34: SCIENCE OF COMMON THINGS. 



Water- wheels. Power of steam. Gunpowder. 



causes to revolve, either by its weight^ by its lateral 

 impulse, or "by both conjointly. 



Fig. 8. Fig 9. 



The most common forms of water-wheels in use are the under-shot and 

 over-shot, or breast-wheels. In the under-shot wheel, Fig 8, a stream of 

 water strikes against the " float-boards " or paddles, placed so as to receive 

 the impulse of the water at right angles to the radii or spokes of the 

 wheel. In over-shot or breast- wheels, Fig. 9, the water is received in 

 cells or buckets on the top or side. In this case the wheel revolves 

 through the agency of the weight of the water. 



103 Upon what does the power of steam depend t 



Upon the tendency which water possesses to expand 

 into vapor when heated to a certain temperature. 



194 What is gunpowder f 



A solid explosive substance, composed of saltpetre or 

 nitre, sulphur, and charcoal, reduced to powder, and 

 mixed intimately with each other. 



105 Upon what does the power of gunpowder depend f 



When brought in contact with any ignited substance, 

 it explodes with great violence. A vast quantity of 

 gas, or elastic fluid, is emitted, the sudden production 

 of which, at a high temperature, is the cause of the 

 violent effects which this substance produces. 



106 Is the power produced in the explosion of powder ever used for 

 propelling machinery regularly f 



It is not, on account of its expensiveness and the sud- 

 denness and violence of its action. It is chiefly applied 

 to the throwing of shot and other projectiles, and the 

 blasting of rocks. 



107' What is (he estimated force of gunpowder when exploded t 



