24 



Prof. F. Elgar. On the 



[Jan. 14, 



longitudinal strength are based ; and rales have, in consequence, been 

 proposed for regulating the strength of the principal component parts 

 of ships' hulls. It is only necessary here to say, that many of these 

 deductions, like the one already noticed, are unsound, and are not 

 consistent with the effects that may be observed of straining action 

 at sea. 



A considerable experience at sea, where the writer has closely 

 observed the effects of straining action caused by twisting moments, 

 and a further experience in investigating the stresses to which the 

 various portions of ships' hulls are subjected according to the theories 

 referred to, and in comparing the results so obtained with the visible 

 evidences of straining action, have convinced him that the stresses 

 caused by twisting moments are much greater than is generally sup- 

 posed, and that no rules for regulating the strength of ships can be 

 satisfactory if based upon hypotheses that exclude all practical con- 

 sideration of twisting moments. 



The straining action which will be considered in this paper is that 

 caused by the twisting moments which operate when a ship rolls 

 from side to side ; and which are caused by differences in the longi- 

 tudinal distribution of the moments of the forces that cause rotation, 

 and those which resist rotation. 



Let a unit of length included between two transverse vertical 

 sections be taken at any point in a ship's length, and let fig. 1 be the 

 section of the ship at that point. The section may be taken as 

 uniform over this short length. The energy of rotation of this unit 



2 



of length will be — wW 1 ; where w is the angular velocity in the upright 



