TJDES AND OCEANIC HIGHWAYS. 349 



CHAPTER IX. 



TIDES AND OCEANIC HIGHWAYS. 



HERE are various movements to which the 

 waters- of the ocean are subject, which are 

 of great importance to navigation, and of 

 high interest to the physical enquirer. They 

 are chiefly the effect of external causes of 

 disturbance, either atmospheric or astro- 

 nomical, which operate with mighty though 

 changeful energy upon the yielding fluid. 

 Waves, tides, and currents are three dis- 

 tinct forms under which its principal agi- 

 tations appear. As water is susceptible of 

 impression from the slightest force, the 

 equilibrium at the surface is disturbed by 

 the aerial currents in contact with it, and 

 upon the particles of the fluid being dis- 

 placed, the adjoining particles immediately 

 rush in to restore the balance. The cause continuing to act, the effect follows, in accord- 

 ance with its duration and potency. In this manner waves are formed, from the power 

 of the atmosphere in a state of motion, displacing the surface waters, and their own 

 tendency to preserve an equilibrium. As the force of the wind varies, its impression 

 upon the ocean varies proportionately. A gentle breeze wrinkles the surface ; a brisk 

 gale produces undulations which rock to and fro the largest vessels upon their bosom ; a 

 storm creates waves of enormous volume and appalling violence. These agitations of the 

 sea may be compared to the waving of a forest, or of a field of grain, where commotion is 

 displayed, apart from any continuous onward movement. They have far less resemblance 

 to the motion of a river current. In a field of corn, under the action of the zephyr or a 

 high wind, we see waves formed by the bending tops of the corn, apparently chasing one 

 another across the field, yet without any advancing motion of the parts that form them. 

 In like manner, the motion of waves of water is not necessarily accompanied by a cur- 

 rent in the same direction, and though a continuous high wind produces this effect, the 

 progress of the water is at a very slow rate, and has no correspondence with the impres- 

 sion which its outward aspect makes upon the mind of the spectator. The proof of this 

 may be easily obtained, by throwing any light substance into the sea, a little beyond the 

 breakers, or into a piece of standing water, the surface of which is ruffled. Such a float- 

 ing body will rise and fall with the motion of the waves, but make little perceptible 

 advance towards the shore. 



Waves vary in their height, form, velocity, and extent. These diversities depend 

 upon the depth of the sea, the size of its basin, and the force of the wind. A wave-sum- 

 mit produced by a breeze from the land, maintains constantly the same height while the 

 impulse is the same ; but the heights increase according to the distance from the shore. In 

 open seas, where the wind blows upon the water in a parallel manner, through a consider- 

 able tract, the waves are generally in the shape of straight and long furrows ; but in more 

 confined situations, they appear in short straight lines, or in arcs of circles, according to 



