66 



MR RUSSELL'S RESEARCHES IN HYDRODYNAMICS. 



nel, a vacancy and corresponding depression of the surface of the fluid, — into this 

 vacancy two currents are determined in opposite directions, the lateral current 

 from the stem towards the stern sent backwards by the pressure of the anterior 

 fluid, encounters near the stern a current in the opposite direction, sent foi-ward 

 by the pressure of that portion of the fluid behind the vessel which has regained 

 its original altitude. The collision of these opposite currents generates around 

 the point where they unite an accumulation of fluid, which performs a series of 

 successive oscillations, until the equilibrium of the fluid under a horizontal plane 

 is at last restored. At each velocity this posterior wave maintains a constant 

 position in regard to the stern of the vessel. The velocity of this wave is equal 

 to that of the vessel, but its position varies with the velocity, approaching nearer 

 to the middle of the vessel at the slower velocities, and falling further behind as 

 the velocity increases, so as to be frequently at a considerable distance behind the 

 stern of the vessel. 



While the velocity of the floating body continues to be small, the stem surge 

 may be recognised in a gentle short undulation following in the wake of the vessel 

 at the stern or near it, and followed at short intervals by a series of smaller waves 

 of the same species. With an increase of velocity the crest of the surge rises in 

 a sharper line, elevated to a greater height above the surrounding fluid, until it 

 forms, at an increased distance, behind the stern, a high crested breaker, which 

 foams and dashes along after the vessel with a loud roaring noise, tearing up the 

 sides of the channel. 



The form given to a fluid in which the velocity of the wave was foimd to be 

 85 miles nearly, is represented in the sections below, which represent the pheno- 

 mena as observed at velocities of 4, 6, and 7f miles an hour, and compared with 

 the fluid in a state of rest. 



Fig. 3. 

 (ut rest.) 



C:^ 



Fig. 4. 

 (at 4 miles an hour.) 



Fig. 5. 

 (at 6 miles an hour.) 



