124 Transactions of the Royal Canadian Institute 



crest, and only a few inches high, — that is, only a few inches between the 

 level of the crest and the level of the hollow. Again, we may have a 

 wave of this length, thirty feet or even forty feet high between the crest 

 and hollow. Even in the stormy North Atlantic, however, such heights 

 are unusual, and forty feet is probably about the limit of regular waves 

 generated in deep water by uTiusually severe and long-continued storms. 

 This exceptional height, however, is liable to be materially surpassed by 

 abnormal waves, the result of superposition of one wave system upop. 

 another. In such case, however, the waves do not manifest themselves 

 with regular crests, but the abnormal heights are more like isolated peaks. 

 The natural tendency when looking at waves is to over-estimate their 

 height, and accurate determinations of wave heights are difficult to 

 make. The excessive heights reported now and then will be found to be 

 estim.ates by eye in every case. The highest wave of which I have been 

 able to discover an accurate record was almost exactly sixty feet high. 



Returning to the question of the strength of ships: If the sea were 

 always at rest, the problem would be very simple. The big strains result 

 from the waves. There is one feat^ire in this connection where a ship 

 differs radically from t^e usual structure . The determination of strength 

 by "rule of thumb" methods and step-by-step development leads in the 

 majority of cases to excessive strength. The natural tendency is to err 

 on the safe side. For fixed structures on shore, and even for many pieces 

 of moving machinery, excessive strength is not necessarily a fault except 

 from the point of view of cost. On a ship, "however, too much material, 

 while not as grave an error as too little, is highly objectionable from 

 other considerations than cost. Every ship is built to perform some 

 distinct service, and, as a floating body, can carry for all purposes only 

 a weight equal to that of the water which it displaces. Every pound 

 which goes unnecessarily into the structure of the ship itself is a dead 

 loss to the disposable weight available for the aim or object for which 

 the vessel is designed. Hence, though the necessity tor adequate 

 strength is paramount, excessive strength or unnecessary weight on a 

 vessel is a grave fault, and the scientific study of this most important 

 question must ever be made with that consideration in mind. 



In spite of the complication of the question, the underlying theories 

 are comparatively simple. Investigation has shown that for a given 

 ship the most severe stresses in waves of a given height are associated 

 with wave lengths from crest to crest the same as the length of the ship. 

 When bow and stern are in the crests of successive waves, and the mid- 

 ship part in the hollow, the vessel is said to "sag," for obviou's reasons. 

 When bow and stern are in hollows and the midship section supported 

 by the crest, the vessel is said to "hog." The strains vary with the height 



