TIMEKEEPING

SUNDIALS

Horizontal sundials are the most common type of timekeeping device in the middle ages. They consist of a flat disc of stone or other material marked with the hours of the day. The central gnomon casts a shadow onto the surface of the dial, the angle of the sun indicating the hour of the day. Because of this, the length of hours in medieval times was not fixed; they were always one twelfth of the time from sunrise to sunset, and thus longer in summer than in winter. This made it very easy to read sundials, but created problems with other types of time-keeping device.

A less common, but more portable, type is the vertical sundial. A flat plate, usually made of metal, is suspended vertically by a chain and is graduated with lines to mark the hours of the day. The gnomon projects horizontally, and the device measures the length of the shadow rather than the angle at which it falls.

Sundials were still widely used in the Renaissance. By this time 'pocket sundials' had also been devised. These consist of a wide ring attached to a chain from which it was hung while in use. There is a small hole in the upper surface of the ring, and when hung facing in the appropriate direction, the sun produces a pin-prick of light on the inner surface of the ring, which is marked with a graduated scale showing the hours according to the angle of the sun.

Sundials have an accuracy of about half an hour.

WATER CLOCKS

The most basic type of water-clock, or clepsydra, is a bowl with a hole in the bottom, through which water flows. By measuring the height of the water remaining in the bowl, a rough estimate of the time can be made.

By the middle ages the constant-flow clepsydra had been devised. The bowl is replaced by a cylinder with a leak-hole lined with a non-corroding metal (usually gold). A second cylindrical container is placed below the leak-hole. The lower cylinder fills with water flowing out of the upper one, and contains an arm and pointer attached to a float. The vertical arm is held on a ratchet to keep it steady and the pointer gives a reading on a vertical scale on a wall by the device.

In order to ensure a steady flow of water, the upper cylinder is fed from a stream or similar source, thus ensuring that the water at the bottom is under a stable pressure. Of course, the lower cylinder must be emptied out when it fills (usually once a day) or it will overflow.

Water-clocks were no longer in any great use by the Renaissance. Like sundials, they have an accuracy of about half an hour.

MECHANICAL CLOCKS

The basic mechanical clock was devised in the early to mid fourteenth century and contains a crown-wheel attached to a descending weight. Projections from a rod, or verge, engage alternately with teeth on the far side of the crown wheel from the weight. The verge is prevented from oscillating by a balance-rod with equal weights at either end, which is attached to it at right angles.

As the crown-wheel rotates, the balance-rod swings round and is stopped, then reverses. This releases the engaged tooth of the verge from the crown-wheel. The crown-wheel rotates further, and a second peg on the far side of the verge engages another tooth, thus reversing the balance-rod and starting the process again.

The verge (also called an escarpment or going-train) is linked to the single hand by a system of gears, and serves to ensure that the clock ticks regularly rather than moving continuously. A later innovation is to have the verge attached to a second wheel notched at various intervals, which controls the number of strikes on a bell. A set of revolving blades act as an air-brake and limit the speed at which the bell is struck.

By the end of the fourteenth century, clocks could also be built which struck a bell at a particular time (useful for waking up priests for their morning worship, for example). Every hour, a lifting device releases a second crown-wheel which is then moved by its own driving-weight and thus operated a second verge-staff with a hammer on the end of it which strikes a bell. A stud on the second crown-wheel stops it after a single revolution and resets the alarm.

Another late fourteenth century innovation was to build clocks with a third train which could strike a bell every quarter hour, giving an accuracy greater than any sundial or water-clock. Other developments consisted of clocks which could simulate the movement of the planets, or control automata.

In the fifteenth century, the driving-weight was replaced by a spring, making the clock small enough to carry. Later, a spirally grooved spindle called a fusee was added which equalised the force transmitted to the gear-train, and thus counteracted the diminishing force of the recoiling spring. A kidney-shaped cam called a stack-feed served the same purpose as it was pressed by a revolving wheel. These improvements meant that such clocks only lost about six minutes a day.

In the late fifteenth century clocks were being built with a second dial. Initially this marked out the quarter-hours, but by the early sixteenth century, it was even possible for the second dial to actually show minutes!