Garden and Agricultural Structures. 



199 



pigs, calves, &c., might 

 be kept in such struc- 

 tures without injurino- 

 them. The circular 

 form would in this case 

 be the best. 



But a more perfect 

 structure of this kind 

 may be erected, having 

 a permanent roof, yet 

 capable of sustaining, 

 at any time, a stack of any kind ; and I see no reason why 

 all garden sheds, or even the buildings (or most of them) of 

 a farmery, might not be thus flat-roofed, and so, in the latter 

 case, answer the double purpose of buildings and rick-stands. 

 Supposing the walls constructed of cast-iron uprights se- 

 cured firmly to foundation stones, and the intervals filled in 

 any of the before-mentioned ways ; the roof is to be con- 

 structed by laying light cast-iron beams or bearers, from one 

 upright to another opposite it, and arching the interspaces with 

 4-inch arches. By these means the building will always be 

 secured from the weather, whether the rick above be removed 

 in part or not. Iron beams have hitherto been constructed 

 generally from formulae which make them much heavier than 

 necessary. A late writer on this subject, Mr. Eaton Hodgs- 

 kinson, has shown, and verified by experiment (vide Trans. 

 Manchester Phil. Soc.\ that, in place of the usual form and 

 proportions of cast-iron beams, viz. the top and bottom ribs 

 nearly alike in scantling, as in j%. 58., the bottom rib should 

 be to the top rib in the ratio of 6 to 1 ; that 

 the vertical or mid rib may be cast as thin as will 

 insure a perfect casting ; that the strength of such 

 beams is inversely as the length, and directly as 

 the depth, and not as the square of the depth, as 

 hitherto supposed; and that the bottom rib should 

 be formed into a parabola on its edges, terminating a few inches 

 within the bearings. 



The formula for the determination of the ultimate strength 

 and scantling of such beams, given by Mr. Hodgskinson, is :, 



W = ; where W is the breaking weight ; a the area in, 



L 



square inches of the mid section of the bottom rib; d the 

 depth of the beam there ; / the length of the beam in inches ; 

 and c a constant quantity deduced from experiment, equal, for 

 beams cast on their sides, to 24 ; and for those cast vertical, 

 to 25. 



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