424 PROCEEDINGS OP THE AMERICAN ACADEMY 



Rule for Hollow Rectangular Pillars. 



Find bj the preceding rules the weights for sohd pillars of the out- 

 side and inside dimensions of the hollow pillar ; subtract one from the 

 other ; the difference will be the safe weight for the hollow pillai*. 



Rule for Solid Cylindrical Wrought-iron Pillars. 



According to Hodgkinson, calling the strength of a cast-iron pillar 

 1000, the strength of a Avrought-iron pillar of the same dimensions is 

 1745. Then to find the safe weight for a solid cylindrical wrought- 

 iron pillar, find, by the preceding rules for cylindrical cast-iron pillars, 

 the weight for a cast-iron pillar of the same dimensions ; multiply this 

 weight by 1.745 ; the product will be the safe weight for the wrought- 

 iron pillar. 



Note. This rule must be confined to pillars very long in proportion 

 to their diameters ; say, in pillars with flat ends, to lengths of not less 

 than GO diameters. Ilodgkinson's experiments indicate that wrought- 

 iron pillars with flat ends, of lengths equal to 30 diameters, are only 

 about one tenth stronger than cast-iron pillars of the same dimensions. 

 In a single experiment on a wrought-iron pillar with flat ends, of a 

 length equal to about 15 diameters, tlie strength was nearly 40 per 

 cent less than that of a like pillar of cast-iron. This great difference 

 in the relative strengths of cast and wrought-iron pillars of different 

 lengths in proportion to the diameters, arises from the greater sti-ength 

 of wi"ought-iron to resist a tensile force, and its less strength to resist a 

 crushing force, the strength of short pillars depending mainly on the 

 power of the material to resist the latter force.* 



Rule for Solid Square Wrought-iron Pillars. 



Find by the preceding rule the weight for a solid cylindrical pillar 

 of wrought iron, whose diameter is equal to the side of the square pil- 

 lar ; add one half to tliis weight, the sum will be the safe weight for 

 the square pillar. The length of the pillars to which this rule applies 

 is subject to the same limitations as in the preceding rule. 



* Strength of cast-iron (Low Moor, No. 3) to resist a crushing force 109,801 

 pounds per square inch ; to resist a tensile force 14,535 pounds per square inch 

 (Hodgkinson). Then crushing strength to tensile strength as 7.55 to 1. 



Strength of wrought-iron to resist a crushing force 36,000 to 40,000 pounds per 

 square inch; to resist a tensile force 60,000 to 70,000 pounds per square inch 

 (llaukine). Then crusliing strength to tensile strength about as 1 to 1.7. 



