Sentence:  How defective the Art of Navigation was in elder Times, when they Sailed by the observation of the Stars, is easie to be imagin'd: For in dark weather, when their Pleiades, Helice, and Cynosura were hidden from them by the <WSD>intervening</WSD> Clouds, the Mariner was at a loss for his Guide, and exposed to the casual conduct of the Winds and Tides. 
Meaning of the word marked with <WSD> tags:  To come between, or to be between, persons or things.