142 THE PHYSICAL PROPERTIES OF THE WOODS 



the relative fuel value will be obtained. A wood free from ash, there- 

 fore, having the specific gravity of 1.000, would represent the unit of fuel 

 value, the specimens being free from hygroscopic water. 



If the values thus obtained are multiplied by 4,000, the results will 

 give very nearly, except in the case of some of the resinous woods, the 

 number of units of heat which a cubic decimetre of the wood is capable 

 of yielding, — a unit of heat being the amount required to raise one kilo- 

 gramme of water one degree Centigrade. The fuel value of any wood is 

 often modified by other conditions than its weight and percentage of ash. 

 Perfect combustion is rarely attainable. Resinous woods, especially, are 

 seldom perfectly consumed, much carbon escaping in the form of smoke. 

 The moisture which always occurs in the firewood of commerce must also 

 be considered. Wood when first cut often contains as much as 50 per cent 

 of its weight of water, and air-dried wood may generally be expected to 

 contain at least 20 per cent. The heat necessary to distil this is, of course, 

 lost in combustion. 



The strength and power to resist compression of the principal timbers 

 produced in the forests of the United States were determined by Mr. 

 Sharpies in the course of the Census investigation. 



A stick being supported at each end and weights being applied to it, 

 it is bent or deflected in proportion to each addition of weight within a 

 certain limit, which differs in different species of wood. This limit is 

 called the elastic limit of the wood. When the elastic limit is exceeded, 

 the ratio of deflection is in excess of that previously produced b\ the addi- 

 tion of similar weight. If the elasticity of a given stick under weights 

 which do not strain it beyond this limit is known, the deflection of any 

 other stick, of the same wood, may be calculated by means of the following 

 formula : — 



PI* 



E-. 



4a6(/»" 



E is the coefficient of elasticity ; P, the weight applied in kilogrammes ; 

 I, the length of the stick in centimetres ; b, the width of the stick in cen- 

 timetres; d, the depth of the stick in centimetres; A, its deflection in 

 centimetres. 



Any five of these being known, the value of the sixth may be calcu- 

 lated. E has been determined for many woods, and its value is given 

 in the fourth column of Table I. 



If the deflection of a stick under a given weight is required, it can be 

 obtained by using the formula 



Pl» 



~ iElxl*' 



