126 Roofs for Farm-Buihlhifjs. 



of Iron. — Iron enters into tlic construction of roofs in two foinis 

 — firstly, in tlie shape of corrugated or sheet iron, as a covering 

 material, which is elsewhere treated of; and secondly, as part 

 of the framework, in the form of cast and wrought iron, which 

 we will now proceed to discuss. 



The relative properties of cast and wrought iron are thus 

 described by Mr, Fairbairn : — 



" Cast iron is a hard, rigid, crystalline, immalleable substance, 

 which presents a great resistance to a force of extension, but a 

 somewhat less resistance to that of compression ; and from its 

 low degree of ductility, it undergoes but little elongation when 

 acted upon by tensile force. On the contrary, wrought iron is 

 a flexible, malleable, ductile substance, which presents a great 

 resistance to a force of extension, but a somewhat less resistance 

 to that of compression. From its high degree of ductility, it 

 undergoes a considerable elongation when acted upon by a tensile 

 hn-ce. When the two metals are released from the action of a 

 tensile force, the set of the one metal differs widely from the set 

 of the other. The flexibility of wrought iron is from eight to 

 ten times greater than that of cast iron. Under the same increase 

 of temperature the expansion of wrought-iron is considerably 

 greater than that of cast iron. While wrought iron yields to a 

 stroke, cast iron is readily broken by a severe collision, or by 

 any vibratory action." 



The slight resistance that cast iron, as compared with wrought 

 iron, offers to tensile strains or sudden influences, and its crystalline, 

 brittle, and inflexible character preclude its employment in long 

 lengths, where yielding elastic qualities are required ; while its 

 great resistance to the force of compression indicates its adapta- 

 bility for struts, braces, and straining-bars. But there are two great 

 drawbacks to the employment of cast iron, even in these forms : 

 the first is the weight, the second is the cost in consequence of 

 the weight. Cast iron weighs about 450 lbs. per cubic foot, 

 while the weight of a cubic foot of Memel fir is only 34Vlbs. 

 The former is consequently about 13 times heavier than w^ood 

 of equal dimensions. The average cost of iron ready for the 

 founder is 1/. 2s. 6c?. per cubic foot, whereas the cost of a cubic 

 foot of Baltic fir ready for the sawyer is 2.?. Id. — thus making 

 the cost of iron 9 times greater than that of wood of equal bulk. 

 In addition to this it must be remembered that the cost of casting 

 iron for struts or straining-bars is three times dearer than the 

 expense of manual labour in preparing wood for like purposes. 



The figures relative to the weight and cost of iron and wood 

 are given by way of preliminary contrast, but do not of course in 

 themselves convey any tangible results, nor supply direct data 



