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Following the Mammystown / Daddystown story it transpired that Divis Mountain has become a National Trust property just recently, making it open to the public for the first time.

Divis is not far away, about 3 miles southwest of the sites of the cottages, possibly even intervisible. (?) What an extension to the Cavehill ! Both Cavehill and Divis have been publicly much - used over many years, still, this is a confirmation.

The same report stated that, to celebrate the event, a light was displayed on the summit of Divis - - - marking the position of the Limelight set over the trigonometrical ( phew! ) station which was / is part of the Ordnance Survey of the British Isles. Divis was first " occupied " for these observations in 1825. This was very interesting, for here, was the physical tie of the locality - - - Mammystown / Belfast to the rest of these islands, and the world - - - begun 180 years ago. There were no cars, lorries, no radio, electricity was not yet discovered, no electronic distance - measurers or power tools. There were horses, donkeys, wooden carts, picks and shovels.

The Limelight on Divis was observed from surrounding stations on numerous mountain-tops - - - in Ireland, Scotland, and the Isle of Man. The Limelight was a device invented by Lieutenant Drummond, Royal Engineers. It was the brightest (artificial) light of the early 1800's, subsequently adopted for " the stage " - - - hence the term " in the limelight ."

The Limelight would be interchanged with the Great Theodolite, a massive survey instrument of 36 inch diameter, weighing 200 pounds. The Limelight was also replaced by a heliotrope ( mirrors ) for day-time work. All of this was subject to the vagaries of the Irish weather, for visibility was critical, work could be halted for days, if not weeks.

Antique Theodolite

The Great Theodolite was the Space-Age technology of the 1700's and 1800's. Three years to make, by Jesse Ramsden in London, it observed over 200 stations from Divis - - - including - - - Slieve Snaght in northwest Donegal, 86 miles away. Slieve Donard 30 miles, Knocklayd 38 miles. Goat Fell on Arran Island 77 miles. A mountain in SW corner of the Isle of Man, about 64 miles. There were many more !

It is said the Great Theodolite was transported in a 4 - wheeled spring carriage, drawn by 2, sometimes 4 horses. A 16 foot portable scaffold came along in a second carriage, to gain height if needed. It was the first survey instrument with the precision to detect spherical excess - - - or curvature of the earth.

Two such Great Theodolites were used in the Principal Triangulation of the British Isles, the Divis one was the second, the earlier instrument survived until 1940, when it was destroyed in the Blitz at Ordnance Survey Headquarters, Southampton. The second ( Divis ) one may still be seen at the Science Museum, London.

The light over the trig. point on the summit of Divis Mountain was a suitable way to mark the acquisition by the National Trust.

From " Ordnance Survey Map Makers to Britain since 1791" by Tim Owen and Elaine Pilbeam. And " A History of the Ordnance Survey ", Edited by W.A.Seymour.

Relevant web links

YP&M Cavehill article