together different kinds of iron was not found to be favorable.  The accidental overheating of a boiler was found to reduce 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 circular inch corresponding to 80 lbs. per square inch) = 6224.1379, and this, divided by 17,800, gives 0.349 as the thickness in inches, instead of 0.3125, or 5/16ths, the actual thickness.  If we take the pressure per square inch instead of per circular inch, we obtain the following rule, which is somewhat simpler:—­multiply