84 ALL AFLOAT 



to stern without a break. In this case the 

 unity of each piece supplies enough longitudinal 

 resistance to strains. But when a vessel is 

 large, and more especially when she is long, 

 the strains known as hogging and sagging are 

 apt to rack her timbers apart. 



A ship is not built for mere passive re- 

 sistance, like a house, or even for resistance 

 only to pressures and vibrations, like a bridge. 

 She is built to resist every imaginable strain of 

 pitching and rolling, and so requires archi- 

 tectural skill of a far higher kind than is re- 

 quired (in the constructional, not the aesthetic, 

 sense) for any structure on the land. When a 

 ship is on the top of a single wave she tends to 

 hog, because there is much less support for 

 her ends than for her centre, and so her ends 

 dip down, racking her upper and compressing 

 her lower parts amidships. When the seas 

 are shorter she often has her ends much 

 more waterborne than her centre, and this 

 in spite of the fact that the extreme ends are 

 not naturally waterborne themselves. Then 

 she sags, and the strains of racking and com- 

 pressing are reversed, because her centre tends 

 to sink and her ends to rise. Now, a series of 

 hogging and sagging strains alternately com- 

 presses and opens every resisting join in every 



