The backbone E F should be quite rigid, but the cross-pieces A B and C D are better if they are slightly curved. A sheet of light paper must now be pasted from A B to C D underneath E F in such a way that it is quite tight under E F, but rather loose between A C and B D.
Fig. 468 shows how the paper should be cut. G H is the exact distance between E and F; J K and L M are wider than distances between A C and B D in Fig. 467, so that when the flaps on the paper are pasted over the cross-bits the paper is loose between A and C and between B and D (Fig. 467). The secret of the balance is to have the flutter at the edges quite equal. Fig. 467 shows how the string is fastened.
Fig. 469
Fig. 470(a)
Fig. 470(b)
Fig. 471