254 OF THE PARTS OF [SECT. vn. 



For boiler tubes, or cylinders under 8 inches iu diameter, 



t 



~ 



_ 



150 n (1 - 0-116 a) ' 



In either case there is a risk of failure when the diameter is less than 7 - 4 inches 

 in the first, and when O116 times the diameter is greater than one in the second 

 case. If the thickness be much more than the rule gives, the risk from unequal 

 expansion increases ; if it be less, the joint effect of pressure and inequality may 

 cause failure. 



529. RULE for the strength of cast iron tubes exceeding 8 inches in dia- 

 meter. Multiply the square of the diameter by the pressure on the safety valve in 

 Ibs. on a circular inch, and divide the product by 150 times the cubic feet of space 

 in the boiler per horse power, multiplied by the difference between the diameter 

 and 7 '4 inches ; the result is the thickness in inches, which should be increased for 

 wear and tear in proportion to the degree of durability required. 



Example 1. The internal diameter of the tube being 10 inches, the cubic feet 

 of boiler per horse power 10, and the load on the valve 36 Ibs. on the circular inch, 

 what should be its thickness ? In this case, 



10 x 10 x 36_ _ , 



1501TLO (HT-T7-4) " 



Example 2. The internal diameter of a cast iron cylinder for a boiler being 

 3 feet, and the force of the steam to be confined to 5 atmospheres, 58 Ibs. per 

 circular inch on the valve, what should be its thickness, the space of boiler for 

 each horse power being 16 feet? In this case, 



36 x 36 x 58 



150 x 16 x (36 - 7-4) 



= I'l inches. 



OF JOINING PIPES AND OTHER PARTS OF ENGINES. 



530. Joints are generally connected by screw bolts passing through flanches ; 

 between these flanches an elastic material of a durable nature is inserted, or a 

 compound called a cement, which unites and forms one mass with the joined 

 surfaces. 



Iron cement is the most valuable of the latter kind ; it may be compounded as 

 follows : To two ounces of sal-ammoniac, add one ounce of flowers of sulphur, and 

 sixteen ounces of clean cast iron filings or borings ; mix all well together by 

 rubbing them in a mortar, and keep the powder dry. When the cement is wanted 

 for use, take one part of the above powder and twenty parts of clean iron borings 

 or filings, and blend them intimately by grinding them in a mortar. Wet the 

 compound with water, and, when brought to a convenient consistence, apply it to 



