TrigonometricalSurvey

A third series branched towards the east, and united with those proceeding westward from the base on Misterton Carr; and a fourth series was carried through Lancashire, Westmoreland, and Cumberland, into Scotland, and connected with another series extending from Misterton Carr base, through Yorkshire and Northumberland, and the east part of Scotland, as far as the north side of the Firth of Forth. Thus, down to 1809, the survey of nearly all England, the south of Wales, and a part of Scotland, had been made. The arc of meridian between Dunness and Clifton was also extended northward to Barleigh Moor, about 3 miles north of Glasborough in Yorkshire; and another meridional length was determined between Dunness and Delamere Forest in Cheshire, about 5 miles north of Tarporley. The triangulation of Scotland was continued first along the east coast to the borders of Ross-shire, and was subsequently extended to the Shetland Islands. A series was also carried from the Cumberland triangles along the western coast, through Dumfriesshire, and to the summit of Ben Lomond, connecting all the remarkable points of Perthshire. From 1811 to 1816 the survey was all but stopped on account of the war. In 1817, a new base of verification was measured with the steel chains on Belhelvie Links, near Aberdeen, the length of which, after making the various reductions, was found to be 26,615.6509 feet. In 1818 and 1819, the principal triangulation of Scotland was proceeded with, but was suspended in 1820. In 1821, it was recommenced in the Shetland, Orkney, and Western Islands of Scotland, and was carried on in these districts in 1822. In 1823, the use of the large theodolites being required in order to proceed with the triangulation in South Britain, the principal triangulation in Scotland was suspended. In 1824, the scene of operations was transferred to Ireland, an accurate survey of that country being considered of more urgent importance.

When the Ordnance survey was first undertaken, it was intended to produce only a military sketch map of the south of England; but as the work advanced it came into favour as a road map, a travelling map, and a general geographical map. When the survey of Ireland was undertaken, it was thought advisable to secure some farther social advantages, especially to make it subservient to the proper assessment and collection of the grand jury cess and other local taxes. Accordingly a committee of the House of Commons, of which Lord Monteagle was chairman, having examined the question, recommended that a scale of 6 inches to the mile should be adopted for Ireland, as the 1-inch scale on which the English maps were constructed was not sufficiently large to admit of the insertion of the townland boundaries and other minute territorial divisions, which were required for the purpose of making a valuation. (A townland is the smallest of the territorial divisions, and is almost always co-extensive with the private estates. There are 60,760 townlands in Ireland, and 2160 parishes.) The original intention was, that the survey should be confined to an exact description of the contents and boundaries of the townlands; but as it advanced, it was found desirable to include the subdivisions of fields and homesteads; and that was accordingly done in the centre and southern parts of the island. The advantages of including the subdivisions of fields were found to be so great that they were afterwards extended to the northern parts.

In 1827, a new base was measured on the east shore of Lough Foyle, in the north of Ireland. Doubts having arisen respecting the accuracy attainable by means of steel chains, Colonel Colby adopted an ingenious compensating apparatus, consisting of two bars, one of iron and the other of brass, 10 feet in length, connected together at their centres, and having at each end an ingenious apparatus for measuring the degree of expansion or contraction of each. (An account of this apparatus will be found in the article FIGURE OF THE EARTH.) The whole line measured was nearly 8 miles in length (exactly 41,640.8873 feet); and it was calculated that the greatest possible error could not exceed two inches. It was also prolonged northward by triangulation to Mount Sandy, whereby two additional miles were given to it. From this base a series of triangles were extended all over the island, and were connected with those formerly observed in Wales, the Isle of Man, Scotland, and the Hebrides.

In 1838, the principal triangulation of Ireland having been completed, that of Great Britain was again taken up. All England, with the exception of the six northern counties, had been surveyed on the scale of 2 inches, and engraved and published on the scale of 1 inch to the mile; and a small portion of Scotland had been surveyed on a similar scale. It was found, however, that the thickly peopled manufacturing, mining, and colliery districts of the northern counties could not be properly represented on the 1-inch scale, and representations were made to the Treasury by various scientific societies and others to that effect. The matter being referred to the Duke of Wellington, the Treasury, in accordance with his advice, issued a minute, of date the 6th October 1840, stating that "My lords are satisfied, from the

consideration they have given to the subject, that the scale on which the English survey has hitherto been conducted falls much short of what is required by the existing circumstances of the country. That scale was originally fixed principally with a view to military considerations, while the demand of the present day is for such a national survey as shall be permanently useful in aiding the improvement of the country, by serving as a basis and guide in the formation of railroads, canals, and other public works, besides assisting in the development of the geological structure of the country, and promoting in various other ways the progress of science and statistical knowledge. . . . . My lords have no hesitation in giving their consent to the remainder of England and the whole of Scotland being surveyed on the Irish scale of 6 inches to the mile; and my lords think themselves fortunate in having their opinion on this important subject confirmed by the concurrence of so high an authority as that of his Grace the Duke of Wellington."

