A DROP OF WATER. gi 
selves up so systematically? Dr. Tyndall says we can, 
and I hope by the help of these small bar magnets 
to show you how he explains it. These little pieces of 
steel, which I hope you can see lying on this white 
cardboard, have been rubbed along a magnet until 
they have become magnets themselves, and I can at- 
tract and lift up a needle with any one of them. But 
if I try to lift one bar with another, I can only do 
it by bringing certain ends together. I have tied a 
piece of red cotton (c, Fig. 22) round one end of each 
c 
FIG. 22. Bar magnets attracting and repelling each other. 
c, Cotton tied round positive end of the magnet. 
of the magnets, and if I bring two red ends together 
they will not cling together but roll apart. If, on the 
contrary, I put a red end against an end where there 
is no cotton, then the two bars cling together. This 
is because every magnet has two poles or points which 
are exactly opposite in character, and to distinguish 
them one is called the positive pole and the other 
the negative pole. Now when I bring two red ends, 
that is, two positive poles, together they drive each 
