Harrison’s precision longcase clocks

During the 1720s John Harrison, working with his younger brother James, built a number of longcase clocks incorporating numerous improvements which gave them such a high level of accuracy that they have become known as his ‘precision’ clocks. His three chief innovations were the grasshopper escapement (an unusual, low-friction escapement) the use of a self-lubricating wood, lignum vitae, for the entire movement, and the compensating pendulum, designed to counteract the effects of expansion and contraction caused by temperature variations.

Compensating pendulums had been constructed before, notably the mercury pendulum created by the Cumberland clockmaker George Graham in 1720, but Harrison’s system achieved a higher level of precision. He used a gridiron composed of thin strips of brass and steel to support the weight, rather than the conventional single metal rod. The different expansion rates of the different metals used in the strips cancelled out the effects of thermal expansion and contraction.

The perfection of such mechanisms required lengthy experimentation, observation and adjustment, which Harrison carried out in his own house, where his workshops were located. Thus, to ensure that the temperature compensation system worked accurately Harrison placed identical clocks in two rooms, one of which was kept very hot and the other very cold, and carried out a series of adjustments until the variations created by the different temperatures had been eliminated.

Harrison’s early clocks were provided with an ‘Equation of Time’ table pasted inside the case. These tables gave the times of sunset and sunrise throughout the year alongside data giving the difference between the variable ‘solar time’ shown by a sundial and the uniform ‘mean time’ given by a mechanical clock. Using this information, the owner of the clock could set it accurately using a sundial. The surviving example of these type of table is in Harrison’s handwriting, suggesting that he compiled it himself. This indicates not only the importance he placed upon the precision and accuracy with which his clocks worked, but also his mastery of a considerable body of detailed astronomical knowledge.

(From Keeping Time a temporary exhibition at Fairfax House, 5th October-31st December 2012)