Boiler tubes are listed and described from the _outside diameter_. This
should be noted, as gas-pipe is described from the _inside diameter_.
Thus a 1-inch gas-pipe is nearly 1-1/4 outside diameter while a 1-inch
boiler tube is exactly one inch. Another difference between the two
consists in the fact that the outside of boiler tubes is rolled smooth
and even; gas-pipe is left comparatively rough and uneven.
When the boiler tubes are new and properly expanded there is a large
reserve or surplus of holding power for that part of the tube sheet
supported by them, this has been proved by experiment made by chief
engineer W. H. Stock, U. S. N., as shown in the following