SECT, x.] OF STEAM NAVIGATION. 289 



has of resisting any change of position when in the water; Secondly, The forms 

 having stability which have the least resistance, and are therefore best adapted for 

 speed ; Thirdly, The different methods of propelling vessels ; and Fourthly, The 

 construction for strength. 



OF THE STABILITY OF VESSELS. 



595. A perfectly spherical ball floating in a fluid has no stability whatever, 

 except that which arises from the friction of the fluid against its sides. On the 

 addition of a small weight to any point of its surface, that point would immediately 

 descend, and become the lowest : such a form would be useless as a vessel. 

 It is obvious, however, that when a weight has been added, and become the lowest 

 point, the sphere possesses a degree of stability depending on the quantity of 

 weight, compared with the weight of the sphere itself. Hence, stability may be 

 given by disposing the weight of a floating body. 



Stability may also be given by the form of the floating body : a spheroid, for 

 example, remains in stable equilibrium when its longer axis is horizontal, and a 

 triangular prism resists change of position with considerable energy from its 

 peculiar form ; so does a thin rectangular prism. 



596. Stability is distinguished by its being longitudinal or lateral; these should 

 be separately considered, and when each is the greatest possible, their joint effect 

 will be a maximum. 



597. For river navigation, the mode of obtaining stability does not appear 

 to be of much importance, but for a sea vessel it must be obtained, so that the 

 vessel may have the least motion possible in consequence of the action of the 

 disturbing forces ; hence, it is necessary to consider that the sea is not a level 

 surface at rest, and that at the time when stability is most important to a vessel, 

 the greatest degree of unevenness occurs. 



598. LONGITUDINAL STABILITY. A vessel at rest would be least disturbed 

 by the motion of the sea, if its surfaces at the water line were vertical ones, and 

 the fore and aft parts of the same figure ; but in motion it is an advantage that 

 the parts should spread above water, both fore and aft, to prevent the vessel 

 burying its head in the wave, or dropping behind as the wave leaves it. The 

 quantity of motion is not increased by this construction, provided the parts produce 

 similar effects ; and the degree of inclination should be proportioned to the velocity 

 the vessel is expected to make. It is also obvious, that the vessel will be more 

 easy in its longitudinal motions, the more gradually it terminates at its extremities. 

 If the vessel be inclined by the action of a lateral force, the longitudinal motions 



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