EXPANSION OF IRON. 123 



Q. Why will the wheelwright's HOOP FIT the 

 nave MORE EASILY, because they are made BED-HOT ? 



A. As iron expands by heat, the hoops 

 will be larger when they are hot; and 

 (being larger) will go on the nave more 

 easily. 



Q. Why will the HOOPS, which have been PUT 

 ON HOT, GIRT the nave more FIRMLY ? 



A. As iron contracts by cold, the 

 hoops will shrink as they cool down ; and, 

 therefore, girt the nave with a tighter 

 grasp. 



Q.. Why does a farrier pat the HORSE-SHOE 

 on HOT ? 



A. That it may stick the closer, when 

 it has contracted by cold. 



Q. Why does a STOVE make a CRACKING 

 NOISE, u'hen afire is very hot ? 



A. The iron stove expands by heat, 

 and (as it swells) the parts rub both 

 against each other, and against the bricks 

 around, driving them further off; and 

 this produces a cracking noise. 



Q. Why does a STOVE make a similar CRACK- 

 ING NOISE, when a large FIRE is TAKEN DOWN ? 

 A. The iron stove contracts again, 



M 2 



