on and brazed
externally. The pipe was not lagged or protected in any manner. After
eight and a half years' service the metal measured where cut 0.32 and
0.375 inch in thickness, showing that the wasting during that time had
been very slight. The interior surface of the tube exhibited no signs of
pitting or corrosion. It was covered by a thin crust of black oxide, the
maximum thickness of which did not exceed 1/32 inch. Where the deposit
was thickest it was curiously striated by the action of the steam. On
the scale being removed, the original bloom on the surface of the metal
was exposed. It would thus appear that the danger from corrosion of iron
steam pipes is not borne out in their actual use; and hence so much of
the way is cleared for a stronger and more reliable material than
copper. So far the source of danger seems to be in the weld, which would
be inadmissible in larger pipes; but there is no reason why these should
not be lapped and riveted. There seems, however, a more promising way
out of the difficulty in the Mannesmann steel tubes which are now being
"spun" out of solid bars, so as to form weldless tubes.

[Footnote 2: Transactions Northeast Coast Institution of Engineers and
Shipbuilders, vol. 7, 1890-91, p. 179.]

TABLE I.--TENSILE STRENGTH OF GUN METAL AT HIGH TEMPERATURES.

--------------+------------+-------------+-------------+------------+
| | | | |
Composition |Temperature | Tensile | Elastic | Elongation |
of | of oil | strength | limit | in |
gun metal. | bath. | per square | per square | length of |
| | inch. | inch. | 2 inches |
--------------+------------+-------------+-------------+------------+
Per cent. | Fahr. | Tons | Tons | Per cent. |
Copper 87 /| 50 deg. | 12.34 | 8.38 | 14.64 |
Tin 8 / | | | | |
Zinc 31/2 \ | | | | |
Lead 11/2 \| 400 deg. | 10.83 | 6.30 | 11.79 |
--------------+------------+-------------+-------------+------------+
Copper 87 /| 50 deg. | 13.86 | 8.33 | 20.30 |
Tin 8 { | | | | |
Zinc 5 \| 458 deg. | 10.70 | 7.43 | 12.42 |
------