310 OF STEAM NAVIGATION. [SECT. x. 



OF THE STRENGTH OF VESSELS. 



643. It was not till 1818 that steam vessels were made to perform regular 

 voyages at sea ; and in proportion as they have had experience, the strength of the 

 vessels has been increased. A vessel should be considered as one slightly flexible 

 frame, and the strength determined so that the greatest possible stress, acting with 

 the most disadvantage probable, would not derange the natural elasticity of the 

 parts, nor disturb the connexions. The want of considering the frame as a whole, 

 has often led to weak modes of construction, and improper modes of bracing. 

 A vessel is also to be considered in the condition where hydrostatic pressure 

 contributes least to its support. The strains reduce to those which would take 

 place in a large hollow beam, of which we have to find the neutralaxis, and then 

 the resisting forces are easily measured. 1 When the timbers are filled in between, 

 it must increase the strength, if it be done in a proper manner ; and this increase 

 might perhaps be obtained with less material, and less addition to the weight of 

 the vessel, but the advantage of leaving no hollow cavities is of much importance 

 both to the durability and cleanliness of a vessel. 2 



644. In respect to timber, fir has the advantage of lightness, and for straight 

 timbers it is stronger than a like weight of oak ; but for curved timbers the harder 

 woods which have greater lateral cohesion are better. 



OF THE APPLICATION OF SAILS. 



645. It is found that sails may be effectively combined with steam power, 

 whenever the direction is not within four points of that of the wind. 



But when the force of the wind becomes considerable, and the sea rough, the 

 wheels often revolve without touching the water in the hollows of the waves, and 

 acquire a great increase of velocity, to be reduced, as soon as they meet the wave 

 again, to less than the ordinary speed. To lessen the abruptness of these changes, 

 it is necessary to diminish the supply of steam, and consequently the power of the 

 engine. 



646. It appears to be impossible to apply so much sail as to give a steam 

 vessel the advantage of being used as an effective sailing vessel, in the event of the 

 engines or coals failing. The proper object of sails in a steam vessel is to save fuel 



1 See my ' Elementary Principles of Carpentry,' Sect. 1 and 2 ; and ' Treatise on the Strength 

 of Iron,' art. 85 a. 



2 See Philosophical Transactions for 1820. 



