March 2, 1896.] 



KNOWLEDGE. 



38 



as if it were rushing upon the ship and would presently 

 overwhelm her; but the vessel herself is borne aloft upon 

 the crest. ( )n the other hand, when ocean waves are forced 

 by violent wind, so that the tops receive a forward motion 

 beyond that which is shared by the more protected part of 

 the billow, the water may strike a blow upon the vessel. 



comparison of the waves of the Mediterranean with those 

 of the ocean. He writes : " Ce n'etaient pas les lames de 

 I'oeean qui vont devant elles et qui se deroulent royalement 

 dans I'immensiti' ; c't-taient des houles courtes, brusques, 

 furieuses. L'ocean est a son aise, il tourne autour du 

 monde ; la Mediterranee est dans un vase, et le vent la 



Fig. 1. — Diagrams of. A, Forcerl Waves; B, Tvep Wave-. 



Such is the wave described by Mr. Ruskin, with " over- 

 whelming crest — heavy as iron, fitful as flame, lashing 

 against the sky in long cloven edge — its furrowed flanks, all 

 ghastly clear, deep in transparent death, but all laced across 

 with lurid nets of spume, and tearing open into meshed 

 interstices, their churned veil of silver fury showing still 

 the calm grey abyss below, that has no fury and no voice, 

 but is as a grave always open, which the green sighing 

 mounds do but hide for an instant as they pass." This 

 passage shows a fine discrimination between the fury 

 of the wind-forced crest and the calm of the sheltered 

 trough. Such minute and accurate word-painting, based 

 upon careful observation, is the literary equivalent of a 

 mathematical formula. According to mathematicians, the 

 highest waves driven by the strongest wind lose entirely 

 the rounded crest ; the flanks being slightly concave and 

 meeting at a sharply-defined angle of one hundred and 

 twenty degrees. The height of this highest possible wave 

 is very nearly one-seventh of its length, it would travel 

 one-fifth faster than a free wave, and the velocity of a 

 particle when at the summit is equal to the velocity of the 

 wave. Its form is shown in the upper curve of Fig. 1, the 

 lower curve being a free wave, or ground swell, of the same 



secoue, c'est ce qui lui donne cette vague haletante, breve 

 et trapue. Le flot se ramasse et lutte. II a Tautant de 

 colere que le Act de I'ocean et moins d'espace." 



The free wave, as we have said, does not strike upon a 

 ship, but its heave and swing may become a source of 

 danger by causing the ship to roll excessively. The an^le 

 through which a ship will roll, so much greater than the 

 angle of pitching, is perhaps one cause of the common 

 over-estimate of the steepness of wave slopes, which in the 

 Atlantic are seldom more than seven degrees to the horizon. 

 The momentary effort of a ship is to place her masts at 

 right angles to the surface of the wave, but owing to inertia 

 the vessel swings beyond this point. If the period of swing 

 of the water harmonizes with the ship's natural period of 

 pendulous oscillation, thou at each roll to starboard the ship 

 rolls further to the starboard, and at each roll to larboard 

 she rolls further to the larboard side, the rolling bein" 

 rapidly and dangerously increased, not by any shock of the 

 waves, but by gravity, which the heaving sea causes to act 

 upon the ship in a succession of well-timed pulls. Fortu- 

 nately it seldom happens at sea that any considerable 

 number of succeeding waves have the same period, the 

 variety of independent systems of waves being thus a boon 



/7. W Ordmnrti .Springs 



,1" 



S'txi Face 



Bar hour Face 



'{_ 



JOoflil 



Scciic 



FtG. 2. — Seiliou of Portland Breakwater, from tlie •'Hfinutes of Proeeediii^s of the lustitiite of Civil Engiiuvrs," Vol. WII., 1S623. 



forced waves that Adam 



wave-length. It was of the 

 Lindsay Gordon wrote — 



'■ • . . the stoutest sliip were the frailest shallop 

 In your liolloir backs." 



The difference of character between the waves in the 

 open ocean and in a closed sea is due to the fact that in 

 the former the transmission of energy from the motion of 

 other waves has greater opportunity to take full effect, 

 whilst the movement of storm waves in a closed sea is more 

 largely dominated by the immediate force of the wind. Tlie 

 difference has been well expressed by Victor Hugo in a 



to the sailor, for the regularly increasing roll of the ship 

 is generally interrupted before it has grown to a dangerous 

 extent. The natural period of a ship's pendulous swing 

 may be reckoned at about two and a quarter seconds for a 

 vessel of ten-foot beam, and about five and a half seconds 

 for a ship of sixty foot beam. The time of swing of the 

 Crieat Ka.'<tirii was six seconds. Remembering that the 

 water completes one swing in the same interval of time 

 as that between the passage of successive wave-crests past 

 a stationary observer, and bearing in mind, also, the 

 connection between wave-length and velocity (due to the 



