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 as the starting 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 triangulation 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 Triangulation, 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 valuation. 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 scale. 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. This led to the employment of civilian surveyors to advance the progress of the map, and it was found necessary at great additional expense, to revise and correct these contract plans. The work did not possess the accuracy demanded by the admiralty in forming the basis of their coast surveys for the Geological Survey or the civil engineers. As a military map its publication during war was suspended, and its continuance became a matter of doubt in time of peace.