344 Introduction to the Study of Science 



II. STABILITY 



151. Stability. Besides floating in water a boat must have 

 stability of a kind to prevent it from tipping dangerously with 

 ordinary shifting of its load. Disaster frequently results if the 

 cargo shifts too much to one side, or if some one " rocks the 

 boat." The problem for the boat builder is to design and con- 

 struct a boat which will not tip over in usual situations. 



You have seen and probably experimented with tumbler dolls, 

 or possibly the bottle imp. The bottle imp is a round bottom 



bottle weighted with some lead shot held in 



place by paraffin. No matter how the imp 

 is placed in water, it will rise of its own 

 accord to a vertical position. So it is with 

 the tumbler doll and other similar devices, 

 such as oil cans weighted to prevent tipping 

 and consequent waste of oil. It is exactly 

 the same principle which is followed in secur- 

 ing stability or equilibrium for boats. 



The following experiments will illustrate 

 \ 17 - L Form the essential facts : 



of board with position 

 of holes for its suspen- 

 sion to find center of Exercise : To find center of weight. Saw out 

 weight. II. Relation a piece of board at least one foot across and ten 

 (C M) andcenter g of ^ nc ^ es deep so as to represent the cross section of 

 buoyancy (C.B.). a boat. Bore three or more holes not far from 



the edge, somewhat as shown in Fig. 117. Hang 



the board on a nail upon which it may swing freely and come to rest 

 without interference. Let a plumb line hang from the same nail. 

 Draw a line upon the board to represent the position of the plumb 

 line. Now hang the board by another hole and designate the plumb 

 line in the same manner. Try then the third hole. Where does the 

 plumb line pass with reference to the other lines? Is the weight of 

 the board apparently centered at the point of intersection of the three 

 lines? How can you prove that it is? 



Attach a weight of several ounces to the part representing the 

 keel of the boat. Proceed as formerly to determine the point at which 

 the weight is centered. The pull of the earth upon an object is not 



