“It may be remarked, that if the bars be heated to white heat a slight contraction does occur, as shown by Experiment No. 4, where a bar of the same dimensions as No. 3 contracted .17 per cent. after the fifth cooling. As, however, the further remarks on this subject have only reference to bars heated to redness and then cooled, the writer would summarize the results of Experiments Nos. 1, 2, and 3, by stating that wrought-iron bars heated to redness permanently contract in their length along the fibre when cooled in water of ordinary temperature; but when cooled in air, they remain unchanged in length.

“To show that this is true as applied to circular hoops, Experiment No. 5 was made upon a wrought-iron bar of 11⁄8 inches square in section, welded into a circular hoop, 57.7 inches outside circumference.

“Experiment No. 5.—Wrought-iron hoop, 11⁄8 inches square by 57.7 inches outside circumference, heated to a dull red, then cooled suddenly in water.

Contraction.Percentage
of original
circumference.		Remarks.
Inches.
After 1st cooling.06.10Red heat.
„2nd„.06.10This was nearly white, but before cooling red hot.
„3rd„.16.28
„4th„.26.45
„5th„.35.61
„6th„.46.80
„7th„.54.93
„8th„.601.04
„9th„.681.18
„10th„.761.32
„11th„.801.38
„12th„.871.51
„13th„.941.63
„14th„1.001.73
„15th„1.081.90
„20th„1.302.25On opposite edge 1.66; hoop splitting.
„25th„1.763.05

“This hoop was heated to redness and cooled in water twenty-five times, the circumference of the hoop being accurately measured after each cooling.[23]

[23] The lengths of circumference were taken, in this and other hoops, after each cooling, by encircling the periphery with a very fine piece of “crinoline” steel, the ends of which were made just to meet round the original hoop. By again encircling the hoop with the same piece of steel the expansion was shown by a gap between the ends, and a contraction by an overlap, either of which was measured with great accuracy by means of a finely divided scale.

“Two wrought-iron bars, 11⁄8 inches square and 30.05 inches long, were selected.[24] No. 1 was of common “Crown” quality; No. 2 of a superior quality known as “Tudhoe Crown.” These bars were heated to redness in a furnace and then plunged into water of ordinary temperature, the length being accurately measured after each cooling. After fifteen heatings and coolings the permanent contraction on No. 1 bar was 2.26 per cent. of the original length, and that on No. 2 bar 1.86 per cent., or an average on the two bars of about .13 per cent. after each cooling, the increment of contraction being nearly equal after each successive operation. It is noticeable that after the first two coolings the better quality of iron did not contract quite so much as the common quality, and that in the latter the contraction was going on as vigorously at the fifteenth as at the first cooling.

[24] In some of these experiments the original sizes of the iron were only measured with an ordinary foot-rule, in which case the dimensions are given in the ordinary fraction used in expressing the mercantile sizes of iron. When accurate measurement was taken decimals are invariably used both in this paper and the Tables of Experiment.

“Similar bars of wrought iron, heated to redness and then allowed to cool in air at ordinary temperature, do not appear to suffer any permanent change in their length.

“Experiment No. 3 was made to verify this.