218 Proceedings of Royal Society of Edinburgh. [sess. 
axis, an ellipse, with its major axis pointing towards the axis of 
the tube. When the walls are thin, the ellipse into which a small 
circular element is changed has its minor axis situated radially. 
The ellipse increases in excentricity as the distance from the axis 
diminishes. In nickel this ellipse is greater in area than the 
original circle ; in iron it is, in a general way, less or greater 
according as X is positive or negative. 
Cobalt behaves very like nickel, hut the elongations are about 
one-third the value. The cubical dilatation is always negative, 
attaining a value of - T8 x 10~ 6 in field 500. 
The paper ends with a discussion of the work done by the 
magnetic forces in straining the metals against the intermolecular 
forces on which the elasticity of the substance depends. 
