the ultimate or maximum
strength of the plates from 65,000 to 45,000 lbs. per square inch of
section, and riveting the plates was found to occasion a diminution in
their strength to the extent of one third. These results, however, are not
precisely the same as those obtained by Mr. Fairbairn.

300. _Q._--What were the results obtained by him?

_A._--He found that boiler plate bore a tensile strain of 23 tons per
square inch before rupture, which was reduced to 16 tons per square inch
when joined together by a double row of rivets, and 13 tons, or about
30,000, when joined together by a single row of rivets. A circular boiler,
therefore, with the ends of its plates double riveted, will bear at the
utmost about 36,000 lbs. per square inch of section, or about 12,000 lbs.
per square inch of section without permanent derangement of structure.

301. _Q._--What pressure do cylindrical boilers sustain in practice?

_A._--In some locomotive boilers, which are worked with a pressure of 80
lbs. upon the square inch, the thickness of the plates is only 5/16ths of
an inch, while the barrel of the boiler is 39 inches in diameter. It will
require a length of 3.2 inches of the boiler when the plates are 5/16ths
thick to make up a sectional area of one square inch, and the separating
force will be 39 times 3.2 multiplied by 80, which makes the separating
force 9,984 lbs., sustained by two square inches of sectional area--one on
each side; or the strain is 4,992 lbs. per square inch of sectional area,
which is quite as great strain as is advisable. The accession of strength
derived from the boiler ends is not here taken into account, but neither is
the weakening effect counted that is caused by the rivet holes. Some
locomotives of 4 feet diameter of barrel and of 3/8ths iron have been
worked to as high a pressure as 200 lbs. on the inch; but such feats of
daring are neither to be imitated nor commended.

302._Q._--Can you give a rule for the proper thickness of cylindrical
boilers?

_A._--The thickness proper for cylindrical boilers of wrought iron, exposed
to an internal pressure, may be found by the following rule:--multiply 2.54
times the internal diameter of the cylinder in inches by the greatest
pressure within the cylinder per circular inch, and divide by 17,800; the
result is the thickness in inches. If we apply this rule to the example of
the locomotive boiler just given, we have 39 x 2.54 x 62.832 (the pressure
per circul