254 HEAT. 



slowly in a new tin dish, and that a polished andiron 

 before a fire is long in becoming hot. 



A concave burning-mirror, which throws the rays of 

 heat to a focus or point, may be made of sheet-tin, by 

 beating it out concave so as to fit a regularly curved 

 gauge. If a foot in diameter, and carefully made, it 

 will condense the rays of heat so powerfully at the fo- 

 cus, when held several feet from the fire, as to set fire 

 to a pine stick or to flash gunpowder {Fig-. 205). 



The reflection of radiated heat may be beautiful- 

 ly exhibited by using two such concave tin mirrors. 

 Place them on a long table several feet apart, and as- 

 certain the focus of each by means of the light of a 

 candle. Then place in the focus of one a red-hot iron 

 ball, or a small chafing-dish of burning charcoal. In 

 the focus of the other place the wick of a candle with 

 a small shaving of phosphorus in it. The heat will 

 be reflected, as shown by the dotted lines {Fig. 206), 



Fig. 206. 



