The Shadow of a Bridge, can never be seen on the Water running underneath it, unless that Water have first lost its transparency, by being troubled and muddy; The reason is, that clear Water having a bright and polish'd Surface, the Image of the Bridge cast on it, is reflected back to all parts, placed at equal Angles, between the Eye and the Body of the Bridge; and even under the Arches, where the Shadow of the Bridge shou'd be cast, instead thereof is exhibited the Image of the Air; which can never happen when the Water is foul and turbid; since its Lustre, and Transparency, to which it owes, that is, it has the effect of a Mirrour, are in that case destroy'd; whence it becomes disposed to receive a Shadow, in the same manner as a Dusty Street.