AIR AND WATER AS SERVANTS OF MAN 117 



When the hot-air balloon is held over a fire the heat 

 expands the air within the balloon and some of this air must 

 therefore escape out of the bottom of the balloon. Since there 

 is less air now in the balloon than there was to start with, the 

 volume of air in the balloon weighs less than the same volume 

 of air outside of the balloon, and therefore the balloon will rise if 

 this difference is greater than the weight of the balloon. 



The principles which have just been explained and illustrated 

 are the ones on which depend the floating of a boat. It seems 

 strange at first thought that a boat may be constructed entirely 

 out of iron and steel, substances which will themselves sink 

 promptly in water, and yet the boat built of them will not only 

 float but will carry a great load of freight. The explanation is, 

 of course, perfectly simple. The boat is not solid steel but is a 

 hollow affair. When it is put into the water it settles down until 

 the weight of the water which it displaces is equal to its own 

 weight. As you load the boat it settles deeper and deeper, 

 displacing an amount of water equal in weight to the weight of 

 the load added. If you continue to load it, it settles until finally 

 the edge of the boat is flush with the water. Then, added load 

 will sink it. All of this may be experimentally verified with a 

 thin glass vessel or a tin pan floated on water and loaded with 

 weights. You may mark the level of the water on the vessel 

 and get the weight of the water it would contain up to that 

 mark. There will be a slight discrepancy between the weight 

 of the contained water and the weight of the vessel and its load, 

 for the contained water measures the volume of the inside of the 

 vessel while the water is displaced by the outside of it. If the 

 glass vessel is very thin this discrepancy will be very slight. 



When the wind hits the sail of a vessel the force with which 

 it strikes is resolved into two factors and one of these serves to 

 drive the boat forward. In a similar way it will be recalled 

 (p. 80) that the force of the wind is broken into two factors as it 

 strikes the kite and one element lifts the kite into the air. If 

 the boat is running before the wind then its sails are set at right 



