OF THE UNITED STATES. 143 



It is often desirable to know what is the ultimate strength of a given 

 stick. This is obtained by the following formula : — 



in which P, I, b, and d have the same value as in the preceding formulas. 

 R is given in the fifth column of Table I. as the Modulus of Rupture. 

 In this formula P will most generally be the unknown quantity, and can 

 be obtained by using this formula, — 



p _ 2brl 2 R 

 31 ' 



"Wood may be compressed in a direction either parallel or perpendicu- 

 lar to its fibres. The latter is known as indentation. When a stick is 

 compressed in the direction parallel to its fibres, if its length does not ex- 

 ceed ten or twelve times its diameter, it generally fails by the crushing of 

 the fibres ; and the force necessary to produce such crushing is propor- 

 tionate to the area of the cross-section of the stick. The figures in the 

 sixth column of Table I. give the weight in kilogrammes necessary to 

 produce such crushing in sticks of the different species one centimetre 

 square. In order to find the weight any given stick will support, the 

 number in the column should be multiplied by the number of square cen- 

 timetres in the end of the stick. The force necessary to sink a punch 

 one centimetre square to the depth of 1.27 millimetres perpendicular to 

 the fibre of the wood of the different species, is given in the seventh col- 

 umn of Table I. The force necessary to produce indentation is propor- 

 tionate to the surface of the punch or the surface exposed to its action. 



For further information in regard to the formulas relating to the 

 physical properties of wood, the reader is referred to : — 



The Materials of Engineering. Part I. pp. 37-153. Robert H. 

 Thurston. New York, 1883. 



Treatise on the Resistance of Materials. De Volson Wood. New 

 York, 1871. 



