John Weeks Moore, Complete Encyclopaedia of Music… (1880)
“ Those flats, and sharps, and naturals which are seen al the commencement of a movement are not accidentals, but are called the Signature, and denote the key in which the piece is to be performed. A sharp, flat, or natural, frequently implies some change of key, or different modulation from that in which the piece commenced, and some consider these signs as equally affecting the key, whether found at the commencement or in the progress of the tune. But we generally designate the flats, sharps, &c., at the beginning, as the signature, and such as come afterwards in the music as accidentals. ”
