Wrought Iron when Heated and then Cooled. 405 



especially those relating to hollow cylinders of wrought iron, which 

 are very instructive. 



Imagine such a cylinder divided into two parts by a horizontal 

 plane at the water-line, and in this state immersed after heating. 

 The under part, being in contact with water, would rapidly cool and 

 contract, while the upper part would cool but slowly. Consequently 

 by the time the under part had pretty well cooled, the upper part 

 would be left jutting out ; but when both parts had cooled, their 

 diameters would again agree. Now in the actual experiment this 

 independent motion of the two parts is impossible, on account of the 

 continuity of the metal ; the under part tends to pull in the upper, 

 and the upper to pull out the under. In this contest the cooler 

 metal, being the stronger, prevails, and so the upper part gets pulled 

 in, a little above the water-line, while still hot. But it has still to 

 contract on cooling; and this it will do to the full extent due to its 

 temperature, except in so far as it may be prevented by its connexion 

 with the rest. Hence, on the whole, the effect of this cause is to 

 leave a permanent contraction a little above the water-line ; and it is 

 easy to see that the contraction must be so much nearer to the water- 

 line as the thickness of the metal is less, the other dimensions of the 

 hollow cylinder and the nature of the metal being given. When the 

 hollow cylinder is very short, so as to be reduced to a mere hoop, 

 the same cause operates; but there is not room for more than a 

 general inclination of the surface, leaving the hoop bevelled. 



But there is another cause of deformation at work, the opera- 

 tion of which is well seen in figs. 2 and 3. Imagine a mass of 

 metal heated so as to be slightly plastic, and then rapidly cooled 

 over a large part of its surface. In cooling, the skin at the same 

 time contracts and becomes stronger, and thereby tends to squeeze 

 out its contents. This accounts for the bulging of the ends of the 

 solid cylinders of wrought iron and the rents seen in their cylindrical 

 surface. The skin at the bottom is of course as strong as at the 

 sides in the part below the water-line ; but a surface which resists 

 extension far more than bending has far less power to resist pressure 

 of the nature of a fluid pressure when plane than when convex. The 

 effect of the cause first explained is also manifest in these cylinders, 

 although it is less marked than in the case of the hollow cylinders, 

 as might have been expected. 



The tendency of the cooled skin of a heated metallic mass to 

 squeeze out its contents appears to be what gives rise to the bulging 

 seen near the water-line in the hollow cylinder of brass. Wrought 

 iron, being highly tenacious even at a comparatively high tempera- 

 ture, resists with great force the sliding motion of the particles which 

 must take place in order that the tendency of the cooled skin to 

 squeeze out its contents may take effect ; but brass, approaching in 

 its hotter parts more nearly to the state of a molten mass, exhibits 

 the effect more strongly. It seems probable that even in the case of 

 brass a very thin hollow cylinder would exhibit a contraction just 

 above the water-line. Should there be a metal or alloy which about 

 the temperatures with which we have to deal was stronger hot than 

 cold, the effect of the cause first referred to would be to produce an 

 expansion a little below the water-line. — G. G. S.] 



