Transportation on Water 



339 



floats as well as, but no better than, a man weighing one hundred 

 seventy-five pounds. If the boy carried a lump of lead weigh- 

 ing twelve and one half pounds, he would weigh one half as 

 much as the man. But would he float? What more than 

 weight must be considered in finding the answer to the ques- 

 tion " Why does a boat float? " 



It is a common saying that some materials float and others 

 sink. Certain kinds of wood sink, as do lead, iron, and steel. 

 But steel and lead are utilized in building ships, while modern 

 dreadnaughts are armored with steel plates that are many 

 times* heavier than the same bulk of wood. What are the con- 

 ditions, then, which determine whether or not a body will float 

 in water? We shall find these conditions readily by the aid of 

 experiment. 



Exercise: Buoyancy. Notch a can, as shown in Fig. 115, to 

 allow water to overflow. Fill the can with water until it overflows. 

 Weigh a suitable block of wood or any kind of 

 material small enough to float in the can. Care- 

 fully putting it into the can, let it float naturally, 

 catching in a large pan the water that overflows. 

 Weigh this water and then calculate the relation of 

 the weight of the water displaced by the floating 

 body to the weight of the body. Does the buoy- 

 ancy depend upon the whole body or only on that 

 part submerged ? When trying to float, does a man 

 keep head and shoulders out of water, or lie so as 

 to keep the back of his head under water? Have 

 you tried to lift a large stone when it is in water 

 and again when it is in the air ? What is the differ- 

 ence in the effort required ? Why ? The important 

 facts will be illustrated in the following experiment. 



Weigh a small stone or other heavy object on a 

 spring balance. The object should weigh 250 grams 

 at least. Lower the stone, while supported by the spring balance, 

 into the water. Take the reading of the balance. Has the actual 

 weight of the object, or, strictly speaking, the pull of the earth upon it, 

 changed? Apparently this is not the case. Another force is now 

 supporting part of the weight which, when in the air, was supported 



FIG. 115. Ar- 

 rangement of alppa- 

 ratus to show buoy- 

 ant force of water. 



