MAGNETIC VARIATIONS IN THE UNITED STATES. [From a Report by C. A. SCHOTT, Assistant U. S Coast Survey]. See Silliman's Journal, May, 1860, p. 335; and U. S. Coast Survey Report for 1859, App. 24, p. 296. W. and E. indicate West and East Declinations. They are given below in Degrees and tenths. Minimum 7.6 6.8 1765 1782 1813 1800 1779 1794 | 1801 1787 1795 1799 1805 1798 1.9 W. 1815 6.13.1 Mobile, Ala. New York City. Oxford, N. Y. Philadelphia, Pa. Washington, DC. W. W. W. W. Williamsburg, Va. ANALYTICAL TABLE OF CONTENTS. PART I. GENERAL PRINCIPLES AND FUNDAMENTAL METHODS. CHAPTER I. Definitions and Methods. 19 (32) Measuring-wheel 24 25 25 (20) Gunter's Chain.. (21) Pins. (22) Staves.. (23) How to chain (24) Tallies.. 19 (33) Measuring Angles 21 (34) Noting the Measurements CHAPTER III. Drawing the Map. (75) Do. into parallelograms 49 (76) Addition of widths 50 (63) First Method, Arithmetically. 43 (77) Third Method, Instrumentally 50 (64) Rectangles (65) Triangles... (66) Parallelograms. (67) Trapezoids.. 43 43 (78) Reduction to one triangle.. 50 54 44 (87) Fourth Method, Trigonometri 44 cally 56 ARTICLE PART II. CHAIN SURVEYING. CHAPTER I. Surveying by Diagonals. (90) A three-sided field. PAGE ARTICLE 58 Keeping the field-notes CHAPTER II. Surveying by Tie-lines. CHAPTER III. Surveying by Perpendiculars. To set out Perpendiculars. (104) By Surveyor's Cross. (107) By Optical Square (108) By the Chain. .... Diagonals and Perpendiculars. Offsets. .... 69 70 Taking offsets.. 75 (110) A three-sided field. (111) A four-sided field 73 (121) When equidistant 80 (112) A many-sided field (113) By one diagonal. CHAPTER IV. Surveying Erroneous rules. Reducing to one triangle 81 by the methods combined. 74 (122) 81 75 (123) (124) 81 CHAPTER V. Obstacles to Measurement in Chain Surveying. (138) The obstacles to Alinement and Measurement.. (139) LAND GEOMETRY...... (140) PROBLEM 1. To erect a perpendicular at any point of a line......... 97 when the point is at or near the end of the line 98 when the line is inaccessible ... 99 4. To let fall a perpendicular from a given point to a given line 99 5. when the point is nearly oppo site to the end of the line... 100 when the point is inaccessible.. 101 when the line is inaccessible... 101 Problems on Parallels. (160) PROBLEM 1. To run a line from a given point parallel to a given line. 102 (165) 2. Do. when the line is inaccessible 103 ARTICLE OBSTACLES TO ALINEMENT. A. To prolong a line ..... PAGR 105 107 (169) By ranging with rods 105 | (174) By transversals (177) Signals lateral... . 108 B. To interpolate points in a line. .. 109 109 | (181) With a single person 111 (178) Ranging 109 (182) On water 111 (180) Over a hill 110 (184) To an invisible intersection. 112 OBSTACLES TO MEASUREMENT. A. When both ends of the line are accessible (186) By perpendiculars.... 113 | (189) By transversals. (187) By equilateral triangles... 113 (190) In a town (188) By symmetrical triangles.. 114 B. When one end of the line is inaccessible..... C. When both ends of the line are inaccessible..... (202) By similar triangles. (203) By parallels.. |