268 



Mr. Harvey on the 



now to be added will enable the young shipwright to distinguish 

 the parts of the diagonal framing subject to extension, and also 

 those subject to compression; and moreover how, by the opera- 

 tion of the extending and compressing forces, the form originally 

 communicated to the framing may be best preserved, and thus 

 prevent, in the greatest possible degree, the arching of the ship. 



' 

 MECHANICAL LEMMA*. 



Through the point in which the sustaining forces meet, let a 

 line be drawn to represent the measure and direction of the 

 straining force ; and on it let a parallelogram be constructed, as 

 a diagonal, having its sides parallel to the sustaining forces. 

 Draw the remaining diagonal of the parallelogram, and, parallel 

 to it, another line through the point where the sustaining forces 

 meet. Then all the parts of the framing on the Sume side of this 

 line, as the straining force, will be in a state of compression, and 

 all those on the other side of the same line, in a state of 'extension. 

 





H .-'' 



* For some interesting applications of this well-known mechanical prin- 

 ciple, see Mr. Tredgold's chapter on the Equilibrium and Pressure of Beams, 

 in hie excellent work on the Principles of Carpentry. 



