CF, which is in the direction of the coast. If CF be not found sufficiently long, proceed as you did with CB, and the line may be extended to any length that may be required, provided the view be not interrupted by any intervening obstacle. Should this method be found difficult in practice, a line of sufficient length may be found from the combination of small triangles, not exactly equilateral. Having determined the position and magnitude of the base line, two persons ready to take angles simultaneously are stationed at its extremities, and a boat being moored successively over every shallow, sounding, sunken rock, or other remarkable spot whose position is required, the angles the alternate extremity of the base and boat subtend are observed, by which the positions of all these places are ascertained. Or planting staves, with conspicuous colours, at the extremities of the base line, then a boat explores all the points at which the bearings of the staves are taken, which furnish so many triangles, from which we can determine the situation of the several points by the rules of trigonometry, or by their graphic representation on paper.

When a correct map of the coast can be procured, the labour is considerably abridged, as then from all the notable points on the surface of the water, the bearing of two known objects on the land are taken, which will furnish data without the trouble of landing. If at the boat the angles subtended by three known objects on the land be observed, the respective situations and distances of these several required points are ascertained. This method, by trigonometry, is however rather tedious and difficult. The construction may be facilitated by the use of an instrument, called a station-pointer, which consists of three brass rulers that open and set at the given angles.

When the positions of the headlands are determined by means of two boats, whose distance asunder is measured on the water, one boat remains stationary while the other passes her to take up a new position, in order to extend the observations along the coast. The first thing to be ascertained, is the latitude and longitude of the starting point, by which the latitudes and longitudes of all the subsequent points may be found from calculation, checked however, as often as opportunity would permit, by astronomical observations.

There is a great variety of circumstances to be noticed in the course of such operations such as the direction and velocity of the tide currents, the height to which the tides rise and fall, the direction of the prevailing winds, the land-marks erected for the mariner's direction, by which he may avoid dangerous rocks, shoals, &c. in approaching the shore. The positions of rocks or islands distant from the land are determined by astronomical observations, as taught in astronomical works.

To enter with sufficient minuteness into all the necessary details of extensive maritime operations would fill a volume of ample dimensions, therefore no more than a bare outline can be expected in the present work, combining, as it does, so many other subjects, which are more immediately the objects the author has in view. Those who desire more information on this particular branch of surveying, may consult the works of Owens, Belcher, Beaufort, and several others who have written amply on the subject. In those works the subject will be seen not only well and largely treated, but technically also. The rules and regulations issued to the different persons composing the surveying staff are judiciously chosen and well arranged in these works. Without due attention to such rules and regulations, no extensive operations could be conducted with regularity or dispatch. Neglecting to enforce this code, would soon lead to confusion and error.

TRIGONOMETRICAL SURVEYING,

AS APPLIED TO A LARGE SEGMENT OF THE EARTH.

Trigonometrical Surveying, according to the common acceptation of the term, comprises the art of measuring large portions of the earth's surface by means of a series of connected triangles.

Topographical surveying comprehends the filling up of the space contained within these large triangles, with the correct position of the remaining objects situated upon, and composing the surface of the country.

Trigonometrical, in connexion with topographical surveying, enables us to ascertain the positions of all the objects within the scope of observation, to measure their relative heights, their mutual distances, and to represent the contours of the country.

Land surveying requires only to mark the boundaries of fields, farms, and estates, and find their contents, without any regard to the inequalities of the ground.

In the preceding part of this work relating to surveying, no regard has been paid to the spherical shape of the earth: as no necessity existed of departing from the consideration, that a limited portion of its surface is a plane.

When a survey does not exceed the limits of a moderate sized county, there is no occasion to depart from the consideration of a plane, as is proved from the fact, that 69 English miles, the length of one degree, onsidered as a curve, measures but little more than 24 feet longer than its chord-a difference so very small as to be inappreciable in a large estate, parish, or even a barony. But when the sides of the triangles employed in more extensive surveys, extend over a larger segment of the earth's surface, then their computation necessarily involves the principles of Spherical Geometry.

A trigonometrical survey is very important in its objects, embracing

1. The measurement of a degree of latitude in various places, with a view to ascertain the figure and magnitude of the earth.

2. The accurate determination of the geographical positions of the principal places, with a view to improve navigation and extend the science of geography.

3. The finding the difference of longitude between two meridians, with a view to improve astronomy and navigation.

The trigonometrical survey at present in progress in Ireland, under the direction of Colonel Colby, embraces still more. It determines not only the boundaries of counties, baronies, parishes, and townlands, but, in the latter part of the survey, the boundaries of fields also. The boundary department is directed by Richard Griffith, Esq., who also conducts the valuation department. The topographical department of this survey developes the natural treasures of the country, such as its mines and minerals, its fossil remains, and its geological formation.* Operations so very delicate, and of

*

This part of the duty has been discontinued, from motives of economya circumstance to be regretted. The talent displayed in the volume already published on this subject, proves how highly qualified the gentleman who had charge of this department, was to discharge the duties of such an undertaking.