lEEEaiJLAR IJNDIJLATIONS. 51 



Sucli perfectly regular waves as these can only be formed in seas 

 exposed to tlie influence of equable winds, sucb as the trade- winds. 

 Wherever the winds are uncertain and shifty, blowing in gusts, it is 

 evident that the waves driven by them cannot assume a regular form 

 or follow in a uniform direction. For aerial currents constantly 

 vary in their sj^eed; being composed of strata of unequal force, 

 which moving at a rate difierent from that of the surface of the 

 sea, alternately increase and diminish in force. Under the influ- 

 ence of these variable atmospheric impulses, the waves must ne- 

 cessarily vary in height and speed, and their crests cannot be 

 developed in a uniform line. The wind also frequently changes its 

 direction ; as if urged by some new impulse,* it commences blowing 

 from another point of the compass, and drives the waves in a dif- 

 ferent direction from that which it had itself given them. Never- 

 theless the first movement is continued by the succeeding waves 

 even while the second is still making itself felt, and from this 

 double impulse an intersection of waves, differing from one another 

 in direction, height, and speed, results. Let the wind shift to another 

 point of the compass, and a third undulation crosses the preceding 

 two. Finally, should the aerial current make the complete circuit 

 of the compass, the ripples of the water pursue one another in all 

 directions, urged from all points of the immense circle. Not a 

 breath is lost on the sensitive surface of the sea, and the variety of 

 its undulations testifies to the diversity of the aerial movements which 

 cause them. 



From a lofty headland or from the mast of a ship, whence a vast 

 expanse of water can be viewed, the beautiful sight may be often 

 enjoyed of two or three systems of waves intersecting each other at 

 various angles. Now they double the natural height of the un- 

 dulations, by piling one wave upon another, and then again they 

 equalize the surface of the water by throwing billows into the 

 furrows. Sometimes the sea is so agitated that it is impossible to 

 discern the direction of all the waves which have aided in producing 

 the violent commotion. As to the voyagers, whom the wave-tossed 

 ship incessantly shakes by its rolling and pitching, it is still more 

 difficult for them to recognise in the intersection of the waves the 

 various impulses communicated to the sea by the atmosphere. The 

 accompanying figure is reproduced from that of an English traveller, 

 of the curves drawn during a single minute by a pencil suspended 

 vertically in the cabin of a ship. At the time when the pencil 



* See below, the chapter entitled, The Air and the Wind. 

 E 2 



