as exhibited in its propagation along plates of glass. 99 
magnetical needle, which has received its polarity by placing 
the north pole of a magnet upon its centre, and drawing it 
several times towards the one extremity without returning 
back again, and afterwards as many times towards the other 
extremity. The indefinite nature of the poles and fringes, 
when the plate of glass is high, as described in Prop. XVI. 
and XVII., and when the heat advances from one edge of the 
plate, is perfectly analogous to the indefinite polarity commu- 
nicated to a steel bar, by applying the pole of a magnet to one 
of its extremities. The same diffused polarity is acquired by 
hot glass, when one of its edges is cooled much more rapidly 
than the other. As two distinct poles, therefore, cannot be 
given to steel, by applying the magnet at one extremity, in 
like manner a distinct polarity cannot be communicated to 
glass, either by heating or cooling it solely at one edge, un- 
less when the height of the plate is very small. 
Such is the resemblance, indeed, between the two classes of 
phenomena, that a description of the state and progress of the 
poles in magnetising a steel bar, is an accurate description of 
the state and progress of the poles in crystallizing a plate of 
glass. 
2. When a heated plate of glass is cooled in the open air, 
and produces the transient fringes described in Prop. XIV., 
the poles are arranged as in Fig. 44. (PI. IV.) where S, S' 
are south poles, and N a north pole in the middle, A and B 
being the two neutral points. This arrangement of the poles 
is exactly the reverse of the preceding, and is the same as 
that which takes place in a needle magnetised in the manner 
already described, but with the north instead of the south pole. 
3. In a plate of glass of the same form and size as Fig. 45. 
O 2 
