WAVES. 11 



use of a regular scale of miles, the degrees and minutes of latitude 

 answering every purpose. 



Waves have been defined as the " alternate rise and fall of suc- 

 cessive ridges of water." They are formed by the action of the 

 wind, or the disturbance of the balance of equilibrium in the sur- 

 face of the water itself. Waves vary in their rapidity of progress, their 

 extent, and their height and breadth, according to the area of water 

 in which they occur, its depth, and the force and direction of the 

 wind ; being smallest in small bodies and largest in large bodies of 

 water. The wave movement primarily is that of simple oscillation, 

 or of repeated risings and fallings without apparent forward or 

 backward motion of any kind ; thus a chip thrown upon a surface 

 of water affected by such motion retains its position while rising 

 and falling as the water of that area rises and falls. Such is the 

 case if the surface of the water is at apparent rest and there be no 

 wind. Let the wind arise, or the sea be affected by currents, or 

 both, and there arises a complexity of wave phenomena at once 

 interesting as it is difficult to study with any degree of satisfaction. 

 The other kind of wave motion is called the wave of translation. 

 It is a long, solitary line of crests, such as would be formed by the 

 pushing forward of a mass of water over and above the level of the 

 surrounding water ; it progresses slow or fast according as the depth 

 about is great or small. The typical wave of translation is the tide 

 wave. The phenomena of the rise and fall of the tide are too well 

 known to have escaped the attention of the ordinary individual. 

 They are produced by the action of the sun and moon, primarily 

 of the moon alone, upon the surface of the earth. They occur 

 with regularity in all gi"eat bodies of water, and consist of the rising 

 and falhng of that body of water effected as a whole, at periods of 

 nearly six hours apart, with a few moments of calm at the end of 

 each period. The period of rising water is called flood tide, and 

 the highest elevation of flood tide high water ; while the period of 

 falling water is called ebb tide, and the lowest ebb tide low water. 

 The period of time between the highest or lowest tide to the next 

 tide of the same kind, or between one high tide and the next high, 



