Sentence:  But as there is a double motion of the stars, that is, under the <WSD>"primum mobile</WSD>", and round the world, by both which, as we have said, they influence, we must consequently suppose, that the significators rule over things subjected to them by this twofold (or double) motion, to wit, under the <WSD>"primum mobile</WSD>", and round the world. 
Meaning of the word marked with <WSD> tags:  The outermost celestial sphere of the heavens in Ptolemaic astronomy, which was believed to cause all the inner spheres to rotate.