252 
On Water and Air. 
[April, 
in mv hand, and draw the magnetic needle towards it, you 
see that the red end is att rafted. There is a certain vivacity 
of vibration which becomes more and more languid as t 
centre of the magnet approaches the needle, and direftly I 
come above the centre of the magnet you have th ® thing 
reversed The blue end of the needle is now attracted in- 
stead of the red. If I bring the magnet down the needle 
turns round, and the red end is again attradte . y 
see that we have one-half of this magnet attracting one end 
of the needle and the other half attracting the other en , 
and we have always attraction and repulsion exerted to- 
ge Now I wish to define a term that I have to use with 
reference to this double force. _ This simultaneous exertion 
of attraction and repulsion is what we call P^ar y. 
Magnetism is called a polar force because of this duplex 
aftion. In the case of the attraction of gravitation we have 
nothing of that kind. The sun attracts the planets, and 
every particle of matter attracts every other particle without 
the exercise of this double force. The attraction of gravi- 
tation is not a polar force. The attraction of magnetism is 
a Now Mr. Cottrell will throw upon the screen the image 
of a small magnet. There you see it (Fig. 27) ; and almost 
every boy has made experiments with iron filings upon mg 
netsf Here we have our magnet, and I will shake some 
iron filings over it, and we shall see that those iron filings 
Fig. 27. 
arrange themselves in a certain definite fashion over this 
magnet. You see how beautiful it is ; that is lovely. ou 
see how beautifully they arrange themselves. Every little 
