The coefficient of linear expansion of oak has been found to be .00000492; radial expansion, .0000544, or about eleven times the longitudinal.

Spruce expands less than oak, the ratio of radial to longitudinal expansion being about six to one.

Metals and glass expand equally in all directions, since they are homogeneous substances, while wood is a complicated structure.
The coefficient of expansion of iron is .0000285, or nearly six times the coefficient of linear expansion of oak and seven times that of spruce[50].
[Footnote 50: See Schlich's Manual of Forestry, Vol.

V.( rev.
ed.), p.

75.] Under ordinary conditions wood contains more or less moisture, so that the application of heat has a drying effect which is accompanied by shrinkage.