51 



tures in the horizontal plane. The spine, however, 

 retains its helical curvature, and there is real action 

 in the perpendicular plane. 



§ 53. We will then suppose that the ground 

 end of each rib is terminated by hoo heveJs, so cut 

 that on one of them — the outer — the rib shall rest 

 when faced to the front (§ 45) and before receiving 

 any secondary turn ; on the other of them — the inner 

 — ichen faced to the rear and under the same con- 

 dition. Then the ribs which are in appui according 

 to § 48 (a) will, before they have received a second 

 turn, rest as follows : those on the anterior part of 

 a convex (being faced to the rear) on their inner bevels 

 — those on the posterior part of a convex (being faced 

 to the front), on their outer bevels. Should the ribs of 

 concaves be put in appui the order would be 

 reversed. 



§ 54. The forces act, primarily, along the con- 

 vexes, and the appuis are normally on the ribs 

 articulated along the two convexes of each S. 

 Should the ribs of a concave take the place of those 

 of a convex as appuis, the forces would act on them 

 only secondarily, i. e., as a sequence of the action 

 along the convexes. 



Supposing the left anterior point of application 

 in action : As the winding liue is formed and the 



