Abstract
For 160 years, the weight of the 1.5 m diameter Edinburgh time ball has been given as 15 cwt (762 kg). This stems from a remark in an 1853 paper by Charles Piazzi Smyth, that appears never to have been questioned. The descent of the ball, with its attached shaft, rack and piston has to be arrested by compression of air in a vertical cylinder having a diameter of 0.2 m. Air is bled from the cylinder during the ball descent in order to damp rebound motion and allow a gentle recovery to the rest position after a fall of 3 m. It is shown by reference to correspondence and announcements by the Astronomer Royal and other nineteenth century authorities, as well as considerations of the forces needed to raise the ball and arrest its descent with the air cushioning cylinder, that the actual weight of the original Edinburgh time ball is likely to have been less than 100 kg. Particular reference is made to the 1873 time ball apparatus that was supplied to Lyttelton, NZ, which has much in common with the Edinburgh arrangement.
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