Evidence from Mechanical Measurements for Flux-Lattice Melting in Single-Crystal Y$ {\mathrm{Ba}}_{2}\( {\mathrm{Cu}}_{3}\) {\mathrm{O}}_{7}$and\({\mathrm{Bi}}_{2.2}\( {\mathrm{Sr}}_{2}\) {\mathrm{Ca}}_{0.8}\( {\mathrm{Cu}}_{2}\) {\mathrm{O}}_{8}\)