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XXIX. On the Figure, Dimensions, and Mean Specific Gravity of the Earth, as derived 
from the Ordnance Trigonometrical Survey of Great Britain and Ireland. Com - 
municated by Lieut.- Colonel James, R.E., F.R.S. 8^c., Superintendent of the Ord- 
nance Survey. 
Received April 30, — Read May 8, 1856. 
i. HE Trigonometrical Survey of the United Kingdom commenced in the year 1784, 
under the immediate auspices of the Royal Society; the first base was traced by 
General Roy, of the Royal Engineers, on the 16th of April of that year, on Houns- 
low Heath, in presence of Sir Joseph Banks, the then President of the Society, and 
some of its most distinguished Fellows. 
The principal object which the Government liad then in view, was the connexion 
of the Observatories of Paris and Greenwich by means of a triangulation, for the pur- 
pose of determining the tiifference of longitude between the two observatories. 
A detailed account of the operations then carried on is given in the first volume 
of the ‘Trigonometrical Survey,’ which is a revised account of that which was first 
published in the ‘Philosophical Transactions’ for 1785 and three following years. 
At the time when these operations were in progress, the Suiwey of several counties 
in the south-east of England, including Kent, Sussex, Surrey, and Hampshire, was 
also in progress, under the direction of the Master Geneial of the Ordnance, for the 
purpose of making military maps of the most important parts of the kingdom in a 
military point of view; and it was then decided to make the triangulation which 
extended from Hounslow to Dover the basis of a friangulation for these surveys. 
It is extremely to be regretted that a more enlarged view of the subject had not 
then been taken, and a proper geometrical projection made for the map of the whole 
kingdom. As it is, the south-eastern counties were first drawn and published in 
reference to the meridian of Greenwich, then Devonshire in reference to the meridian 
of Butterton in that county, and thirdly the northern counties in reference to the 
meridian of Deiamere in Clieshire ; but there is a large intermediate space, the maps 
of which ai’e made of various sizes to accommodate them to the convergence of the 
meridian. 
In 1799 the Royal Society gave further proof of the interest it took in the pro- 
gress of the Survey, by lending to tl)e Ordnance its great 3-foot Theodolite, made by 
Ramsden, for the purpose of expediting the work of the Survey; and I have great 
pleasure in stating, that although this instrument has been in almost constant use 
for the last sixty-seven years, during which time it has been placed on the highest 
MDCCCLVI. 4 L 
