the ascertained difference in the case of No. 7 being 1.332, thus sufficiently accounting for the discrepancy between specific gravity and change of volume by the scaling.
“By Experiment 7 it has been shown that the loss of thickness due to scaling after fifty immersions was .0284 inch over the whole surface (sides and edges.) Therefore, assuming this scaling as uniform over the surface, the girth, whether measured lengthwise or breadthwise, should be eight times .0284, or .23 inch less after immersion than before. Now the gross loss of girth is:—
| Lengthwise. | Breadthwise. | ||
| Inches. | Inches. | ||
| In No. 6 | 1.3 | 8 | .86 |
| In No. 7 | 1.2 | .52 | |
| Or for both experiments a mean of | 1.2 | 9 | .69 |
| Deducting from them the loss of girth due to scaling | .2 | 3 | .23 |
| Net contraction after fifty immersions | 1.0 | 6 | .46 |
| Or in percentage of original girths, which were | 25.4 | 6 | 13.43 |
| per cent. | per cent. | ||
| We have a percentage of | 4.1 | 6 | 3.42 |
| Or for each immersion an average of | .0 | 83 | .07 |
“Comparing these results with those of Experiments Nos. 1, 2, and 5, we find that the contraction of the skin of the plate is less for each immersion than that of a bar or hoop, in the proportion of .125 to .083. This is what might be expected, as the contraction of the plate is resisted by the volume of heated matter inside, which is eventually displaced by bulging, while the bar finds relief endwise without having to displace the interior.
“We have now before us the following facts, substantiated by the experiments described:—
“1. That in heating to redness, and then cooling suddenly in water at ordinary temperatures, bars and plates of wrought iron, a reduction of specific gravity takes place, the amount being about 1 per cent. after fifty immersions, and 1.57 per cent. after one hundred immersions, further heatings and coolings not appearing to produce further change.
“2. That a reduction of the surface takes place after each heating and cooling, this being due to two causes:—
“a. The scaling of the surface, which is shown to amount to a film over the (sides and edges) entire area of .00057 inch in thickness for each immersion, or 0.284 inch for fifty immersions (Experiment 7).
“b. A persistent contraction, which takes place after each immersion. This varies according to the form of the iron, being in plates from .07 per cent. to 0.83 per cent. (Experiment 6), while in long bars it varies from .122 to .15 per cent. (Experiments 1, 2, and 5). This contraction continues vigorously up to fifty immersions, and probably much farther.
“3. That in the case of plates a bulging takes place on the largest surfaces, increasing the thickness towards the centres, although the edges diminish in thickness.