At this time the county of Wigton, and one half of the counties of Ayr and Kirkcudbright, had been surveyed and drawn for the 1-inch scale in Scotland. In 1840, therefore, the survey of Scotland was commenced anew for the 6-inch scale; but the work for some years proceeded very slowly. The old base on Salisbury Plain not being found to agree very accurately with that of Lough Foyle, it was resolved to re-measure the former with the compensation bars employed in the measurement of the latter. The re-measurement was effected in 1849, and either from the guns which marked the extremities of the base having shifted their position, or from the original measurement having been defective, the result obtained was about a foot in excess of the previous measurement. This difference in a length of nearly 7 miles may seem small, but it sufficed to remove the greater part of the discrepancy which had been detected between the English and Irish bases.

The general work of the survey was proceeding so slowly, that in 1851 only Lancashire and Yorkshire in England, and Wigtonshire, Kirkcudbrightshire, Edinburghshire, and the Isle of Lewis in Scotland, had been surveyed. In that year, therefore, a committee of the House of Commons, Lord Elcho being chairman, was appointed to inquire into the state of the survey, and in particular to determine whether a 1-inch or a 6-inch scale should be adopted. The committee, in their report, came to the unanimous conclusion, that the scale of 6 inches, and the system of contouring, ought to be abandoned, on the ground that the 6-inch map alone was not of sufficient public utility to justify the large expenditure of public money, and that the 1-inch map was better adapted to geographical purposes. They appear also to have been influenced by the belief that the latter would be much more speedily executed. Orders were accordingly issued by the Treasury and Ordnance, in conformity with this report, that the remaining four counties of England, and the rest of Scotland, should be done on the 1-inch scale. These instructions produced great dissatisfaction, both in Scotland and the unsurveyed parts of England, and numerous representations were made by various public bodies and others, in 1852 and 1853, to induce the Treasury to rescind the orders in favour of the 1-inch scale. Some of these memorials were referred by the Treasury to the Board of Ordnance in June 1852; and a determination was meanwhile come to by these departments, that the survey of Fife, which was then going on, should be conducted on the 6-inch scale, and that the question as to the scale of the rest of the country should be reserved for future consideration.

By this time the desirableness of even a larger than the 6-inch scale was agitated in influential circles, and in February 1853 Lord Elcho, then a lord of the Treasury, drew up a very able memorandum on the survey, and suggested a scale of 2½ inches as that best adapted for correct plans of estates, and for other civil purposes. This, with numerous letters on the subject, was transmitted, with a Treasury circular letter, dated 16th April 1853, to a large number of scientific societies and experienced persons in the kingdom, requesting them to state their opinion on the comparative merits of a 6-inch, as contrasted with any larger scale, for the purposes of a national survey. At the same time, an order was issued to the survey department to carry on the survey of Ayrshire and Dumfriesshire "with that degree of accuracy which would admit of the plans of the cultivated districts being hereafter drawn on the scale of 24 inches to a mile if desired." Of the replies to the circular letter, 20 were in favour of a 6-inch scale, and 120 in favour of a larger scale, among whom were the Registrar-general, the President of the Geological Society, Sir Henry de la Beche, Mr A. C. Ramsay director of the Geological Society, the President of the Geological Society, the Society for the Amendment of the Law, the Metropolitan Commissioners of Sewers, the Ecclesiastical Society, the Poor-Law Board, the Board of Supervision for Relief of the Poor, the Commissioners of Woods, the General Board of Health, the Statistical Society, the Statistical Conference, &c. The weight of authority being thus decidedly in favour of a larger scale, and generally for a scale, in the manuscript plans, of 24

Trigonometrical Survey. 1854.

or 26$\backslash frac\{1\}\{2\}$ inches to a mile of the rural districts, and 120 inches to a mile of the towns, and half of these scales for the engraved plans, a second Treasury letter was issued to the same parties on the 16th of January 1854, requesting them to state whether the 24-inch, 25-inch (properly 25$\backslash frac\{1\}\{2\}$ inches, or $\backslash frac\{1\}\{2\}$ of the linear measure of the ground, or 1 square inch to the acre), 26$\backslash frac\{1\}\{2\}$ inch (i.e., an inch to 3 chains) scale would be preferable for the rural districts, and whether the scale of $\backslash frac\{1\}\{2\}$ of the linear measure = 10 feet 6$\backslash frac\{1\}\{2\}$ inches to a mile would be preferable for the towns. The replies to this circular were submitted to a committee, consisting of Sir John Burgoyne, Mr Blamire, and Mr Rendel, who unanimously reported that the weight of evidence in the correspondence before them was in favour of the $\backslash frac\{1\}\{2\}$ of the scale for the rural districts, and $\backslash frac\{1\}\{2\}$ for the towns. A treasury minute was accordingly issued, of 15th July 1854, directing that the surveys of Ayrshire and Dumfriesshire should be drawn on the scale of $\backslash frac\{1\}\{2\}$ of the scale, and that, until a final determination should be arrived at, the same scale should be applied to the other districts. This order, however, contained no instructions as to the scale for the towns; and Colonel James, who had been newly appointed superintendent of the survey, addressed a letter to the Treasury, of date 26th August 1854, requesting instructions upon this head, and recommending the scale of $\backslash frac\{1\}\{2\}$. On 20th July 1854, a commission of eminent persons, Lord Ellesmere being chairman, appointed to consider whether the advantages of contour lines to the public are commensurate to the cost of their production, recommended that levels should be marked upon permanent objects, as churches, bridges, &c., and stated that such levels having been provided contour lines to a certain extent, would be a useful addition to a map on the 6-inch scale, and that contour lines selected from those obtained for the large surveys (if of a different colour) would be a useful addition to the 1-inch map. They further urge the extreme importance of expediting, in the greatest possible degree, the completion of the general 1-inch map. On 18th May 1855, a Treasury minute was issued, directing the uncultivated districts to be drawn on the scale of 6 inches to a mile; the cultivated districts on the scale of 25$\backslash frac\{1\}\{2\}$ inches to a mile; towns containing more than 4000 inhabitants on the scale of $\backslash frac\{1\}\{2\}$, or 126$\backslash frac\{1\}\{2\}$ inches to a mile, and that the 1-inch map should be proceeded with as rapidly as possible.

