<P> Word clocks are clocks that display the time visually using sentences . E.g.: "It's about three o'clock ." These clocks can be implemented in hardware or software . </P> <P> Some clocks, usually digital ones, include an optical projector that shines a magnified image of the time display onto a screen or onto a surface such as an indoor ceiling or wall . The digits are large enough to be easily read, without using glasses, by persons with moderately imperfect vision, so the clocks are convenient for use in their bedrooms . Usually, the timekeeping circuitry has a battery as a backup source for an uninterrupted power supply to keep the clock on time, while the projection light only works when the unit is connected to an A.C. supply . Completely battery - powered portable versions resembling flashlights are also available . </P> <P> Auditory and projection clocks can be used by people who are blind or have limited vision . There are also clocks for the blind that have displays that can be read by using the sense of touch . Some of these are similar to normal analog displays, but are constructed so the hands can be felt without damaging them . Another type is essentially digital, and uses devices that use a code such as Braille to show the digits so that they can be felt with the fingertips . </P> <P> Some clocks have several displays driven by a single mechanism, and some others have several completely separate mechanisms in a single case . Clocks in public places often have several faces visible from different directions, so that the clock can be read from anywhere in the vicinity . Of course, all the faces show the same time . Other clocks show the current time in several time - zones . Watches that are intended to be carried by travellers often have two displays, one for the local time and the other for the time at home, which is useful for making pre-arranged phone calls . Some equation clocks have two displays, one showing mean time and the other solar time, as would be shown by a sundial . Some clocks have both analog and digital displays . Clocks with Braille displays usually also have conventional digits so they can be read by sighted people . </P>

Who invented the first mechanical clock in the renaissance