248 National Geographic Magazine. 
There the matter remained in abeyance until, after renewed hos- 
tilities with France and Spain, peace was negotiated in 1783. 
The trigonometrical survey of Great Britain may be said to 
have been begun one hundred and six years ago. 
Astronomers of that day were desirous that the difference of 
longitude between the Greenwich and Paris observatories should 
be ascertained by trigonometrical measurement ; and under the 
auspices of the king and of the Royal Society, General 
Roy, R. E., in April, 1784, began the task by the measurement of 
a base line on Hounslow Heath which was to serve us the start- 
ing point of a series of triangles to be extended to Dover and 
across the channel. 
This work was carried out, a connection with the French trian- 
gulation being established in 1786. 
Soon after this the government decided on having a “general 
survey made of the entire kingdom, on the scale of one inch to 
one mile for military purposes, and General Roy’s triangulation 
in the southeastern counties became the basis of the Great Tri- 
angulation, which was gradually extended over the whole of the 
British Isles and finished in 1853. 
The one-inch survey was carried northward through England 
and Wales under the successive superintendence of artillery and 
engineer officers, and by 1824 had reached the southern borders 
of Yorkshire and Lincolnshire. 
At this time it became necessary that a survey of Ireland 
should be made on a large scale as a basis for general land valua- 
tion. On the recommendation of Colonel Colby, then director, 
the scale of six inches to one mile was agreed upon ; the work in 
England was suspended and the force transferred to Ireland. 
It appears from a report of Colonel Colby, in 1840, that the 
purposes for which the English and Irish surveys were designed 
were gradually developed and not all originally known. 
The principal triangulation, on which the survey of South 
Britain had been based, was partly designed for astronomical 
purposes, and partly for a map on small seale. 
The detail plans were commenced by officers of the Royal 
Engineers, partly for the purpose of practicing them in military 
drawing, and partly for the purpose of making plans for the use 
of the Ordnance. 
The publication of some parts of this map on the scale of one 
inch to one mile created a desire among the public to possess 
better maps than had formerly existed. 
