The Ordnance Survey of Great Britain. 257 
a view of the country from above at a range of 5,000 feet or 
nearly one mile, and a town plan, an image at 1,000 feet or a pos- 
sible view from a series of Eiffel towers. 
This suggestion of an observer stationed in a balloon will not 
have been valueless if it draws attention to the fact that vastly 
more information is given on the map than it would be possible 
for any single observer to discover from an elevated station with 
an unobstructed view, the map being the compilation of the re- 
sults of hundreds of observations by many workers, and that its 
scale and the amount and character of the detail shown have been 
specially designed to meet definite ends. 
It is beyond the limits of the paper to enter into the theory or 
practice of surveying, or to say more than a few words of the 
delicate and refined operations necessary in carrying out the 
geodetic or trigonometrical work of a national survey which 
binds together the many parts to make a complete whole. 
The principal triangulation of the British Isles was begun in 
1784 and finished in 1852. Two magnificent 3-feet theodolites 
made by Ramsden, one for the Royal Society, the other for the 
Master General of the Ordnance, an 18-inch theodolite also by 
Ramsden, and 2-feet theodolite by Troughton and Simms were 
used in these observations. 
In the principal triangulation of Great Britain and Ireland 
there are 218 stations, at 16 of which there are no observations, 
the number of observed bearings is 1554—and the number of equa- 
tions. of condition, 920. 
In order to avoid the solution of this enormous number of 
equations, containing 920 unknown quantities, the network cov- 
ering the kingdom was divided into a number of blocks, each pre- 
senting a not unmanageable number of equations of condition. 
These calculations, all in duplicate, were completed in two 
years and a half, an average of eight computers being employed. 
Many of the sides of the principal or primary triangulation are 
of great length, 66 of them exceeding 80 miles, while 11 measure - 
more than 100 miles, the longest being 111 miles, that from Sea 
Fell to Sheir Donard. So great, however, had been the accuracy 
of the observers’ work, that the average amount of correction 
of the observed angles was no more than 0.6, and the measured 
length of the Salisbury base differed from its length as com- 
puted from the Irish Base, 350 miles distant, by a difference of 
only five inches. 