1855.

In March 1855, a select committee of the House of Commons was appointed to report on the ordnance survey in Scotland. It found that, as general attention had come to be more immediately directed to the survey, there had been a gradual progress of public opinion in favour of the larger scale of 25 inches to the mile. The Lord Advocate of Scotland gave valuable evidence as to the facilities which a 25-inch scale would afford in the transference of landed property, by superseding written descriptions, and introducing "a system of conveyancing infinitely more simple, as well as more secure, than anything we have hitherto had." The committee found that delays had been occasioned by the want of a definite idea of what sort of a survey is required, which had been productive of much unnecessary labour, confusion, and expense; and that the time had now arrived when it was "necessary for Parliament to consider and finally determine the comprehensive question, What is the nature of the survey they are prepared to sanction as the one which will most conduce to the general interest of the nation at large?" "Your committee," they continue, "have come to a decision in favour of the $\backslash frac\{1\}\{2\}$ of the scale for a cadastral survey of the country. In doing so they are influenced by the great advantages which may be anticipated from the prosecution and completion of such a national work, on principles which have been successfully acted upon in the surveys of France, Bavaria, Switzerland, and other countries in Europe." The difference of the cost of surveying between the 25-inch and the 6-inch surveys, they found, would be comparatively immaterial; and that, while it was possible to reduce, it was impossible to enlarge a parent map.

On 19th June 1855, a discussion took place in the House of Commons upon the report of the committee; and a motion by the Right Hon. E. Ellice, for a reduction in the amount to be voted for the survey, on the ground of his objecting to the 25-inch scale, was rejected by a majority of 91. On the 18th June 1857, another debate took place in the House of Commons, on the motion of Sir D. Norreys, when the house decided by a majority of 10, that the plans of the cultivated districts on the 25-inch scale should be discontinued, and that the survey should be carried out on the 6-inch scale only, as was done in Ireland, in Yorkshire and Lancashire, and in several counties of Scotland. A treasury letter was accordingly issued (2d July 1857), directing that the parish plans on the 25-inch scale be discontinued after the completion of those parishes then actually in progress, and that the 6-inch scale was to be adopted in the further progress of the survey.

Royal Commission, 1857.

On the 24th December 1857, a royal commission was appointed to inquire into and report upon—

1. The principal purposes which the national survey should subserve.

2. The progress which has been made in the survey, and the scales upon which the maps and plans have hitherto been drawn and published.

3. The change or changes in those scales, or in any of the details of the survey, which, according to your judgment, should be made.

4. The estimated cost of completing the surveys of England and Wales, Scotland, and Ireland respectively, on the scales and in the manner which you recommend to be adopted.

