332 Rural Architecture. 



somewhere near 5-6. Now in order thai this may take 

 place there must be, with white pine, according to (402) a 

 tensile stress at the upper edge of ten thousand pounds to 

 the square inch, and if the board is one inch thick the upper 

 inch should resist a stress of 10,000 pounds at any point 

 from 5 to 1 ; but we know that no such load will be carried 

 at W. The reason for this, and also for its breaking at 5 

 rather than at any other point, is found in the fact that the 

 load acts upon a lever arm whose length is the distance 

 from the point of attachment of the load to the breaking 

 point, wherever that may be, and this being true the great- 

 est stress comes necessarily at 5. 



If the board in question is 48 inches long and 6 inches 

 wide, it will, in breaking, tend to revolve about the center 

 of the line, 5-6, and the upper three inches will be put 

 under the longitudinal strain but, according to (402), is 

 capable of withstanding 



3 X 10, 000 Ibs. = 30, 000 Ibs. 



without breaking; but in carrying the load at the end as 

 shown, this cohesive power is acting at the short end of a 

 bent lever whose mean length of power arm is one-half of 

 45 or 1.5 inches, while the weight arm is forty-eight 

 inches in length. It should therefore only be able to hold 

 at W 937.5 pounds, for 



asPXPA=WX W A, 

 we have 30, 000 X 1.5 = W X 48. 



whence W = ^[^ = 937.5 ib s . 



4o 



When a board, in every respect like the one in A, Fig. 

 144, is placed under the conditions represented in either B 

 or C, Fig. 144, it should require just four times the load to 

 break it, because the board is practically converted into two 

 levers whose power-arms remain the same, but whose 

 weight-arms are only one-half as long each. 



405. The Transverse Strength of Timhers Proportional to 

 the Squares of their Vertical Thicknesses. Common experi- 

 ence demonstrates that a joist resting on edge is able to 



