124 



Domestic Science 



amounts of other forms do exist as a result of the trans- 

 formations undergone by the original supply. The 

 production of sound, light, heat, and electricity are 

 all manifestations of the change of the original energy 

 into these widely varying forms. 



80. The change of kinetic energy into heat is well 

 illustrated by the familiar schoolboy's experiment of 

 rubbing a button vigorously on a wooden seat. The 

 button may be easily rendered sufficiently hot by this 

 operation to cause considerable shock to anyone 



Fig. 34. 



receiving it unwarily from the experimenter. Skilful 

 blacksmiths are said to be able to make a horseshoe 

 red-hot by striking it repeatedly with a sledge-hammer. 

 Another illustration of this change of kinetic energy 

 into heat is afforded by the method of obtaining fire 

 still practised by some uncivilised tribes. A piece of 

 hard wood, pointed at the end, is rotated briskly 

 between the hands, with the pointed end pressed firmly 

 against the surface of a small piece of wood of a softer 

 character. By skilful manipulation the operator soon 

 makes the softer wood begin to smoulder and glow, 