The commissioners found that the purposes which a national survey should subserve may be divided into two classes. "The first includes the wants of the State either for military purposes, for levying taxes or rates on real property, or in carrying into effect any legislative measure relating to land over which the State may exercise a direct superintendence, as, e.g., measures for the registration of the titles to lands, measures giving facilities to the transfer of landed property, and so forth." "The second class includes the wants of the public as individuals, where such wants are not confined to any particular section of the community, and where they cannot be satisfied by private enterprise." The principal among these is the necessity for a general geographical map of the country; next, the supply of commercial data, or definitions of the positions of well marked points by which private surveys may be facilitated, and surveys of limited districts may be connected as parts of one great plan; third, such surveys of special districts as are likely to produce in their contingent applications important public benefits, whether as aiding in other surveys, in tracing the course of railways or canals, in giving materials for plans in reference to water supply to towns, drainage, and so forth, or in promoting the sciences of geology, geodesy, and others. When the Tithe Commutation Act passed in 1836, the Ordnance Survey maps were found to be too small for the purposes contemplated in the Act, and hence a demand arose for first class plans of a scale as large as 20 to 26$\backslash frac\{1\}\{2\}$ inches to the mile. About two millions sterling are said to have been expended in procuring these plans, which were for the most part hastily and carelessly got up, so that many of them are imperfect and inaccurate, and only about one-sixth of them are described as being first class plans. They had, moreover, been constructed on no uniform principle, and could never be juxtaposed so as to form a national cadastre. The commissioners found further, that owing to the frequent alterations in the instructions issued to the superintendent, the survey was in a very unsatisfactory state as regards uniformity. Thus, all England, with the exception of the six northern counties, had been surveyed for and mapped on the scale of one inch to the mile only. Of the six northern counties, the whole of Yorkshire and Lancashire had been plotted and engraved and published on the 6-inch scale, while Durham, the greater part of Westmoreland, and the southern part of Northumberland, had been plotted on the 25-inch scale. In Scotland, the counties of Edinburgh, Haddington, Fife, Kinross, Wigton, Kirkcudbright, and the Island of Lewis, had been plotted for, engraved, and published on the 6-inch scale; the county of Linlithgow had been published both on the 6 and 25 inch scales; and the counties of Ayr, Dumfries, Renfrew, Peebles, Berwick, Lanark, Forfar, Roxburgh, and Selkirk, had all been plotted for the 25-inch scale, and some of those last named were in course of publication on the 25 and 6 inch scales. In Ireland the whole country had been plotted, engraved, and published on the 6-inch scale, and seven of the counties in which accurate details had not been taken in the first instance, have been revised and made complete for valuation and assessment purposes. Armagh alone remained for revision. In Ireland also, it had been necessary to enlarge the plans of about 700 towns from the 6-inch scale, on account of its insufficiency for valuation purposes. In the case of transfers of land also, under the Encumbered Estates Act, the 6-inch plans of upwards of 2,000,000 acres had to be enlarged for the purposes of sales. Several towns throughout the United Kingdom had been surveyed, plotted, and published on various scales, from about 60 to 126$\backslash frac\{1\}\{2\}$ ($\backslash frac\{1\}\{2\}$ of the scale) inches to the mile, and some of these, and indeed of other surveys performed by the Ordnance Corps had been paid for by the parties requiring them. They recommended, 1st, That the 1-inch map of the United Kingdom be forthwith completed, engraved, and published, as beyond all question the most important object in a national point of view. 2d, That the survey of the northern counties of England and the counties of Scotland proceed contemporaneously, and be completed and published,—the cultivated districts on the 25-inch scale, and the whole on the 6 and 1 inch scales, except the Highlands of Scotland, to be surveyed on the 1-inch scale only. 3d, That the revision of the 6-inch plans of Ireland be completed. 4th, That the final determination of the question as to the expediency of extending the survey on the $\backslash frac\{1\}\{2\}$ of the scale to Great Britain or the whole of the United Kingdom, be left to the decision of the legislature, when the contemplated measures with which it is more immediately connected may have been adopted. The expense required for the 25-inch scale exceeded by so small proportion that required for the 6-inch,

Trigonometrical Survey. as to leave no doubt that when either is adopted, the preference should be given to the former, which will also furnish materials for the 6 and 1 inch scales. The estimated expense of carrying out the various plans was—

The sums expended on account of the survey up to 31st March 1858, were—

On the receipt of the report of the Royal Commission, the Treasury, at the suggestion of the Secretary-of-State for War, directed the readoption of the 25-inch scale for the cultivated districts in the meantime, till the matter could be brought before parliament. Some doubts having been expressed in parliament as to the accuracy of the method of reducing the plans by means of photography, a committee of scientific gentlemen was appointed to inquire into and report upon the subject. They stated in their report that, in their opinion, the system of making reductions by photography is more accurate than by the pentagraph or any other known means of reducing plans; that, as regards time, it is for plans of rural districts only one fourth, and for plans of towns only one-ninth, of the time required to make reductions with the pentagraph; that the saving effected by the introduction of photography has been at the rate of L.1615 per annum, and that the saving which will be effected by its introduction during the progress of the survey will amount to at least L.31,952.

The present state of the survey will be best shown by giving a few extracts from the last published report of its progress, which comes down to 31st December 1859:—"The surveys of the six northern counties of England and Scotland are proceeding according to the recommendation of the royal commissioners on the survey, and the orders contained in the Treasury minute of the 11th September 1858. The detail survey of the six northern counties of England will be finished in the present year, and with the exception of the hill sketching of a small portion of country, we shall this year complete the 1-inch map of England. We shall also complete this year the outline 1-inch map of Ireland; 108 sheets are already published, and a great number of others are in progress. By the 31st March next we shall have finished the detail survey of the counties of Dumfries, Stirling, and Clackmannan. All the southern counties of Scotland are surveyed, and we are now engaged in Perthshire and Forfarshire. With the exception of Aberdeen, all the great towns of Scotland have been surveyed, and as the greater part of the remainder of Scotland consists of mountainous country and very open work, I have no doubt but that we shall complete the survey of Scotland within the time, and at a cost within the estimates which I gave the royal commissioners on the survey." He strongly recommends that the subject of proceeding with the survey of England on the 25-inch scale should be taken into consideration by Government and Parliament this year, that he may be enabled to make the preliminary arrangements such as ascertaining the boundaries of the townships, parishes, &c., for proceeding without any costly delays with the survey of the remaining three-fourths of England and Wales, the estimated cost of completing which, on the scales now adopted, being estimated at L.1,450,000. "If," he adds, "the remaining three-fourths are not finished in the same way, it will be the only portion of the United Kingdom without a cadastral survey, and we shall be almost the only state in Europe without one." As compared with 1857, the year ending 30th September 1859 exhibited a reduction on the actual cost of the plans of from 11.49d. to 9.47d. per acre, or about one-sixth on the $\backslash frac\{1\}\{12\}$ scale; and from 6.2d. to 5.5d. per acre, or about one-seventh on the 6-inch scale. As regards Scotland, the estimates by these

two items alone will be reduced by L.70,000. The saving effected by means of photography had also been increased, and was estimated to amount to L.35,000 on the cost of the survey.

