A mechanically operated device in which the revolutions of the Earth and the other planets around the sun are represented on a dial or dials; they may be hand-operated or clock-driven. Richard of Wallingford's astronomical clock of 1336 was also a planetarium, for although the part of his manuscript describing it is missing, John Leland, who saw the clock during the reign of Henry VIII, states that the planets were depicted. The first planetarium of which we have full knowledge was the machine designed and made by Giovanni Dondi of Padua between 1348 and 1364. The weight-driven clock movement was contained in a seven-sided bronze vertical frame above which were mounted seven main dials on which were displayed the movements of the sun, moon, and five other known planets, moving in epicycles according to the Ptolemaic system. In 1561 Eberhart Baldewein of Kassel made to the order of Wilhelm IV of Hesse a somewhat similar planetarium mounted in a square vertical gilt-metal case, with planetary dials on all four sides, still on the geocentric principle. This fine example had a celestial globe on top. The first English planetarium after Wallingford's was one built nearly 350 years later by Samuel Watson of Coventry. It was ordered by Charles II in 1681 and completed by Watson six years later; this also is geocentric. A relatively simple but effective heliocentric planetarium was made for Christiaan Huygens in 1681 by Johannes van Ceulen of the Hague. It is spring-driven and controlled by a balance with spiral balance-spring, as invented by Christiaan Huygens in 1675. Another elaborate planetarium was made in England by Henry Bridges. It has a geocentric upper dial and a planetary heliocentric lower dial which shows the eccentricity of the planetary orbits, the latitude of each planet and the position of its perihelion and aphelion. During the present century elaborate and effective planetaria employing optical projection have been made, mainly by the firm of Carl Zeiss, for showing to large audiences. Spectators sit below a dark dome on which images of stars, sun, moon and planets are projected from an instrument situated at the centre of the dome, with very realistic effect.