He used instruments such as the astrolabe and sextants to aide his studies of navigation. By the late 18th century, navigators replaced their prior instruments with octants and sextants. There are two types of astronomical sextants, mural instruments and frame-based instruments. Most sextants also include a vernier on the worm dial that reads to 0.1 minute. Sextants with aluminum frames and brass arcs have also been manufactured. Whole-horizon sextants use a half-silvered horizon mirror to provide a full view of the horizon. Most sextants mount a 1 or 3-power monocular for viewing. There are examples of sextants made with wood, however most are made from brass. All of these instruments may be termed "sextants". Brass is lower-expansion than aluminium, but aluminium sextants are lighter and less tiring to use. Many commercial sextants use low-expansion brass or aluminium. These large sextants are made primarily of wood, brass or a combination of both materials. High-precision sextants have an invar (a special low-expansion steel) frame and arc. Some scientific sextants have been constructed of quartz or ceramics with even lower expansions. Sextants were first implemented around 1731; they replaced the backstaff.