ARCHITECTURE OF THE SKULL 15 



The wall plate is a portion of timber built into 

 the wall, to which a transverse or tie-beam is 

 attached by carpentry. This cogging, as it is 

 termed, keeps the wall in the perpendicular, and 

 prevents any lateral pressure of the roof. 1 We 

 sometimes see a more clumsy contrivance, a clasp, 

 or a round plate of iron, upon the side of a wall ; 

 this has a screw going into the ends of a cross- 

 beam, and by embracing a large portion of the 

 brick-work, it holds the wall from shifting at this 

 point. Or take the instance of a roof supported 

 on inclined rafters, A B : 



FIG. 2. 



Were they thus, without further security, placed 

 upon the walls, the weight would tend to spur or 

 press out the walls, which must be strong and 

 heavy to support the roof ; therefore, the skeleton 

 of the roof is made into a truss (for so the whole 

 joined carpentry is called). The upper cross-beam, 

 marked by the dotted lines C, is a collar-beam, 

 connecting the rafters of the roof, and stiffening 



1 In the second Treatise on Heat, the reader will find an ac- 

 count of the manner in which the expansion of iron by heat, and 

 its subsequent contraction on cooling, is used in order to cog great 

 buildings. 



