90 Ocean Waves 



being that of 90*^ it forms with the horizontal line cZ 6 e an 

 angle of 45*^, which is therefore the maximum slope angle 

 at the point e.* 



24. The whole motion of the body is compounded of 

 three oscillations^ viz. : a bodily horizontal oscillation, or 



sway, thus ^^ ^ ; a bodily vertical oscillation, or 



? t 

 heave, thus ; and an angular oscillation, or roll, thus 



I' i: 



^ around the wave axis or centre of gravity. 



^ V 



In the upper semicircle (figure 5, paragraph 20) the bodily 

 sway is from right to left, the angular motion of the mast 

 from left to right, and these are reversed in the lower 

 semicircle. Hence we have for general rules that — 



When a body rises on a wave it is swayed horizontally 

 first against the wave and then with it, but when falling on 

 a wave, first with it and then against it. 



A floating body on the upper part of a wave is swayed 

 with the wave ; on the lower part against it. 



The sway is equal to the heave or vertical rise and fall on 

 approach and departure of the wave. 



During the passage of a wave, there are two oscillations 

 of the plummet equal to twice the wave angle ; or four half 

 oscillations equal to the wave angle. 



These laws are universal, for though they are deduced 

 on the assumption that the floating body is inconsiderable 

 in magnitude with respect to the wave, they hold in a 

 modified degree in the case of large bodies. For instance, 

 if one end of a vessel rises to a wave, it is certain that the 

 part which rises sways first against the advancing wave and 

 then with it, and in falling, first follows the receding wave 

 and then leaves it. 



25. A ship, one diameter of whose plane of flotation is 

 greater than the other, buoyed upon a wave or different parts 

 of a wave, is acted on variously by their forces, which, as the 

 vessel is a rigid body, in a great measure neutralise each 

 other : her actual motion of course is in accordance with the 



* By slope angle in future will he intended the greatest inclination or 

 maximum angle attained by a vessel of anj' magnitude during an oscillation. 

 It is customary for shipwrights to denote this angle by 6 as above. 



