114 LECTURES TO SCIENCE TEACHERS. 



the excess of pressure at the tail end will be converted by 

 the friction of the particles of fluid into defect of pressure, 

 and so will destroy the balance between the forward and 

 backward pressures, thus causing eddy-making resistance. 



Case V. A body moving through frictionless fluid, but 

 at or near the surface. The direct pressures on the surface 

 of the body are altered .by the operation of the wave 

 system which has been created, thus destroying the balance 

 of forward and backward forces, and introducing wave- 

 making resistance. 



Case VI. A body moving through frictional fluid at or 

 near the surface. Here, surface-friction, eddy-making resist- 

 ance, and wave-making resistance will act in combination, 

 and will together make up the total resistance. 



Having this reviewed the several operations which will 

 combine to cause resistance to a ship moving at the surface 

 of the water, it will be interesting to see in what propor- 

 tion they are combined in an actual ship of ordinary form ; 

 and to take a single instance I show the " curves of resist- 

 ance," as they are called, of the SS. Merkara, a mercantile 

 ocean steamship of 3,980 tons. It is perhaps necessary to 

 explain that a curve of resistance is a diagram constructed 

 to show at a glance the resistance at any speed, so that if 

 any point be taken on the scale of speed forming the base- 

 line, the ordinate or vertical height from the point to the 

 curve above, measured by the scale of force, will show the 

 amount of resistance at that speed. Thus, in Fig. 18, where 

 the uppermost line represents the total resistance of the 

 ship, we see that at a speed of twelve knots the resistance 

 as indicated by the height up to the line is 9 '3 tons. 



The plain line on Fig. 18 is the curve of total resistance 

 of the Merkara, deduced from experiments made with a 

 model of that ship. The lowest of the two dotted lines is 

 the curve of surface-friction resistance of the ship, calcu- 

 lated from experiments made upon the resistance of thin 

 planes moving edgeways through water. The space between 

 the foregoing line and the dotted line immediately above 

 it, represents the amount of resistance due to eddy-making, 

 deduced from data which it would take too long to describe 

 here. The space between this upper dotted line and the 

 plain line above it is the wave-making resistance. 



