18 ANIMAL MECHANICS 



consequently where the space must be free in the 

 middle, we find that the scientific workmen are 

 careful, by a transverse beam, to protect the points 

 where the principal thrust will be made in carry- 

 ing up the masonry : this beam does not act as a 

 tie-beam, but as a straining-piece, preventing the 

 arch from being crushed in at this point. 



The necessity of strengthening certain points 

 is well exhibited in the carpentry of roofs. In 



this figure it is clear, 

 that the points A A 

 will receive the pres- 

 sure of the roof, and 



if the loining of the 

 FIG. 4. 



puncheons * and raf- 

 ters be not secure, it will sink down in the form 

 of the dotted line. The workmen would apply 

 braces at these angles to strengthen them. 



In the arch, and at the corresponding points 

 of the parietal bones, the object is attained by 

 strengthening these points by increase of their 

 convexity and thickness ; and where the work- 

 man would support the angles by braces, there are 

 ridges of bone in the calvaria 2 or roof of the skull. 

 If a stone arch fall, it must give way in two 

 places at the same time ; the centre cannot sink 



1 The puncheons are the upright lateral pieces, the rafters are 

 the timbers which lie oblique, and join the puncheons at A A. 



2 From the Latin calva, or calvaria, a helmet. 



