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Home arrow Historical arrow Technology arrow Signalling arrow A General's View

A General's View Print E-mail
Written by Administrator   
Tuesday, 25 November 2008
Line of Sight The 22nd November historical reconstruction of the Monte Socorro signals mast - and the good weather - enabled a direct test of  the effectiveness of Wellington's communication system.

Recent pruning of the trees at the rear of Quinta da Póvoa has given a clear line of sight to the summit of Monte Socorro and direct observation using a contemporary pocket telescope from General Sir Brent Spencer's room (Wellington's second in command) showed just how good it must have been. 

From Spencer's window 

The signal "257" - interpreted from one code book as "Victory!" - was clearly visible. 


The interesting point as to the distance at which a signal (or other activity) could be clearly seen using telescopes available at the time has been debated by  several authors. Appendix IV of the "Warre Letters" book contains some information in relation to observations during the seige of Almeida and also other more general points. Warre wrote on 29th August 1810:

"I hurried up to the Telegraph in front of Freixedas and from what I could see through the glass I had not a doubt of the event [the surrender of Almeida following the explosion], as I could see people coming in and out and the rampart crowded over the gate."

 Warre and other officers had apparently seen movements of men on the walls and outside the gate of Almeida at a distance of nearly eleven miles as the crow flies, and even with the sun behind them, which it would have been in the afternoon, that is a good long distance. It seems probable that 'the glass' referred to was a fixed telescope mounted on a stand at the telegraph point at Freixedas.

On "the Lines" the longest distance between telegraph stations (Monte Socorro to Ponte do Rol) is about 10km (6.2 miles) so seems well within the capabilities of a stand-mounted "glass" of moderate size and magnification.

In humans, 20/20 vision is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc which as a linear distance is just under 3cm at 100m.  At 10km, therefore, an object would need to be (in theory) at least 100 times bigger (3m) to be distinguishable by eye. The (approximately) 1m diameter signal "balls" separated by 1m (2m centre-to-centre) thus would seem to be easily distinguishable by a reasonably sized telescope of moderate magnification at this distance.

The "Spencer telescope" (a hand held instrument) has a 50mm objective, a 10mm eyepiece, and a total draw (four tubes) of 770mm. Its theoretical best resolution (Rayleigh limit) is thus 138/50 = 2.76 seconds of arc - quite sufficient for the task! (but that of course assumes "perfect" conditions. In practice Wellington and his generals had the "Gallopers " for back-up and bad weather...) The summit of Monte Socorro is about 1400m in a direct line from the General's window.

 There's more about telescopes and their resolution here, and about larger stand-mounted instruments here (opens a new window).

We also have a view from the summit

Last Updated ( Monday, 17 August 2009 )
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