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.

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