CHAPTER I. GENERAL OUTLINE OF THE SYSTEM OF CARRYING ON A TRIGONOMETRICAL SURVEY

MEASUREMENT OF A BASE LINE.

Description of the different Methods that have been adopted to ensure its correct Measurement.—Combined Iron and Brass Rods used on the Ordnance Survey.-Visual Contact with reading Microscopes.-Reduction of a Base measured on any elevated Plain to its Value at the Level of the Sea.-Prolonging and verifying a Measured Base by Triangulation.

TRIANGULATION. Choice of Stations.—Method of rendering distant Stations visible-by Reflection of the Sun's Rays-Argand Burners-Drummond's Light.Method of increasing the Length of the Sides of the first Triangles directly from the Measured Base.—Secondary Triangles.-Assumed Base.—Instruments used for observing Angles on the Continent and in England.-Reduction to the Horizon.-Spherical Excess.-Reduction to the Centre.Adjustments of a Theodolite.—Method of discovering lost Stations.Laying down a Triangulation upon Paper.—Position of Trigonometrical Stations also ascertained by astronomical Observation

INTERIOR FILLING-IN OF A SURVEY, EITHER ENTIRELY OR

PARTIALLY, BY MEASUREMENT. Method of Filling-in the Detail entirely by Measurement, as practised on the Ordnance Survey.—Levelling Marks and Forms of Field-Books, &c.— Measurement of Roads by the Chain and Theodolite.-Computing the Contents of Enclosures directly from the Field-Book.--Filling-in the Interior, partly by Sketching.-Road Surveying.–Variation of the Compass.—Sketching between Trigonometrical Points and Measured Lines.Practical Methods of avoiding Obstacles and determining inaccessible Heights and Distances in the Field.-Station Pointer.-Surveys for Railways