The engraved plans of Ireland on the 6-inch scale have been found of great value for many important purposes connected with the valuation of property, &c.; but for the transfer of land under the Encumbered Estates Court, they were found to be utterly insufficient. The judges of the court therefore applied to the Lords of the Treasury to have the plans required for the transfer of property under the authority of the court made as part of the ordnance survey, and on the $\backslash frac\{1\}\{12\}$ scale; and by Treasury letter of 11th September 1858, their lordships approved of this arrangement, provided that effectual care be taken that the whole of the expense incurred in preparing the plans is repaid by the Encumbered Estates Court. The plans are first drawn on the $\backslash frac\{1\}\{12\}$ scale by means of the notes of the original 6-inch survey, and are then subjected to a rigid examination on the ground in order to correct them to the present date, and also to perfect the details in a manner suitable to the increased scale.

The sums stated as requiring to be provided for in the estimates for 1860-61, were as follows:—

The number of royal engineers, civil assistants, and labourers employed on the surveys in each of the three kingdoms were—

The principal triangulation of the United Kingdom, Principal which was commenced in 1783 under General Roy, has re-triangulated been completed, and an account of all the operations connected therewith is given in a volume entitled, Account of the Observations and Calculations of the principal Triangulation; and of the Figure, Dimensions, and Mean Specific Gravity of the Earth, as derived therefrom. Drawn up by Captain Alexander Ross Clarke, R.E., under the direction of Lieut.-col. H. James, R.E., F.R.S., &c. London, 1858, 4to. pp. 782. It consists of a series of great triangles, extending over the whole country, connecting and showing the distances and bearings of all the principal points. There are in all 218 points in the principal triangulation, and the observed bearings amount to 1554.

The following table gives the lengths of the several bases as measured and reduced to the same standard, and their lengths as shown in the triangulation, in accordance with the adopted scale of linear measure:—

¹ About L.3000 of this sum is required to meet the expense of surveys made for the Landed Estates Court, Ireland, which will be repaid.

² This amount will be covered by the sale of the maps. In the year ending 30th June 1859, the sum so received was L.5365, besides the value of maps supplied to public departments during the same period, amounting to L.908, being in all L.6274.

³ The large number of assistants in England arises from the head-office being at Southampton.

The base lines from which all the trigonometrical distances have been computed are those which were measured on Salisbury Plain, and on the shore of Lough Foyle in Ireland. They are respectively 6.93 and 7.89 miles long, and the difference between their measured lengths and their lengths as computed through the triangulation from each other is only about 5 inches. This difference was divided in proportion to the square roots of the lengths of the measured bases, whence was obtained the mean base which has been used in the triangulation, and there is therefore a difference of about $2\backslash frac\{1\}\{2\}$ inches between the measured and computed length of each of these bases. Of the four other base lines measured, the greatest difference between the measured lengths, and the lengths as computed from the mean base, does not amount to 3 inches in three of them. The fourth base, on Rhuddlan Marsh in North Wales, was measured in an unfavorable position, and the difference between its measured and computed length is 1.596 feet.

The mean length of each side of a triangle, in the principal triangulation is 35.4 miles. There are 37 sides, whose lengths are between 30 and 90 miles, 18 whose lengths are between 90 and 100 miles, and 11 exceeding 100 miles in length. The longest side in the triangulation is 111 miles. The sum of all the distances or sides in the principal triangulation is 206,710,000 feet, or, in round numbers, exactly ten times the radius of the earth. The horizontal angles, as well as the azimuthal bearings of the stations, have been principally obtained from observations with Ramsden's great three feet theodolites; and the perfection of these instruments may be judged from the fact, that the sum of the angles in the triangles rarely differed 3.4 from the true sum. For an account of these, and the other instruments used in the survey, see the work already referred to. One of the operations frequently demanding great labour, was the setting up of the theodolite, which had sometimes to be raised by scaffolding to a great height; sometimes a solid foundation had to be made for it in a bog; and sometimes a site had to be prepared for it on the rocky summit of a mountain. In exposed situations on the tops of mountains, a stone wall had generally to be built close round the lower part of the observatory,—a precaution which on one occasion saved the instrument from certain destruction. In placing the framework for receiving the feet of the instrument, every care had to be taken to have it as nearly concentric with the station as possible: the circular mahogany tray upon which the instrument rests was then placed on the table, and accurately brought over the centre mark by means of a plumb-line suspended from the orifice in its centre. The next step, after placing the instrument in position, was to select a spot for the referring object,—an object which, from its position, should be visible under all circumstances, and to which the bearings of all other objects could be referred. It was so constructed as to present a fine vertical line of light of any required breadth, and it was usually placed at the distance of one or two miles. As the adjustments of the theodolite are liable to be deranged by travelling, the first operation at a station was to correct any deficiencies of this nature. The general adjustment consists in this, that the line of collimation of the telescope should be perpendicular to its axis of rotation; this axis perpendicular to the revolving axis of the instrument; and the latter perpendicular to the plane of the horizon. The runs of the micrometers should also agree with the divisions on the limb. The observations then proceed as follows:—The instrument being perfectly levelled, and the lower part or body firmly clamped to the table, the observer

