1C4 Transactions of the American Institute. 



rest with its spear-end 73° below the horizontal line. This deflection 

 of the needle below the horizontal plane is called its " dip." 



We have now in onr possession a most valuable instrument for 

 exploring the magnetic condition of the earth, and, with similar 

 needles, explorers have traveled over all the accessible regions of the 

 earth, and have carefully noted how its inclination to the horizon 

 changed with various stations. At New York the north end of the 

 needle dips 73°. Carrying it up the Hudson, we find that at Catskill 

 it has increased to 74° ; and at Saratoga it is 76°. Proceeding north 

 and west we find the needle dipping more and more, until we reach 

 latitude 70°, and a longitude that brings us in the center of 

 the North American continent, where the needle points in the direc- 

 tion of the plumb line. Retracing our path to the south, we see 

 the needle continuously lifting its north end, until we again have it 

 at New York dipping seventy-three degrees. At Philadelphia it 

 points a little below seventy-two degrees, and when we reach Wash- 

 ington it is at seventy-one degrees. Its north end gradually rising, 

 we pass over the end of Florid/i, where it dips about fifty-five degrees. 

 At the mouth of the Amazon, directly on the equator, it is yet 

 twenty-five degrees below the horizontal line ; but when we have 

 reached latitude 17° south of the equator, and are about 12° 

 in longitude west of the coast of Brazil, we see the dipping 

 needle with its length parallel to the horizon. Here we have 

 reached a point of the earth's magnetic equator correspond- 

 ing to the point midway between the poles on the magnetic disc. 

 But does the position we have reached correspond to such a position 

 in reference to the earth's magnetic poles ? I again take the terres- 

 trial globe, and on it I draw a circumference of the sphere, which I 

 pass from this point west of Brazil through the North American mag- 

 netic pole, and extending this circle' beyond, I pass it round on the 

 other side, until it has girdled the sphere. It very nearly cuts 

 through the other pole, whose position we marked on Wilkes' Land. 

 I now take a string and stretch it along the line from the Boothia pole 

 to the point off the coast of Brazil where the dipping needle is hori- 

 zontal. I then apply this same length from the southern pole towards 

 the same point of Brazil, and I find that this point is only one degree too 

 far south to be exactly midway between the two terrestrial magnetic 

 poles. The coincidence is as near as we can expect with a sphere com- 

 posed, like our earth, of such varying materials. We will now trans- 

 port ourselves to the other side of the earth, on the line which we 

 drew around the globe, and we find that the needle takes a horizontal 



