THE AECH. 



117 



have all been added on the crown. But it must be re- 

 collected, that although our equilibrated line of thrust 

 may have been correctly drawn, the arch will not yet 

 give way unless wo have at the same time drawn it in the 

 strongest equilibrated form that the arch as constructed will 

 admit of. 



Lot us now take a glanco at the question of fancy arches ; 

 that is, of archoa built in some fanciful form in order to 

 please the eye, but not in the truly equilibrated form. 



Take first the flat Gothic (see plate XVII.). If an arch 

 of this sort is to stand, we must first draw the strongest 

 equilibrated arch we can got into the pattern, and having 

 made it of the necessary thickness to carry the 8uporimi)osed 

 load, with a fair margin of safety, we can add both above 

 and below this such additional thicknesses as we wish in 

 order to give the Gothic form to the structure. In the 

 sketch the dotted lines show the real arch that bears the 

 load, and the full lines the pieces added to givo tho 

 required form. These last merely signify so much moro 

 load when added underneath, but the "lino of thrust" 

 still passes along the dotted arch. 



In the semicircle tho equilibrated arch under a horizontal 

 load-line would pass into the abutments much above tho 

 nominal springing, and tho " line of thrust " would then 

 leave the apparent arch and i)ass into the backing. Tho 

 angular ])iocos put in to complete the circular form would 

 be merely suspended from the real arch above; but the 

 bedding of the arch stones should bo square to tho " lino of 

 thrust " and not to the circle. 



The platband, properly constructed, contains the true 

 arch principle. Let us su])pose we wish to make a drawing 

 for tho best form of platband, havuig a span of 6 feet and a 

 hoiglit of 2 feet (see plate XVII.). AB is 2 feet and DE 