directs his telescope upon the referring object, and, having carefully clamped the upper limb, brings the intersection of the cross-hairs, by the motion of the tangent-screw, to bisect the vertical line of light in the referring object. He then reads the degrees, minutes, and seconds given by the divided circle and each of the micrometer microscopes. The upper limb is then unclamped, and the telescope directed to the next object to be observed, which is bisected, and the readings recorded. Similarly all the other points to be observed, and then, finally, the referring object is bisected and read again. The agreement of this last reading with that at the commencement of the series is some test of the care of the observer and the steadiness of the instrument. Each such series is called an "arc," and they are numbered consecutively from the commencement to the close of the station. The first arc being completed, the telescope is reversed in its Y's, the horizontal circle turned through 180°, and the levels adjusted if necessary. The second arc is then proceeded with precisely in the same manner as the first, closing as well as commencing with the referring object. The second arc being completed, the lower limb of the instrument is moved through a small angle, 20° or 30°, and clamped, so as to get readings on another part of the circle; and in this position the third arc is taken. This being completed, the telescope and horizontal circle are reversed, as in the second arc, and so on. As the readings of the microscopes at each observation are read out by the observer, they are recorded—invariably in ink—by the booker in the observation-book. A copy of the day's work is made every evening, and, being carefully compared by the observer with the original, is transmitted to the head-office every two or three days. The original record of the observations is retained by the observer until the book is filled. From each point the angular distances of all the principal stations visible from it are observed, and in this way a number of values are obtained, which correct one another. Each of these values, again, is the result of numerous repetitions of the same observation, and thus the greatest possible degree of accuracy is secured. In order to obtain the mean of a number of observations, so as to give due weight to those of them which there is reason to believe to be most nearly correct, the "method of least squares," a result of the theory of probabilities, is adopted. See article PROBABILITY, § X.

Besides determining the directions and distances of the principal points of a country, it is necessary for the construction of a map to determine the latitudes and longitudes of the several points, and the inclinations of the sides of the triangles to the meridian. In order to effect this, the latitude and longitude of one station at least, and the inclination of a side of one of the triangles to the meridian, must be determined astronomically, and hence the geographical latitudes and longitudes of all the other stations, and the inclinations of their several sides to the meridians which pass through their extremities, may be computed. As, however, a small error in the computation would lead to errors of considerable magnitude in the latitudes and longitudes deduced from it, it is necessary to verify the computed results by a number of astronomical observations at different stations in course of the survey.

The latitudes of thirty-two of the stations were determined in course of the survey with Ramsden's zenith sector of 8 feet radius, which was destroyed in the fire at the Tower in 1841, and with Airy's zenith sector of 20.5 inches radius by Troughton and Simms. From the very close approximation of the observed zenith distances of the stars with each instrument, and the perfection of the instruments

¹ In Airy's Figure of the Earth the equatorial radius is 20,923,713 feet, and the polar radius 20,853,810 feet, giving an ellipticity of $\backslash frac\{225\}\{228\}$; but from the Ordnance and other more recent surveys abroad the axes are found to be 20,926,500 feet and 20,855,400 feet respectively, and the ellipticity about $\backslash frac\{212\}\{214\}$. The mean density of the earth, as determined from observations at Arthur's Seat, is 5.316.

themselves, the surveyors have every confidence in the results obtained.

Longitudes. The longitudes of the different stations were in the first instance calculated on Airy's figure of the earth,¹ by using the observed latitude of Greenwich, and the observed azimuth of the meridian mark, and transferring this azimuth with the differences to each station in succession; the observed latitudes and the observed azimuths were then used to determine corrections to Airy's Figure, and the Greenwich latitude and azimuth; the longitudes and latitudes then received their necessary corrections.

The longitudes were also determined chronometrically by the transmission of chronometers, and by transit observations; the difference of longitude obtained in this way between the extreme points, viz., Greenwich and Feaghmaan in the island of Valentia, including an arc of about 10°, was found to correspond to a difference of 461 feet in the position of the station as determined geodetically.

Altitudes. The altitudes of many of the stations have been determined by actual levelling with the spirit-level, and are all referred to the mean tidal level at Liverpool; but the vertical angles or zenith distances of the stations were also reciprocally observed with the theodolites.

Minor triangulation. Each triangle of the primary triangulation is broken up into a series of smaller triangles, the sides of which are from 5 to 10 miles long, and this secondary triangulation is again broken up into smaller triangles, whose sides are about 1 mile long, and which form the tertiary or minor triangulation. The length of each side of the minor triangles are then actually measured on the ground, the men noting in their "field-books" every fence, stream, or other object they may cross. They then measure cross lines from one side of the triangle to another, and by taking offsets from the measured lines to every object on the face of the country, they obtain in their field-books the data for plotting accurate plans of the country upon any scale which may be required. The plotting is done in the office from the field-books. The survey is first laid down upon paper, and is then traced on thin tracing-paper. A man is then sent to walk over the ground to see that the surveyor has made no omission, and that everything which should be supplied is there; and also with the view of putting in the character of the trees, the woods, the gardens, the names, and so on; in fact to put in that detail which enables the draftsman in the office to make a perfect plan of the country. The computed trigonometrical length of each side of a triangle is thus checked by actual measurement; and the accuracy of the lines within each triangle is checked by the plotting, so that no errors which may be made by the surveyor can escape detection. Of course, the same extreme care is not necessary in dealing with the smaller triangles as is required in the principal triangulation, as errors here are more easy of detection, and cannot be propagated beyond very narrow limits; while the calculations are also comparatively simple. The difference, therefore, in the expense of surveying for a large and a small scale is comparatively slight, a fact which seems to have been lost sight of by those who raised a cry in parliament against the larger scales on the score of expense. It is of course equally laborious and costly to measure the distances of a given number of objects on a large or on a small scale; the additional expense arises only on the last operations, and between a 25-inch and a 6-inch scale, it is less than ten per cent.

