This portable timer was purchased by the Royal Observatory circa 1825. In principle it enables the user to measure short intervals to an accuracy of one three-hundredth of a second.

Its maker, William Hardy, was awarded the Gold Vulcan prize of thirty guineas by the Society for the Encouragement of Arts, Manufactures and Commerce (now known as the Royal Society of Arts) for this design. In his description of the timer, published in volume XLIII of the Society’s transactions, Hardy mentioned its use in ballistic studies by artillery officers at Woolwich. At the Royal Observatory a timer such as this would have been very useful for timing star transits.

The movement and dial are enclosed within a brass drum case with a screw bezel and domed glass. The keyless wind and hand-setting knob are located on the underside with three brass feet to allow the timer to be set firmly on a table.

The spring-driven movement has a going barrel and cylinder escapement. The escape wheel has unusually square-formed teeth, which beat five times per second. For each tick, the centrally placed seconds hand will advance by one fifth of a second along the outermost scale.

Further division of the second is made possible by means the semi-circular scale. The blued-steel hand is attached directly to the balance staff and so does not make a full rotation, instead, it wags back and forth with the motion of the balance for each beat. The timer is stopped by a sprung blade that makes contact with the balance rim; as such the hand will indicate the point at which the balance was arrested on the 0-60 semi-circular scale. This fine division scale is a good indicator but is not entirely accurate because the motion of the balance is not simple: as the balance vibrates back and forth it speeds up and slows down and so the linear division of the scale cannot give a true division of the time.