

 than the VALLEY beneath, although it be two or 

 three miles nearer to the sun ? 



A. 1st Because the air on a moun- 

 tain is less compressed, than the air in a 

 valley. 



2ndly It is more rarefied : and 

 Srdly It is less heated by reflection. 



Q. Why is air COLDER on a mountain 

 " because it is LESS COMPRESSED ?" 



A. As the air in a valley is more 

 compressed (by the mass of air above) 

 than that on the top of a mountain, 

 therefore more heat runs out ; just as 

 more water runs from a sponge, the 

 closer it is squeezed together. 



Q. Why is a mountain-top COLDER than a 

 valley, " because the AIR there is MORE RAREFIED ?" 



A. As the air is more rarefied, its heat 

 is diffused over a larger space and is less 

 intense ; just as a candle would show less 

 light in a large room, than in a small one. 



