170 Transactions of the American Institute. 



called La Bible Guyot, written by Guyot, of Provins, about 1190, in 

 which he speaks of it as well known. 



Guyot tells how a needle which has been rubbed by the rnajriniere 

 will point to the pole-star, and in the dark nights, without star or 

 moon, will guide the mariner on his course. 



The fact that the needle does not point to the true north, at one 

 place, was early known, but the discovery that it changed its direction 

 with a change of place is generally attributed to Columbus ; but this 

 is incorrect, for the needle's departure from the geographic meridian 

 (called its variation, or declination) is marked down for different 

 points of the sea, on the atlas of Andrea Bianco, which was made in 

 the year 1436 ; but what Columbus really did discover was a line of 

 no variation 2£° east of the Island of Corvo, in the Azores, on the 

 13th of September, 1492. 



Some time after this, about 1620, it was found that the needle did 

 not keep one line of direction even in the same place, but slowly 

 moved year after year. Thus it was found that in the year 1580 the 

 north end of the needle at London pointed 11° 15' east. In 1622 

 only 6° east ; while in 1660 the needle pointed due north and south. 

 There in 1730 it pointed 13° west ; in 1765, 20° west ; in 1818, 24° 

 41' west ; in 1850, 22° 30' west, and in 1865, 21° 6' west. Here we 

 see in the needle a most remarkable motion, governed by some cause 

 acting regularly through a long period of time, which, after having 

 given it a swing from the meridian to its extreme westerly position, 

 in 158 years, is now slowly, year after year, throwing it to the meri- 

 dian on its easterly swing. Thus, in about 320 years, it makes one 

 oscillation ; and what is yet more remarkable is this — it follows the 

 same kind of motion as a pendulum ; for from the figures, you see 

 that it moves faster and faster, until it gains the meridian ; then it 

 slacks its velocity and gradually comes to rest at its extreme easterly 

 or westerly position. 



Surely this is a most noteworthy fact and well worthy of being 

 studied and remembered. To impress this on your minds I have 

 devised an experiment which shows this phenomenon in all the pecu- 

 liarity of its varying velocity. It is shown to you not only to exhibit 

 this great law, but also to illustrate to you the extreme mobility, the 

 constant fluctuation, the regular ebb and flow of some mysterious 

 unknown action which we call magnetism. 1 have here a heavy 

 brass ring of eighteen inches in diameter, weighing several pounds. 

 Springing up from this ring are two metal semicircles which meet 

 exactly over its center. Through this point of meeting passes a steel 



