Sentence:  ... But when we find that they [volcanoes] are but few in Number, and the <WSD>chiefeſt</WSD> of thoſe too near the torrid Zone, and from their Tops to iſſue forth, now clear Fire, then thick, black Smoke, and ſometimes little or nothing at all; we muſt conclude, that they are only particular Fires, probably of the "Sun'''s kindling at firſt, and ſince continued by the caſual and incidental Applications of that "Pabulum", which thoſe Part of the Earth adminiſter to them. 
Meaning of the word marked with <WSD> tags:  Primary; principal.