Fig. 1433.—Condition of the hoop after the twentieth cooling.
“In No. 8 a circular hoop of wrought iron was forged out of a 31⁄2-inch by 1⁄2-inch bar, the external diameter being about 18 inches, the breadth, 1⁄2 inch, being parallel to the axis of the hoop. This hoop, [Fig. 1432], was heated to redness, then plunged into cold water half its depth, the upper half cooling in air. The changes in the external circumference of the hoop were accurately measured after each of twenty successive coolings, at the end of which the external circumference of the water-cooled edge had increased 1.24 inches, or 2.14 per cent. of its original length, and the air-cooled edge had contracted 7.9 inches, or 13.65 per cent.
“Experiment No. 8.—Wrought-iron hoop, 31⁄2 inches by 1⁄2 inch by about 18 inches in diameter, or exactly 57.85 inches in circumference at top, and 57.95 inches at bottom edge.
| Top Edge. | Bottom Edge. | Remarks. | |||||
| Contraction. | Percentage of original circum- ference. | Expansion. | Percentage of original circum- ference. | ||||
| Ins. | Ins. | ||||||
| After | 1st | dip | .50 | .86 | .08 | .14 | |
| „ | 2nd | „ | .99 | 1.71 | .08 | .14 | |
| „ | 3rd | „ | 1.47 | 2.54 | .26 | .45 | |
| „ | 4th | „ | 1.92 | 3.32 | .30 | .52 | |
| „ | 5th | „ | 2.30 | 3.97 | .34 | .59 | |
| „ | 6th | „ | 2.60 | 4.49 | .40 | .70 | Slight crack in expanded edge. |
| „ | 7th | „ | 2.94 | 5.25 | .44 | .76 | |
| „ | 8th | „ | 3.40 | 5.98 | .50 | .86 | |
| „ | 9th | „ | 3.70 | 6.39 | .56 | .96 | |
| „ | 10th | „ | 4.40 | 7.60 | .62 | 1.07 | |
| „ | 11th | „ | 4.42 | 7.64 | .66 | 1.14 | |
| „ | 12th | „ | 4.85 | 8.40 | .70 | 1.22 | |
| „ | 13th | „ | 5.24 | 9.02 | .78 | 1.34 | |
| „ | 14th | „ | 5.74 | 9.92 | .80 | 1.39 | |
| „ | 15th | „ | 6.00 | 10.37 | .86 | 1.49 | |
| „ | 20th | „ | 7.90 | 13.65 | 1.24 | 2.14 | After deducting for a crack .06 inch wide which appeared at sixth dip. |
“It will be observed that we have here two remarkable phenomena: 1. The reversal of the expansion and contraction as described. 2. The very large amount of contraction on the upper edge compared with what was exhibited in Experiment 5 of entire submersion.
“The table showing Experiment 5 gives a contraction of 2.25 per cent. after the twentieth cooling, whereas the contraction on the air-cooled edge of Experiment 8 is 13.65 per cent., or six times the contraction of an entirely submerged hoop.
“To ascertain whether these unexpected phenomena had any connection with the circular form of the hoop, Experiment 9 was made with a straight bar of iron 31⁄2 inches deep by 1⁄2 inch thick by 28.4 inches long.
“Experiment No. 9.—Wrought-iron bar, 31⁄2 inches by 1⁄2 inch by 28.4 inches long, heated to a dull red, then quenched half its depth in water.