Transportation on Water 45 



Ctclually centered at any point, but is represented as the resultant of 

 the many pulls or the weight of the object acting at a point called the 

 center of gravity. Draw or imagine a water line crossing the board 

 which is taken as the shell or hollow hull of a vessel ; you will find 

 that the center of weight is not in the solid hull, but in the inclosed 

 space. This is not puzzling, if you consider the boat with its air space 

 as a solid substance. Find the center of weight or gravity of a wooden 

 or iron ring, as a barrel hoop. Where does the center lie, in the solid 

 material or not? 



Exercise : To find center of buoyancy. Cut out of heavy card- 

 board a form to represent a cross section of a yacht with a deep keel. 

 Determine its center of weight. Tip the form to an angle which 

 would be dangerous for a boat, and draw a line to indicate where the 

 water line is supposed to come. See diagram II, Fig. 117. Cut off 

 the part above this line. Now find the center of buoyancy of the lower 

 part, just as you found the center of weight. When you have done 

 this, reattach the cut-off part by means of gummed labels ; and find 

 out whether or not the boat would tip over at the position which you 

 find it has when at the center of buoyancy. What is the relative 

 position of the center .of weight to the center of buoyancy? Is it 

 above, below, or by the side of, the latter ? Suppose the keel weighted 

 with lead as a ship is with ballast. Where now is the center of gravity? 

 What is its position relative to the center of buoyancy? 



152. Relation of centers of weight and buoyancy. The ex- 

 periments have brought to your attention the important facts 

 of weight and buoyancy. It is necessary now to inquire what 

 must be the relative positions of the two centers of weight and 

 buoyancy to give the boat the greatest degree of stability. 

 Study of the diagrams of Fig. 118 will help you to answer the 

 question. 



Exercise : Stability of boats. Diagram I represents the center of 

 gravity or weight as vertically above the center of buoyancy. Will 

 the boat tip readily to either side? Why? In diagram II, the center 

 of buoyancy is located to the right of that of the weight, and it is also 

 represented as at the center of the submerged part of the boat. What 

 will be the tendency of the boat as determined by the two forces acting 

 thus upon it? Remember that weight is the downward pull of the 

 earth upon a body ; that buoyancy is the upward, vertical push of the 

 water ; and that both of these forces, the pull and the push, may be 



