Rockets For Longitude

 

But what really endears Robinson to me was the way he set out to find out where he was.  At the 1834 meeting of the British Association for the Advancement of Science in Edinburgh, and at the 1838 meeting in Newcastle, he and others agreed to measure very accurately the longitudes of Cambridge, Oxford, Dublin and Armagh, basically to check the maps.  I find it hard to think about this – we now assume that our maps are dead accurate – but 150 years ago there was some uncertainty about the exact positions of places.  So they wanted to measure their longitude, and they wanted to do it with two varying and different methods.

 

They persuaded the famous clockmaker, Mr E J Dent, to visiot them with 15 chronometers.  They were all synchronised at Greenwich; then Mr Dent carried them in a wooden box on his knee, 500 miles by train in England, 275 miles at sea by steamship, and 190 miles by stagecoach in Ireland.  The whole journey took only a week, being performed as Robinson remarked, “With the marvellous rapidity of modern improvement.”

 

But he also wanted to use direct observation; so his second method was to set up a signal that could be seen from both Armagh and Dublin at the same time.  Then they could be sure that their clocks were exactly synchronised.

 

There isn’t a mountain high enough to be visible from both places, but the highest mountain between Armagh and Dublin is Slieve Gullion, and from Slieve Gullion they could fire rockets high enough for the explosions to be visible from 18 miles away in Armagh and 51 miles away in Dublin.  He sent up his son to organise the rockets, with assistance from Lieutenant Thomas Larcom of the Ordinance Survey, who provided them with maps, tents, and most important, rockets.

 

They set up camp on the mountain on 13 May 1839.  The Rector of Forkhill provided remarkable hospitality, even for Ireland, including food and two policemen, in case of trouble.  The only trouble came from the weather; as it snowed, and for two whole nights that week they couldn’t fire any rockets.

 

Robinson sat in the observatory in Armagh; his friend William Rowan Hamilton sat in Dublin, and they both had to measure as accurately as possible the time of the rocket bursts.  Robinson also trained three assistants, by getting them to write down the times of shots from a flintlock pistol.

 

Robinson sat in the dark by his telescope, and every now and then looked at the precise grandfather clock, by the light of a candle, and he counted the seconds in his mind.  “Five past one, and ten seconds, eleven… 12… 13… 14… 15… 16… 17 FLASH – there it goes – about 17.4 seconds – write it down.”

 

They started firing rockets at 10 o’clock at night – obviously they could see the signal better in the dark.  They fired 15 rockets the first night, at five-minute intervals, 13 the next night, and more at the end of the week.  Each rocket provided one precise observation in Armagh, and one in Dublin.

 

They both knew their local time accurately, by observations.  After watching the rocket explosions they could synchronise their clocks, and measure precisely the difference between local time in Dublin and local time in Armagh.

 

The result of this heroic set of experiments was completely to revise the difference in longitude between Dublin and Armagh from 1 minute 14.220 seconds to 1 minute 14.258 seconds.  In other words, by letting off several pounds of high explosive into the night sky, Thomas Romney Robinson showed that Armagh was further west of Dublin than anyone had suspected by a distance of nearly 50 centimetres!