Scientific Lectures. 165 



position about latitude 6° north. Again stretching the string 

 from northern and southern magnetic poles to this point, we find that 

 it is six degrees loo far south to be exactly midway between these 

 poles. Here is a discrepancy, but yet a sufficient approximation to 

 serve in the broad argument we are making. 



Standing on the east coast of Brazil, at Porto Seguro, in south 

 latitude 16°, the dipping needle is horizontal. Let us 

 town travel eastward from this point around the world, and in our 

 progress keep such a direction that the needle always remains hori- 

 zontal. The line of our course will have marked out what is called 

 the earth's magnetic equator. This line, starting from the coast of 

 Brazil, tends north towards the coast of Africa, cutting the geographic 

 equator about east longitude two degrees, and then enters the African 

 continent at the Bight of Bennen ; hence, leading slightly north, it 

 strikes the parallel of ten degrees north, and keeps this line directly 

 east to Cochin, on the western coast of Hindostan ; now gradually 

 bending south, it again cuts the equator in west longitude 170°, 

 and thence treads southerly to South America, and meets that con- 

 tinent at about south latitude 7°, on the west coast of Peru, and from 

 this point it bends southerly to reach the place of our departure at 

 Porto Seguro. Such is the sinuous path of the earth's magnetic 

 equator. 



We have only roughly tracked this one line, and have approximately 

 given the position of its poles, but the necessities of navigation 

 require a minute knowledge of the magnetic condition of the earth, 

 and charts have been prepared from the observations of magnetic 

 observatories, and from the results of voyages and travels of discovery 

 that, by means of similar lines, exhibit at a glance its action on a 

 magnetic needle placed anywhere on its surface. This action mani- 

 fests itself in giving to the needle (1) a direction in a horizontal plane 

 (2) a direction in a vertical plane ; and (3) by causing the needle to 

 persist in these planes with intensities increasing from the magnetic 

 equator towards its poles. 



These charts, however, show only the earth's superficial magnetic 

 action. But did not the experiments with our magnetic disc show 

 that wherever in that bright circle we brought our tiny needle, it 

 there placed its length in the line of a curve emanating from the 

 magnet? and does not this fact show us where to "let the imagina- 

 tion go, guarding it by judgment and principle, but holding it in and 

 directing it by experiment V [Faraday.] You see the disc on the 

 screen with a diameter of about six feet; imagine this the 8,000 



