. St. Nicholas [serial] . desrigid. The photograph givesa good idea of the two-post bridge. This wasmade in about fortyminutes by the lads inthe picture. And thatbrings us to the THREE-POST BRIDGE This one reminds youmuch of the two-post,only the middle section,which was a rectanglewith the two-post, hasbeen lengthened outconsiderably, so thatthere must be a third post, this one connecting the middle of thefloor-timber with the middle of the brace con-necting the tops of the two end triangles. You see that now the side is divided intofour parts, so that when you want to find thelength of the s
. St. Nicholas [serial] . desrigid. The photograph givesa good idea of the two-post bridge. This wasmade in about fortyminutes by the lads inthe picture. And thatbrings us to the THREE-POST BRIDGE This one reminds youmuch of the two-post,only the middle section,which was a rectanglewith the two-post, hasbeen lengthened outconsiderably, so thatthere must be a third post, this one connecting the middle of thefloor-timber with the middle of the brace con-necting the tops of the two end triangles. You see that now the side is divided intofour parts, so that when you want to find thelength of the slant sides of the end triangles. you must square half the length of the bridgeand proceed as before. Suppose the bridgeis to be 20 feet long. Half of this is squared is 100. Half of this is 50, andthe square root of this, roughly, is 7 feet. Thatgives the length of the slanting side—the long-est side—of the two end triangles, and also oftwo new triangles which the diagram shows inthe middle part of the A THREE-POSTER. THE SIDES A LITTLE FLAT. AS THE MATERIAL IS HEAVY I hardly have to explain how to go aheadwith this bridge—it is all so simple. The toptimber connecting the tops of the end trianglesis half the length of the bridge. And it is fromthe center of this that the third post connectsthe upper timber with the middle of the floor- 736 FOR BOYS WHO DO THINGS [June, timber. This divides the big rectangles of eachside into squares, and you place a diagonal ineach square, running from the top of the centerpost of the bridge to the opposite corner, meet-ing the floor-timber where it is joined to thefirst post. These two extra diagonals keep thecenter of the bridge from sagging downward,and so must be very firmly nailed, both to thetop of the middle post and to the floor-timbers. The three-post is a very strongly bracedbridge, and, by using heavy timbers, you can make quite a long span with it. And of course,having now three posts on a side, there mustbe t
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Keywords: ., bookcentury1800, bookdecade1870, bookidstnicholasserial4721dodg
