144 



OUR PHYSICAL WORLD 



Powder and other more powerful explosives are essential in 

 blasting out coal, the ores of the metals in the mine, the stone 

 in the quarry. Air compressed by powerful engines is sent 

 down by iron pipes and hose into mines and quarries to furnish 

 the power for drills that make the holes in which the charge of 



FIG. 57. A drill operated by compressed air in a quarry 



explosive is placed to smash the rock into pieces that can be 

 handled (Fig. 57). The hand-power air pump is familiar to 

 most boys and girls who have ridden a bicycle and to the auto- 

 mobile driver, for by its aid the tires are inflated. The principle 

 of operation is very simple. A piston-head fits a metal cylinder 

 tightly. In this head is a valve that lets air into the cylinder on 



