Part 7 (1/2)

[Ill.u.s.tration: 159]

[Ill.u.s.tration: 160]

An upthrust, then, must not only be an ending ridge rising at a sufficient degree from the horizontal plane, but there must also be a s.p.a.ce between the ending ridge and the ridge immediately beneath it.

_This, however, is not necessary for a short upthrust or spike, or any upthrust which rises perpendicularly._

In connection with the proper cla.s.sification to be a.s.signed to those borderline loop-tented arch cases where an appendage or spike is thrusting out from the recurve, it is necessary to remember that _an appendage or a spike ab.u.t.ting upon a recurve at right angles in the s.p.a.ce between the shoulders of a loop on the outside is considered to spoil the recurve_.

If the appending ridge flows off the looping ridge smoothly in such a way that it forms a bifurcation and not an abutment of two ridges at a right angle, the recurve is considered as remaining intact. The test is to trace the looping ridge toward the appendage, and if, when it is reached, the tracing may be continued as readily upon the appendage as upon the looping ridge, with no sudden, sharp change of direction, the recurve is sufficient. Figures 161 to 184 should be studied with this in mind.

[Ill.u.s.tration: 161. Tented arch.]

[Ill.u.s.tration: 162. Tented arch.]

[Ill.u.s.tration: 163. Tented arch.]

[Ill.u.s.tration: 164. Tented arch.]

[Ill.u.s.tration: 165. Tented arch.]

[Ill.u.s.tration: 166. Tented arch.]

[Ill.u.s.tration: 167. Tented arch.]

[Ill.u.s.tration: 168. Tented arch.]

[Ill.u.s.tration: 169. Loop.]

[Ill.u.s.tration: 170. Loop.]

[Ill.u.s.tration: 171. Loop.]

[Ill.u.s.tration: 172. Loop.]

[Ill.u.s.tration: 173. Loop.]

[Ill.u.s.tration: 174. Loop.]

[Ill.u.s.tration: 175. Loop.]

[Ill.u.s.tration: 176. Tented arch.]

[Ill.u.s.tration: 177. Tented arch.]

[Ill.u.s.tration: 178. Tented arch.]

[Ill.u.s.tration: 179. Loop.]

[Ill.u.s.tration: 180. Loop.]