Part I 

 SURFACE AND LONG WAVES 



Chapter I 



General Remarks on Waves 



1. Introduction 



In this volume we shall concern ourselves chiefly with such phenomena of the 

 oceans which have as their main characteristic a period, in contrast to the 

 ocean currents, which were discussed in Vol. I. This means that the movement 

 repeats itself after a certain time so that observations for this time interval 

 suffice to determine the process. 



The surface of a motionless fluid must satisfy the condition that at every 

 point it is perpendicular to the forces acting on that liquid. When the fluid 

 is at rest, gravity is the only acting force and, therefore, the surface at rest 

 must coincide with a level surface. However, the ocean surface, even when 

 we only consider small parts of it, is never completely at rest. Many factors, 

 especially winds blowing over the ocean, generate waves which are present 

 most of the time; close to the shores we nearly always have a surf. A mirror- 

 like surface is an exception, whereas a sea covered with waves is customary. 

 Besides these short waves, which are easily visible, there are much larger 

 waves which, because of their great dimensions, cannot be detected by visual 

 observation. Over a certain length of time we can observe their effects, and 

 from the combination of such observations we can conclude as to the wave 

 character of these disturbances. These are the waves following earthquakes, 

 volcanic eruptions, etc. (dislocation waves), submarine earth slides and the 

 great phenomena of the tides which affects all the oceans. Each water particle 

 in these wave motions is not subjected to any large local displacement. The 

 water-masses shift relatively little backwards and forwards in a horizontal 

 direction. After a certain time they will generally return to their original 

 position; and in this they differentiate themselves from ocean currents, in 

 which a permanent transport of water takes place in a definite direction. 

 This pendulum movement of the separate water particles and their return 

 to their original position, which applies to nearly all waves, can be con- 

 sidered as oscillations around a steady position. 



Before entering upon the nature of the waves proper, we will consider 

 some of their fundamental physical properties and establish some definitions 

 which we will have to use quite often. 



In general there are two forces acting on a water surface which is in 



