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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.
Acknowledgements
I am indebted to Jan Titus and Bruce Carr of the New Zealand Historic Places Trust for their support throughout the work, notwithstanding the devastating earthquake and aftershocks that almost destroyed the Lyttelton time ball apparatus in 2011. I am grateful to Alan Marshall for permission to use photographs that he took during restoration of the Edinburgh time ball apparatus. I also thank the Science and Technology Facilities Council and the Syndics of Cambridge University Library for permission to use material from the Royal Greenwich Observatory archives in this paper.
Notes
1 Roger Kinns, ‘The Early History of the Edinburgh Time Ball and Time Gun’, International Journal for the History of Engineering & Technology, 81.2 (July 2011), 294–319.
2 Alan Marshall, ‘Nelson Monument, Calton Hill, Edinburgh’, Architectural Heritage Society of Scotland Magazine (2009).
3 Ibid.
4 Charles Piazzi Smyth, ‘Notice of the Time Ball at the Royal Observatory at Edinburgh’, Select Papers of the Society of Arts, iv (1856), 191–96. ‘Given at the request of the Council on 12 December 1853. A working model and diagrams were exhibited.’
5 Ibid.
6 Maudslay, Sons & Field, letter to G. B. Airy, 6 November 1856, Royal Greenwich Observatory (hereafter RGO) 6.613, Section 6, Leaf 64.
7 Henry Wood ‘Time Ball on Semaphore at H.M. Dock Yard Portsmouth: Answers to Questions submitted by Professor Airy’, 4 September 1856, RGO 6.613, Section 7, Leaves 72–73.
8 H. C. F. C. Schjellerap to G. B. Airy, 2 February 1857, RGO 6.145, Section 8, Leaf 89.
9 G. B. Airy, letter to H. C. F. C. Schjellerap, 10 February 1857, RGO 6.145, Section 8, Leaf 90.
10 Kinns (2011).
11 Derek Howse, Greenwich Time and the Longitude, Millennium Edition (Philip Wilson Publishers Ltd, 1997), pp. 100–01.
12 H. C. Russell (Sydney), letter to Sir Charles Todd (Adelaide), 21 March 1899.
13 G. B. Airy, Letter to H. C. F. C. Schjellerap, Copenhagen (as n. 9).
14 Roger Kinns, ‘Time-keeping in the Antipodes: A Critical Comparison of the Sydney and Lyttelton Time Balls’, Journal of Astronomical History and Heritage, 12.2 (July 2009), 97–107.
15 ‘The Time-Ball’, The Scotsman, Saturday 19 November 1853, p. 2; Kinns (2011).
16 Kinns (2009).
17 Russell.
18 Bruce Carr (Christchurch, NZ), email to Roger Kinns, 19 November 2013.
19 Andrew Laing (Edinburgh), email to Roger Kinns, 2 November 2010.
20 Kinns (2009).
21 Discussion with Jan Titus (Lyttelton time ball manager) in Christchurch, NZ on 29 October 2013.
22 Newspaper article, probably 1970s (courtesy of Bruce Carr, Christchurch, NZ, 31 October 2010).
23 ‘Museum assembles city’s lost icons’, The Press (NZ), 4 February 2012: <http://www.stuff.co.nz/the-press/news/christchurch-earth quake-2011/6365035/Museum-assembles-citys-lost-icons> (accessed 29 May 2014).
24 Editorial, ‘The Electric Time Ball in the Strand’, The Times, 12 August 1852.
25 Gareth Wright, A Compilation of World Time Signals, Past and Present, rev. edn (New Zealand Historic Places Trust — Lyttelton Time Ball Station, 12 November 2007), p. 48.
26 Edwin Clark, ‘The Electric Telegraph Company’, letter to the Editor, The Times, 8 September 1852.
27 Howse.
28 ‘New Time-Ball at the Royal Observatory, Cape of Good Hope’, Notice signed by Thomas Maclear. RGO 6.615, File 7, Leaf 148.
29 Thomas Maclear, letter to G. B. Airy, 16 June 1863, RGO 6.615, File 7, Leaves 145, 6; letter to G. B. Airy, 18 June 1863, RGO 6.615, File 7, Leaf 147.
30 Sandys & Co., letter to G. B. Airy, 31 August 1863. RGO 6.615, File 7, Leaves 151–52.
31 Russell.
32 Roger Kinns and Lesley Abell, ‘The Contribution of Maudslay, Sons & Field to the Development of Time Balls in Australia’, International Journal for the History of Engineering & Technology, 79 (2009), 59–90.
33 Nick Lomb (Sydney Observatory), email to Roger Kinns, 21 December 2010.
34 Nick Lomb (Sydney Observatory), email to Roger Kinns, 14 February 2011.