18 Expei-iments on the Utrength and Stiff ness of Acacia, 



E = — jjj = 4609000, still exceeding the oak. If we adopt 



the modulus of elasticity according to Mr. Brvan, viz. by ex- 

 pressing the deflecting weight by the length H of a column 

 of wood of the same section and specific gravity as the spe- 



cmien experimented upon, we have m = ,, ^, = 373842&, 



4 a 6 



■which is less than Mr. Bevan's number deduced from his ex- 

 periments on acacia. This, however, was evidently an inferior 

 specimen compared with those employed in the former expe- 

 riments, being so near the outside as to take in some part of 

 the sap-wood. 



Mr. Bevan expresses a wish that I had given the modulus 

 of elasticity instead of the value of E, and observes, " had 

 this been done it would have appeared that the stiffness of 

 Memel Fir, compared with its weight, is greater than for the 

 other woods." I do not, however, with every respect for the 

 well known talents of Mr. Bevan, see what advantage is gained 

 by considering the weight of the timber, except in the parti- 

 cular case where the question is the deflection of a beam from 

 its own weight; a case which very rarely occurs in the con- 

 struction of buildings. 



To give an example of what is stated above, the value of 

 E, or the elasticity of Tonquin Bean, is one and a half time 

 gx'eater than that of Memel Deal; that is to say, it required 

 one and a half time the weight to produce the same deflec- 

 tion, and is accordingly expressed in my table by a number 

 bearing the same proportion ; but the modulus is less when 

 we use Mr. Bevan's formula. 



It will however be observed, that the two formulae are esta- 

 blished on the same principles; viz. that the deflection varies 

 in the ratio of the cube of the length divided by the breadth 

 into the cube of the depth ; so that in all cases Mr. Bevan's 

 number may be obtained from mine, by multiplying by 576, 

 and dividing by the specific gravity. 



It is necessary here to mention, that an error in the for- 

 mula in my former paper may possibly have misled Mr. 



l^ 11) 

 Bevan. In the head of the sixth column, instead of E= tt-;, 



it ought to have been E = — ,,^: the numbers, however, are 



'^ ad^o 



perfectly correct. 



Some other queries of Mr. Bevan, I am sorry I cannot re- 

 ply to. The price per cubic foot of the different woods I have 



