342 Introduction to the Study of Science 



when brought to the surface explode. Experiments have been 

 made in deep places of letting down with a sinker an empty, 

 flat bottle tightly corked. The bottle is crushed by the enor- 

 mous pressure. 



Application to boat building. Builders of ships have a 

 practical knowledge of the facts which the experiments have 

 illustrated. They know the density of materials of which ships 

 are made, and can compute beforehand the weight of a vessel 

 and its equipment ; how much water it will displace ; and to 

 what depth it will sink in displacing its own weight. They also 

 determine what load it will carry and to what depth it will 

 sink when loaded. 



Exercise. Suppose you are making a plan for a boat of the com- 

 mon flat-bottom sort, with the dimensions 14 feet long, 3 feet wide, 

 and 16 inches deep. If the materials of which the boat is made weigh 

 200 pounds, how deep will the boat sink? We know that one cubic 

 foot of water weighing about 62.5 pounds exerts a pressure at a depth 

 of one foot of 62.5 pounds per square foot. The area of the boat 

 bottom is 14 feet X 3 feet or 42 square feet. The total pressure 

 necessary to cause the boat to sink one foot is 62.5 x 42 or 2625 

 pounds. But the weight of the boat is 200 pounds. Therefore it 

 will sink into the water only ^\fo or .076 of a foot. Is this more 

 or less than one inch? How much? What must be the weight 

 of a load to cause the boat to sink to the depth of six inches ? Ten 

 inches ? 



150. Density and relative density. The density of water 

 is approximately 62.5 pounds per cubic foot. The important 

 items in this are the weight and the unit volume. The density 

 of any other substance is determined in the same manner, that 

 is, its weight per unit volume. This may be expressed in so 

 many pounds per cubic foot, or grams per cubic centimeter. 

 The standard of density in all scientific measurements is pure 

 water with a temperature of 4 C., or 39 F. Its density is 1.00. 

 The density of other substances is stated in relation to the density 

 of water. Pine wood, for instance, has a relative density of 0.5, 

 that is, it is just one half as dense as water. 



