Compared to the old ways of measuring time - gnomons, meridians, rings, etc - mechanical clocks, developed mainly between the XVIth and XIXth centuries, offered significant advantages.
According to Derek de Solla Price the first mechanical clocks appeared in Italy and England in the XIIIth century. They were the work of the Paduan Giovanni de Dondi (1318-89) - who between 1348 and 1364 built the first astronomical clock - and the abbott Richard of Wallingford (c. 1292-1336), the main aim of which was to follow the movements of the Sun, the Moon and the planets, for purposes of forecasting and learning.
The oldest mechanical clocks worked by the drop of a weight attached to a cord, that acted in opposition to a braking mechanism. The advent of the pendulum, introduced in practice by Christiaan Huygens (1629-1695) and described by him in his 1673 work Horologium Oscillatorium, led to a great improvement in accuracy. Irregularities dropped from several minutes a day to just a few seconds.
The detailed study of the isochronism of the pendulum produced by its cycloidal trajectory led Huygens and successive craftsmen to design a pendulum that struck the seconds, longer and slower (and hence subject to less perturbations), gaining still further in accuracy. It was in this way that the long-case regulator clocks developed, indispensable instruments in astronomical observations for almost two centuries.
The invention, again by Christiaan Huygens and Robert Hooke, of the hairspring - a very thin spiraled recoil spring attached to the balance wheel which powered the mechanism in place of the pendulum weight - encouraged the development of portable clocks.
Of all the instruments that have been used in the Bologna Specola, it is the clocks that have been most depleted in number. The 1983 work of E. Baiada and A. Braccesi (op.cit.) lists those clocks that today are no longer with us but which were mentioned in the inventories or observation records.