under the consideration of Government to make a map of the whole island from actual survey, to which the map just mentioned was to be made subservient; the execution of the whole was to be committed to General, then Colonel Roy, whose experience, acquired in the Scottish survey, had been improved by as constant an exercise in the operations of practical geometry and astronomy as the duties of his profession would admit, and whom his love of such pursuits, and his indefatigable ac tivity, pointed out as eminently qualified for this service.

Circumstances, however, which it is easy to conceive in general, but which it would be useless to know in detail, prevented any step from being taken toward the execution of this design, till after the peace of 1783, when a memoir drawn up by Cassini de Thury was presented to our Government by the French ambassador, setting forth the advantages that would accrue to geography and astronomy from carrying a series of triangles from Greenwich to Dunkirk, (to which place the meridian of Paris had already been extended,) so that the relative position of the two most celebrated observatories in the world might be ascertained by actual measurement.

This memoir having been communicated by the secretary of state (Mr Fox) to Sir Joseph Banks, and the plan proposed in it having received the ap

probation of the Royal Society, the execution of it was committed to General Roy, who was at that moment engaged in a survey of London and its environs, for the purpose of connecting together the different observatories in and about that metropolis; a work which, with his usual ardour and activity, he had undertaken for his own amusement.

As a series of triangles was now to be extended from about Greenwich through Kent, and across the Channel to Calais and Dunkirk, the first thing to be done was to measure a base, from the length of which the lengths of the sides of all the triangles might be inferred. Such a line was accordingly traced out on Hounslow-heath, extending from a point near Hampton Poor-house, to a place called the King's Arbour, a distance of more than five miles, which was measured with the most scrupulous exactness. The description which General Roy has given of this measurement, deserves the attention of every one who is concerned in the operations of practical geometry, and who wishes to be made acquainted with the utmost resources of his art. He will perhaps see with surprise, that many of the things which he is accustomed to do with very little expence, either of time or of thought, require, when they are to be done with precision, no small proportion of both; that to make two rods exactly of the same length, to place them in the same straight line, and to make the beginning of

one coincide with the end of another, demand much skill and patience; in a word, that the most common matter, when executed with extreme accuracy, becomes difficult; and that science and art must combine to discover and to remove those minute obstacles, of which the bulk of mankind do not even suspect the existence.

The measurement of the base was first undertaken with deal rods of twenty feet in length; but though these were made of the best seasoned timber, from an old mast cut up on purpose, though they were perfectly straight, and secured from bending in the most effectual manner, yet the changes in their lengths, produced by the moisture and dryness of the air, were so considerable, as to take away all confidence in the results deduced from them. Glass rods were therefore substituted in their room, consisting of straight tubes twenty feet in length, enclosed in wooden frames; and these had the advantage of being susceptible of alteration only from heat or cold, according to laws which could be accurately ascertained. The base measured with these rods was found to be 27404.08 feet precisely, or 5.19 miles.

We refer for the particulars to the account itself, where General Roy has described the apparatus used, and the precautions taken to ensure the suceess of the operation. The detail, though minute, is interesting, and must be highly instructive to

those engaged in operations any way similar. We would particularly recommend his description of the deal rods, the method of laying off their lengths, of the stands for supporting them, of the boning telescope, &c.

As the measurement of lines by a chain is, however, much more convenient and expeditious than by any other means, it was thought desirable to ascertain how far the accuracy of such a measurement could be depended on, and how near, in the present instance, it might approximate to that by the glass rods. For the purpose of this experiment, Ramsden had prepared a chain of the very best construction, made of hardened steel, one hundred feet in length, and jointed somewhat like a watch-chain. General Roy having measured a part of the base with this chain, and with the glass rods at the same time, found that the results differed by a quantity wholly inconsiderable. Several years afterwards, the whole base was measured with the steel chain; and the difference between that and the measurement by the rods was found not to exceed two inches and three quarters of an inch, a difference on the length of five miles that is plainly of no account. Hence it was inferred, that measurements made with such a chain as has just been mentioned, and with due precautions, viz. stretching it always in the same degree, supporting it on troughs laid horizontally, allowing for change of

temperature, &c. are as much to be relied on as those made in any other way whatsoever. This experiment, therefore, involved the determination of a material question with respect to the conduct of all future surveys. *

General Roy was assisted in these operations by Mr Isaac Dalby, a mathematician of eminence, and now Professor of the Mathematics in the Military College at High Wickham. A party of soldiers was also attached to the survey, for the purpose of doing such parts of the work as were merely laborious, and had a small encampment on the heath. The performance of this great experiment, for so it may very properly be called, could not fail to draw the attention of the men of science about London. The Master-General of the Ordnance, the President of the Royal Society, the Astronomer-Royal, and many other distinguished persons, frequently witnessed the skill and attention employed in conducting it. The mensuration of the base (including the repetition of it, and several collateral matters, as well as delays from bad weather) took up from the middle of June to the end of August 1784.

*

The extremities of the base were then

It may be proper to remark, however, that Legendre, after having considered the method of the steel chain, seems still to prefer that by rods of metal, because of the difficulty of giving the chain always a sufficient and uniform degree of

tension.