Of the extensive and scientific Geodesical Operations described in these latter works, the present Treatise professes to give nothing beyond a brief outline, as their detailed account would be far too voluminous to be condensed in so small a compass. The cadets at Woolwich and Addiscombe are taught the use of the Chain and Theodolite, and to calculate the contents of the different portions into which the ground is divided by natural and artificial boundaries; they are also rendered conversant with Plane Trigonometry and Mensuration, and with sufficient Spherical Trigonometry for the solution of the ordinary cases of Spherical Triangles. Such preliminary knowledge is consequently assumed as being already acquired. It is, however, in the power of any individual to make himself master of the necessary theoretical part of this knowledge, by the study of one or other of the numerous excellent works on Trigonometry and Mensuration; and the practice of Land Surveying can be acquired in a few weeks in the Field, under any competent Instructor, or even without this assistance, by the careful study of some elementary work on the subject. ROYAL ENGINEER ESTABLISHMENT, CHATHAM, .PREFACE TO THE SECOND EDITION. In consequence of finding, on my recent return to England, that this work had been for some time out of print, and that considerable portions had been extracted by different authors, a second edition has been prepared, in which, beside many alterations, improvements, and omissions of parts since deemed not sufficiently practical, will be found a separate chapter devoted to Surveying in the Colonies, with reference to the marking out of waste lands for future occupation; the result of nearly ten years' experience obtained during the superintendence of the Survey of South Australia. BRIGHTON, April, 1850. TABLE OF CONTENTS. CHAPTER I. GENERAL OUTLINE OF THE SYSTEM OF CARRYING ON A TRIGONOMETRICAL SURVEY Page 1 CHAPTER II. 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 . . 5 CHAPTER III. 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 CHAPTER IV. INTERIOR FILLING-IN OF A SURVEY, EITHER ENTIRELY OR 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 13 30 CHAPTER V. Particular objects of a Reconnaissance under different circumstances.— Correction for Curvature of the Earth-for Refraction.-Average Amount of these Corrections.-Reciprocal Angles of Depression and Elevation for determining the Amount of Refraction at any particular period.-Method of taking Sections of Ground with a Theodolite.-Cross Sections.-Trial Sections.-Check Levels.-Spirit Level and its Adjust- ments. French Water Level.-Boning Rods.-Reflecting Level.-Method of taking Sections with the Spirit Level, or other Instrument adapted for tracing Horizontal Lines. - Plotting Sections. - Sectio-planography.— Sections for Railways.-Method of Tracing Contour Lines.-System of Contouring practised on the Ordnance Surveys.-Data afforded by Contour Plans for determining the most available directions for Roads, Railways, Lines of Drainage, &c.-Construction of Models.-Problems determined by 70 Method of ascertaining Altitudes with the Mountain Barometer.— Aneroid. Substitute for the Barometer.-Determination of Altitudes by |