QUANTITY AND TRANSMISSION OF HEAT 143 



which is a very poor conductor of heat. The packing 

 prevents convection currents and retards radiation and 



conduction. Food to 

 be cooked is first 

 heated and then 

 placed in the cooker 

 where the cooking 

 continues, since the 

 heat cannot escape. 



It is quite easy for 

 anyone who is at all 

 handy with carpenter 

 tools to make a very 

 good fireless cooker 

 after the plan of the 



Sawdust 



one shown in the cut. 

 The Thermos Bot- 

 tle. The thermos 

 It is made of two 



FIG. 123. A Homemade Tireless Cooker. 



bottle is similar to the fireless cooker, 

 glass bottles which are sealed together at 

 the top after the air in the space between 

 them has been exhausted. This prevents 

 the loss of heat from the inside bottle by 

 convection and conduction. The outside 

 bottle is lined on the inside with an excel- 

 lent reflecting surface which prevents a 

 loss of heat from the inside bottle by 

 radiation (Figure 124). 



The Davy Safety Lamp. If a piece 

 of wire gauze be held above an open gas 

 jet and a match applied below the gauze, 

 the flame will burn below the gauze as FlG - 12 , 4 -~ Cross 



' Section of Thermos 



in ligure 125 (o), but it will not pass Bottle. 



