HEAT 



81 



EFFECT OF HEAT ON AIR 



As the air is heated it expands and 

 escapes in the form of bubbles. 



heated to the same temperature as the ball, the ball will 

 again pass through it. Why? What use does the black- 

 smith make of this same 

 principle? 



In summer the tele- 

 graph and telephone 

 wires sag because of in- 

 crease in length due to 

 expansion. In winter 

 they contract and are 

 then stretched tight and 

 produce the characteris- 

 tic humming noise when 

 the wind shakes them. 

 On a hot summer day 

 the concrete walks will 

 sometimes expand so 

 much that they will crack or crumble. This is not 

 usually noticed until cold weather when the concrete has 

 contracted, leaving open spaces where 

 it broke during the summer. In order 

 to avoid much of the breaking of walks 

 by the action of heat, expansion creases 

 are put in about every three or four 

 feet, when the walk is laid. The cracks 

 between the ends of railroad rails also 

 provide room for the linear expansion 

 of the rails. When are these cracks 

 HEAT AND METALS between the railroad rails almost closed 

 and when are they open? Why? 



Along the base of rock cliffs there are 

 quantities of small stones which have 

 fallen from above because they were moved from their 



When the brass 

 ball is heated it will 

 not pass through 

 the brass ring. 



