66 FIRST YEAR SCIENCE 



Sound has been found to be a wave motion in a material 

 medium. If a scratch is made on the end of a long log, 

 it can be heard if the ear is placed at the other end of 

 the log, when it cannot be heard if the ear is away from 

 the log. In this case the medium is 

 the wood. 



If a stone is dropped into a quiet 

 pond, the rippling waves developed 

 will extend often to the farthest shore 

 of the pond, but a chip floating near 

 where the stone fell will not be moved 

 from its position except up and down. 

 Fig. 40. Thus the waves traveled outward from 



the point of origin, but there was no outward movement 

 of the water. If a long rope, attached at one end and 

 held in a horizontal position, is suddenly struck with a 

 stick, a wave motion will travel along the rope from end 

 to end, but the particles of the rope will simply move up 

 and down. It is in a similar way to this that the sound 

 waves travel, but the particles which transmit the sound 

 only move back and forth through small distances. 



Summary. All energy upon the earth is due to the 

 sun. There are two kinds of energy, kinetic and poten- 

 tial. Energy may be changed in countless ways but it 

 cannot be destroyed. 



Heat is a form of molecular energy. Heat is shown in 

 changes in temperature and these are measured by ther- 

 mometers, of which the Fahrenheit and the Centigrade are 

 the commonest. Heat affects the state of matter: the 

 same substance may be solid, liquid or gaseous, depending 

 on the amount of heat to which it has been subjected. 

 This is shown in ice, water and steam. Heat is trans- 

 ferred by conduction, convection and radiation. The 



