DEVELOPMENT OF HOUSE HEATING 115 



place in the bonfire. As soon as the fresh air reaches the 

 heated portion of the fire it is greatly expanded. The coal 

 in the stove or furnace produces a still higher temperature. 

 The air passing up through the bed of glowing coke is so 

 heated at that moment that it is increased to three or possibly 

 five times its volume as it enters the damper. It is this heating 

 of the air and the consequent expansion, or increase in volume, 

 which produces CONVECTION CURRENTS. Just why and how 

 this is so we must learn. 



133. Floating Bodies and Buoyancy. We all know that a 

 cork or a piece of wood weighs less than a piece of iron of the 

 same size. We also know, that if we place the cork or piece 

 of wood in water, it will float, while if we place the iron in 

 water, it sinks. What makes the cork float? Just why 

 does the iron sink? Does the iron have any tendency to float? 

 Answers to these questions will help us to understand CON- 

 VECTION CURRENTS. 



Exercise 36. Floating Bodies and Buoyancy 



(a) Place a cork in a basin of water. Does it float entirely upon the 

 surface? If any portion of the cork is below the surface of the water, 

 about how much of it is so? Does "floating upon the water" 

 mean that all of the body is above the level of 

 the water? Does the cork have any tendency to 

 sink? 



(6) Set a small pail in an empty basin. See 

 that the pail is exactly level. Fill it exactly full 

 of water. Take care that no water runs over 

 into the basin. Now tie a cord securely around 

 a stone. Weigh the stone by means of a spring 

 balance. While the stone is still suspended FIG. 88. Buoyancy, 

 from the balance, lower it into the pail of water 

 till it is entirely covered by water. At the same time the stone 

 must not touch the bottom of the pail. See how much the stone 

 now seems to weigh (Fig. 88)? How much has it lost in weight? 



(c) Carefully remove the stone from the water. Remove the pail 

 from the basin. You now have in the basin the water which ran over 

 when the stone was immersed in the pail. Empty the water out of the 

 pail and pour the water from the basin into it. Weigh the pail and 