Engraving. But not only are all the surveying operations conducted under the Board of Ordnance, but also all the operations connected with the engraving and printing of the maps. The head-quarters of the survey of the United Kingdom is at Southampton, and from thence all orders connected with the administration and conduct of the survey are issued, and all the plans and maps of Great Britain are there engraved and printed; those of Ireland being en-

graved and printed at the survey-office in the Phoenix Park, Dublin. The superintendent resides at Southampton; and is assisted by an officer in charge of the general correspondence, and the publication of the maps of Great Britain at Southampton; an officer in charge of the trigonometrical branch of the work, and of the initial levelling; and an officer in charge of the examination and zincography of the manuscript plans, and of their reduction to the 6 and 1 inch scales. There is also an officer in charge of the office and publication of the maps in Dublin. Division officers are employed upon certain districts, and have under them numerous parties of surveyors, draftsmen, and computers, acting under the direction of non-commissioned officers; and by this arrangement the whole force is systematically and effectually regulated. The division officers are supplied from the head-office at Southampton, with the trigonometrical distances, and the positions of the trigonometrical stations on the plans; they personally examine the plans on the ground, and certify to their accuracy before they are forwarded for publication. Each officer makes up the accounts of his division, which, after being examined at the head-quarter office, and certified by the superintendent, are forwarded to the accountant-general.

The plans of parishes on the 25-inch scale are published by lithography, zincography, or the anastatic process; the plans of the towns, counties, and the present map of the kingdom, are engraved on copper.

Zincography is now generally adopted in preference to lithography, on account of the facility of handling thin zinc plates rather than lithographic stones, which are necessarily very heavy, and are constantly liable to be broken. "We find," says Colonel James, "not only that the plates are cheaper than the stones, and not liable to break, but that the impressions are darker and clearer; and as we can at all times resort to new tracings, or to the anastatic process, for producing any additional copies that may be required, the same plates, after being cleared of the impressions, are again and again used for taking the impressions of other maps."

The peculiar advantage of the anastatic process consists in the means which it affords of reproducing from a drawing or print, however old, which has been made with a greasy ink, a new plate or any number of new plates of zinc, from which new impressions may be taken. It is first ascertained, by rubbing a piece of thin paper over some part of the drawing, whether the ink is so fixed as that no trace of it will come off by pressure; and if this is the case, the drawing is immersed for a few minutes in a hot solution of strontia (1 oz. of strontia to a quart of water), which has the effect of loosening the ink. It is then partially dried, and afterwards immersed in a solution of nitric acid (one to six of water). If the print be comparatively new, the strontia bath is not required, and it is only necessary to immerse it in the nitric acid. The drawing is then ready to be transferred to a zinc plate, previously polished as finely as possible with powdered emery, and etched by placing a sheet of paper over it damped with nitric acid, and passing it through the press. The transfer is effected by passing the plate through a copperplate printing-press, after which the drawing is removed, and the plate wiped over with gum-water. It is then charged with printing-ink, and subsequently etched with phosphoric acid, a few drops of which are mixed with gum-water, after which it is ready to be printed in the usual manner.

Plans of towns on the $\backslash frac\{1\}\{25\}$ scale, and of the cultivated districts on the $\backslash frac\{1\}\{50\}$ scale, are reduced to the 6-inch scale for photography. "The advantages," says Colonel James, "derived from the introduction of this method will readily be understood by those who are at all conversant with the tedious methods of reducing plans of towns by the pentagraph or the eidograph, by proportional compasses or

