Sentence:  It is contended by some that their [cyclones'] general movement is at right angles to the magnetic meridian. If this should prove to be correct, then in all that part of the United States lying west of the <WSD>Agonic</WSD> Line, the general direction whence local rain, hail, and snow-storms come, will be as many degrees north of west as the declination of the needle is east of north; and in all that part of the territory lying east of said <WSD>Agonic</WSD> Line, they will come from a direction as many degrees south of west as the declination of the needle is west of north. 
Meaning of the word marked with <WSD> tags:  Having a magnetic deviation of zero.