Sentence:  An object appears to present a difficulty to a flame that cannot surround it, or approach the so doing. This a mere upper-surface ignited affords little prospect of doing, an undersurface much more, an under edge still more, and an <WSD>underpoint</WSD> (never mind Euclid's definitions) again still more, being capable of being nearly surrounded by the smallest flame. 
Meaning of the word marked with <WSD> tags:  A downward facing point on the underside.