Sentence:  This [earthquake] was felt at Mexico at precisely the same hour, lasting there about one minute and a half, the motion there being "undulatory", but at Acapulco <WSD>"trepidatory</WSD>". 
Meaning of the word marked with <WSD> tags:  Of an earthquake, having a vertical, up-and-down motion, as opposed to a horizontal, side-to-side motion.