148 LECTURE 



dimensions each way, or for posts subjected only to a statical strain, it 

 is admirably adapted. For beams, or portions of beams, especially 

 •where it will be subjected to varying strains, to vibrations, and to the 

 action of intense cold, it should be used with extreme caution. 



When a single casting has some portions much thicker than others, 

 most dangerous strains are induced by unequal cooling and contrac- 

 tion ; parts being in this way subjected to tensions, which a small 

 added load will render sufficient to cause total destruction of the cast- 

 ing. Square corners and square openings in a casting are peculiarly 

 dangerous in this respect, and should be most carefully avoided. 

 Again, in a casting which is somewhat irregular, bubbles of air are 

 apt to be entangled, and they cause holes or flaws, which frequently 

 cannot be detected on the outside, even by the aid of the hammer. 

 The iron being deficient at these points in the cross section, weakness 

 is the result. • 



Under a sharp sudden blow cast iron breaks instead of bending, 

 and great cold seems to render it brittle. 



Wrought iron, either hammered or rolled, is tough, elastic, and 

 homogeneous, and resists sudden blows and vibrations much better 

 than cast. It bears a crushing strain up to 60,000 pounds per square 

 inch, and a tensile strain of about the same. 



In practice it has been found necessary to give the upper flange about 

 twice the area of the lower one, since a thin wrought-iron flange, being 

 soft, yields by buckling, although its resistance to compression per 

 square inch of section, is nearly equal to, or, perhaps, a little greater 

 than its resistance to extension. As I have elsewhere stated, beams 

 are now rolled in this country in one piece, with the two flanges of equal 

 areas, and with care in proportioning them, this is an economical 

 form. 



Since wrought iron is brought to its form by hammering or rolling, 

 there can be no flaws in it from air bubbles or similar causes, except 

 in the very rare case of some foreign matter being inclosed by accident 

 in the mass. For the same strength as a beam it has less than half 

 the weight of cast iron, an important consideration in very large struc- 

 tures, of the foundations of which the slightest suspicions are enter- 

 tained. 



Its sujjerior elasticity enables it to resist sudden shocks, or the 

 strains caused by the unequal settling of adjacent parts, and its tough- 

 ness, enables us to make fastenings to resist a tensional strain with 

 great facility. 



Fairbaim lias shown that, at English prices, a wrought-iron beani, 

 to sustain a given weight, can be made for nearly the same price as 

 one of cast iron, with the advantage of much less weight. His state- 

 ment is as follows : 



Cast-iron beam, 31 feet 6 inches long, 22 inches deep, weighs 4,480 

 pounds $65 00 



Wrought-iron beams, 31 feet 6 inches long, 22 inches deep, 



weighs 1,834 pounds $65 50 



To bear a weight of 25.5 tons in middle, or 55 tons distributed uni- 



