Sentence:  Let a <WSD>Gnomon</WSD>, being firſt ſharpened into a Point, be ſhaped, and faſtned in ſuch wiſe, that it no way hinder either the draught of the Horizontal Line, or the Point of the ſhadow from having free access to the Dial at all times of the Year. ... Upon the Superfices of the Dial, obſerve the Point of the Shadow of the <WSD>Gnomon</WSD> (making a mark at it) and the Sun's Altitude, both of them at the ſame inſtant of time. 
Meaning of the word marked with <WSD> tags:  An object such as a pillar or a rod that is used to tell time by the shadow it casts when the sun shines on it, especially the pointer on a sundial. (from mid 16th c.)