Sentence:  To find the surface of a spherical <WSD>zone</WSD>. / Rule.—"Multiply the altitude of the <WSD>zone</WSD> by the circumference of a great circle of the sphere, and the product will be the surface" (Book VIII. Prop. X. Sch. 1). 
Meaning of the word marked with <WSD> tags:  The curved surface of a frustum of a sphere, the portion of surface of a sphere delimited by parallel planes.