Scientific Lectures. 168 



with the periphery of the circle. In like manner behaves the needle 

 in this room, as I will presently show yon. Here we have progressed 

 so far south that the needle sets its length parallel to the circumfer- 

 ence of the circle; thus the needle rests when moved about 90° from 

 the magnetic pole of the earth. We are now traveling over the other 

 side of the magnetic equator, and you observe that the end of the 

 needle, which, in the former semi-circumference, pointed toward 

 the disc, now points away from it ; this also represents exactly what 

 occurs when the needle is carried to the south of the terrestrial mag- 

 netic equator. As I proceed over the circle, this end points more 

 and more toward it ; now it stands at right angles to its surface, 

 and is directly over the other magnetic pole; likewise points the 

 needle when carried toward the magnetic pole of the southern 

 hemisphere. 



It is only possible for us to examine together the dip here at New 

 York ; all the rest we have to take on faith from the trustworthy 

 travelers and voyagers who have reported in a minute manner on 

 what I have given in a very general way. But, desirous that my 

 work before you this evening should leave a permanent impress of 

 that repose which always follows a thorough demonstration, I will, 

 at least, show you the earth's directive action in this room in a very 

 satisfactory manner. I have in my hand a disc of glass on whose 

 margin is photographed a graduated circle, and at its center is a freely 

 moving axis which passes through the center of gravity of this 

 nnmagnetized needle ; thus supported, the needle will remain indiffer- 

 ently at any division of the circle to which I may direct it. I now 

 place it in front of the condensing lens of this ordinary lantern, and 

 the axis of the needle is horizontal, and the latter can move in the 

 same vertical plane in which it would place itself if suspended by a 

 fiber. I now place the needle at various angles of the circle, and 

 you observe it remains immovable at all of them. Now its ends 

 point to the zeros of the circle, which shows that its length is in a 

 horizontal line. I will now magnetize it, and you will observe imme- 

 diately that it is then endowed with a decided preference for a certain 

 angular division of the circle. You now see on the screen end of the 

 magnetic bar passing over the needle from heel to point. Now I 

 have drawn it over several times, and I will take away my fingers 

 which have held it in a horizontal position ; see how it is swinging ; 

 no longer indifferent to its position in reference to the horizon, its 

 spear or north end shows a decided preference for the lower part of 

 the circle, and now, after several oscillations, it has finally come to 



