n6 OUR PHYSICAL WORLD 



top of the block as far as the water is concerned is evidently 

 equal to the weight of a column of water i centimeter square and 

 as high as the distance from the top of the block to the surface 

 of the water. The upward pressure of the water on the underside 

 of the block is equal to the weight of a column of water i centi- 

 meter square and as tall as the distance from the underside of 

 the block to the surface of the water. The upward pressure on 

 the underside of the block, therefore, exceeds the downward 

 pressure on the top of the block by the weight of 5 cubic centi- 

 meters of water, which, as we have seen, is 5 grams. 



If now the immersed object weighed less than 5 grams, the 

 pressure on the underside of the block would evidently force it up 

 to the surface, and it would rise out of the water until the portion 

 of it in the water displaced a volume of water equal in weight 

 to the weight of the object. If you float a cube of cork on water, 

 and mark the line at which the surface of the water stands on 

 the cork, then cut the cork in two along this line, you will find 

 that the weight of the cork is the same as the weight of a volume of 

 water equal to the part of the cork that was below the water-level. 



It is now easy to see why a soap bubble filled with hydrogen 

 gas, which is lighter than air, rises, but one more thing must be 

 explained before the rise of the hot-air balloon is clear, that is, 

 that heat expands things. Unscrew a nut from a bolt and heat 

 the screw end of the bolt until it is real hot. Now try to put 

 the nut on. It is evident that the end of the bolt has enlarged, 

 expanding in the process of heating. Fill a small flask one-third 

 full of water to which a little red ink has been added to color it. 

 Bore a hole in a cork that will fit the flask and insert in this hole 

 tightly a small glass tube, so that when the cork is in the flask 

 the end of the tube will dip into the water. Insert the cork in 

 the flask tightly. The colored fluid will rise part way up the 

 tube. Now hold the flask in your warm hand and watch the 

 level of the water in the tube. It is very evident from this 

 experiment that the air in the flask as it warms expands and 

 forces the water higher up the tube. 



