great theoretical and mechanical achievement which
the Borghesi clock represents has been most adequately covered
elsewhere.[3] Consideration of the development of equation and
astronomical clocks is required here only for the purpose of relating
the Borghesi timepiece with the other significant developments in this
branch of horology.

The invention of the anchor escapement in about 1670, and the consequent
greater accuracy in time-telling, led to increased preoccupation with
precision. Daily differences in time as recorded by sundials and clocks
became more noticeable. Finally, in the second half of the 17th century,
some attempt was made to construct mechanical clocks combined with
sundials as well as astronomical clocks.

With the improvement of precision time-telling, it became necessary to
reconcile the actual difference between true and mean time. Although a
great variety of time-equation tables were produced, there was a
considerable margin for error in their use. This led to the construction
of mechanical clocks in which the equation of time was automatically
accomplished. A few were produced late in the 17th and early 18th
century at considerable cost and, consequently, with little popularity.
Equation sundials were also developed which were elaborately ingenious,
but they were not completely practical. Inevitably, they were supplanted
by the mechanical equation clock.

Probably the first documented mention of an equation clock is in the
diary of John Evelyn who recorded that in 1666 he visited the Royal
Society where he witnessed a curious clock, which showed the equation of
time, being presented by a certain Mercator. More data on the subject
appeared in the first two decades of the 18th century, when Henry Sully,
Joseph Williamson, Daniel Quare, and Thomas Tompion--who were among the
foremost English clockmakers of all time--produced elaborate examples of
these timepieces. Another significant maker was Dowe Williamson, who
became Court Clockmaker to Emperor Charles VI of Austria. In London,
Joseph Williamson produced some of the finest astronomical timepieces of
this type that have been known. The interest in the subject next shifted
to France where many fine examples were produced during the first half
of the 18th century.

Just after the middle of the 18th century, the subject of astronomical
clocks suddenly became a major horological preoccupation in another
region, namely, Austria, where the work in