Sentence:  This muſt be performed by help of a "Spherick <WSD>Gnomon</WSD>", (as "Blaew" &#91;w:Willem Blaeu 
Meaning of the word marked with <WSD> tags:  An object such as a pillar used by an observer to calculate the meridian altitude of the sun (that is, the altitude of the sun when it reaches the observer's meridian), for the purpose of determining the observer's latitude.