Trigonometrical Survey. squares, or any of the methods formerly employed; and it is no exaggeration to say, that the reduction of a plan of a large town on the $\backslash frac\{1\}\{25\}$ scale may be and is now made by photography, first to the $\backslash frac\{1\}\{100\}$ scale, and from this to the 6-inch scale, at one-hundredth of the cost at which we were previously able to do this work; and, what is scarcely less important, the reductions are made and copies printed so rapidly by this process, that no delay takes place in the publication of the same district on the different scales ordered—a most important consideration, when it is borne in mind that the survey is now proceeding at a rate of about 1,500,000 acres per annum, and that, in the conduct of so large a work, the rapid closing up and the final disposal of the work as it is produced are essentially necessary to success in the conduct of it. As a proof of the facilities which photography gives us in making the reductions, I may mention that during the last week one man, with the assistance of a printer and a labourer, reduced 32,000 acres from the 25-inch to the 6-inch scale; and that he produced 3 copies of 45 sheets, or 135 impressions, in six days, besides some other work. One hundred draftsmen could not have produced so much work. "I have made experiments to ascertain the relative power of the different colours for producing photographs, and I found the following scale of colours produced a scale of shades from nearly perfect white to jet black: blue, purple, red, orange, yellow. I have, therefore, had all the houses on the MS. plans coloured yellow."—Report on the Ordnance Survey of the United Kingdom for 1855-6. The lens of the camera is a single achromatic meniscus $3\backslash frac\{1\}\{2\}$ inches in diameter, with a principal focal length of 24 inches. The required scale of the reduction is obtained by tracing on the ground glass of the camera a rectangle corresponding on the reduced scale to the rectangle of the plan to be reduced. The curvature of the image and indistinctness of outline from spherical aberration are both remedied by reducing the diaphragm in front of the lens to a small aperture. The negative copy is taken upon glass by the collodion process, and is then placed in the printing-frame in contact with sensitive paper, and in this way as many positive prints are taken in succession as may be required. Recently, in place of nitrate of silver, a coating of bichromate of potash, gum, and lamp-black, or any other pigment, has been employed in preparing the printing-paper. The action of light on a coating of this composition has the peculiar effect of rendering it insoluble in water, and consequently, when a sheet of paper coated with it is placed in the printing-frame under the collodion negative, the outline of the plan is rendered insoluble in water, and remains on the paper when all the remainder of the composition is washed away; and thus a positive plan, in ink of any colour which may be required, is obtained. The prints, however, obtained by this process are less clear and sharp in their outline than those obtained from nitrate of silver; but, by the following mode of treatment, sharp, clear lines are produced:—The ink of the print, after being soaked in a saturated solution of caustic potash or soda, becomes, so to speak, disintegrated, and is then in a state in which the print may at once be rubbed down, and the outline transferred to the waxed surface of a copper plate for the engraver. This promises to be of great importance, as, after obtaining the photographed reductions of the maps, they have hitherto been obliged to make tracings from them in ink, for the purpose of transferring the plan to copper; the expense and delay of which will now be saved, whilst no risk is run of any error being made by the draftsman.

Photo-zinography. A new method of printing from a negative, extremely simple and inexpensive, has recently been tried, and promises to be of great use. By means of it the reduced print is in a state to be at once transferred to stone or zinc, from which any number of copies can be taken, or to the waxed surface of the copper plates. To effect this the paper, after

being washed over with the solution of the bichromate of potash and gum and dried, is placed in the printing-frame under the collodion negative, and after exposure to the light, the whole surface is coated over with lithographic ink, and a stream of hot water then poured over it. As the portion which is exposed to the light is insoluble, whilst the composition in all other parts, being soluble, is easily washed off, the outline of the map, in a state ready to be transferred either to stone, zinc, or the copper plate, is at once obtained, or the photograph can be taken on the zinc at once. "From the perfect manner," says Col. (now Sir) H. James, "in which we are able to transfer the impressions to zinc, we can, if required, print any number of faithful copies of the ancient records of the kingdom, such as Doomsday Book, the Pipe Rolls, &c., at a, comparatively speaking, very trifling cost. I have called this new method photo-zinography, and anticipate that it will become very generally useful, not only to government, but to the public at large, for producing perfectly accurate copies of documents of any kind."—Report for 1859.

A considerable saving in the cost of engraving is effected by using steel punches to cut the woods, figures, rocks, &c., on the copperplates. A portion of the writing, also, is engraved by machine (Becker's patent); and the parks and sands are ruled by a machine with a steel dotting-wheel, the pressure of which, and the interval between the lines of dots, being regulated according to the tint required to be produced.

The process of electrotyping has been found to be eminently useful for the purposes of the survey. Instead of printing from the copperplates, which speedily become obliterated in parts, and thus lead to a constant expense for repairing or re-engraving, an electrotypic copy of the plate is taken, from which the impressions are printed. The original plate is thus preserved in its perfect state, from which, when required, another electrotypic copy can be taken, or rather from the "matrix" of the original. Another great advantage is the facility it affords for making alterations, it being much easier to scrape off obsolete details, &c., from the electrotypic cast of an engraved plate where they are in relief, than to cut them out from the original copperplate. Copies can also in this way be obtained from the plates in their different stages of progress, so that different classes of information can be engraved upon a map the same in all other respects, as for instance, one copy of a map may be obtained with contours, boundaries, &c., another with the hill features, a third with the geological lines, &c. This process has also been very usefully employed in joining two or more engraved plates together, so as to form a single copperplate for printing. In this way maps of several of the counties of Scotland, to serve as indexes to the 6-inch county maps, have been formed out of the copperplates of the 1-inch map. In one instance, no less than seven plates have been thus joined together.

The peculiar and practical advantage of the 25-inch scale of the parish plans over the smaller scales is, that it enables the acreage of every single enclosure and garden in the country to be given with a distinct reference to it. Besides, as every square inch represents an acre, any one may, in point of fact, ascertain exactly what each field contains. Tables of areas of each separate enclosure are also published with these plans. Each sheet contains 960 acres, being a mile wide and $1\backslash frac\{1\}\{2\}$ mile long. The reference, therefore to any property is exceedingly simple, and may be described as consisting of the fields numbered so and so, represented on sheet No. — of the Government Survey of the County of —. Sixteen of the plans on the 25-inch scale go to form one sheet on the 6-inch scale, which therefore comprises 24 square miles, being 6 miles long by 4 wide. On the towns scale of $\backslash frac\{1\}\{100\}$, or 10 feet 6 inches to a mile, not only is every house laid down, but the actual divisions of each (on the ground floor) are also given. (D. K.)