190 FERGUSON'S LECTURES. 



tiply the number of cubic feet which are under the 

 surface, by 62.5, the number of pounds in one foot of 

 fresh water ; or by 64.4, the number of pounds in a foot 

 of salt water ; the product will be the weight of the 

 ship, and all that is in it. For, since it is the weight of 

 the ship that displaces the water, it must continue to 

 sink until it has removed as much water as is equal to 

 it in weight ; and therefore the part immersed must be 

 equal in bulk to such a portion of the water as is equal 

 to the weight of the whole ship." 



To prove this by experiment, let a ball of some light 

 wood, such as fir or pear-tree, be put into water con- 

 tained in a glass vessel ; and let the vessel be put into 

 a scale at one end of a balance, and counterpoised by 

 weights in the opposite scale : then, marking the height 

 of the water in the vessel, take out the ball ; and fill 

 up the vessel with water to the same height that it stood 

 at when the ball was in it ; and the same weight will 

 counterpoise it as before. 



From the vessel's being filled up to the same height 

 at which the water stood when the ball was in it, it is 

 evident that the quantity poured in is equal in magnitude 

 to the immersed part of the ball ; and from the same 

 weight counterpoising, it is plain that the water poured 

 in, is equal in weight to the whole ball. 



In troy weight, 24 grains make a pennyweight, 20 

 pennyweights make an ounce, and 12 ounces a pound. 

 In avoirdupoise weight, 16 drams make an ounce, and 

 16 ounces a pound. The troy pound contains 5760 



Note 53. A reference to the difference between the specific gravity 

 of salt and fresh water will account for the curious fact of a vessel 

 floating higher in salt water than in fresh. The greater buoyancy of 

 the former enabling it to support a body higher in the water : so that 

 if a vessel receives its cargo at a salt-water port, it will be necessary 

 to make a certain allowance in the freight, should it afterwards be 

 removed to river water, the specific gravity [of which is, as our Author 

 has shewn, less. 



