LEVELLING AND ITS GENERAL APPLICATION - 1914 - PREFACE TO THIRD EDITION - HAVING been asked to prepare for the press this third edition, I have endeavoured to follow as closely as possible the simple, yet practical explanations of the author. I have carefully read over the whole work, and have made a few add tions and alterations merely with the view of bringing it up to date, as some improved methods have been introduced since the previous edition was published. - CONENTS - CHAPTER I. IMPORTAN O C F E T HE ART OF LEVE LLISG-DIFFEKESCE BETWEEN THE APP. IREST OR OPTICALL EVEL A XD THE TRUEL ETEL-CI-RVA. I. R OF E TIIE EAR-111- UES--IGKS OF LEPELLISG - 1 FFECTED BY THIS.. .... 1 CHAPTER 11. THE SPIRITL EVEL P URE ATD SIMPLE-BOXING1 x0 s - MECHANICS LEVELS-LEVEL WITII FOLDISG SIGHTS-REFLECTIXLGE VELS . .......................... 7 CHAPTER 111. THE LEVELLINGST AFI . ......................................... 15 CHAPTER IV. INTRODUCTIO OF N THE DUIIPY LEVEL-ADVICE TO INTENDINPGU RCHASER OF S LEVELS-THE ACJUST AIENT 01 THE DUAIPYL EVEL .. ......................... 19 CHAPTER VI. THE DATUME XPLAINED-EXPLANATI O O F N T HE TERM BENCH MARK -LEVELS TAKEN AND A SECTION MADE ON A GIVEN LINE ................................. 42 CHAPTER VII. CHAPTER VIII. CHAPTER IX. CHAPTER X. CHAPTER XI. LEVELLISG A IDED BY THE APPLICATIO O N F THE SUB TEXSE OR STADIA RE, DING TELESCOPE-THE GRADIOLIET . E .. R ... .......................................... 102 CHAPTER SII. LEVELLIL-W G I TH THE THEODOLIT . E ... . . . . ................... 112 CHAPTER XIII. Tows LEVELLING-BENC M H A RKS REVERTED TO .. .... 119 CHAPTER XIV. LEVELLING AND ITS GENERAL APPLICATION. . CHAPTER I. IMPORTAN O C F E THE ART OF LEVELLIN - G DIFFERENCE BETWEEN APPARENT OR OPTICAL LEVEL AND TRUE . LEVEL - CURVATURE O F THE EARTH - QUESTIONS O F LEVELLIN A G F FECTED BY THIS. T HE art of levelling is one of those generally useful branches of engineering which, associated as it is with the law of gravitation, applies itself in a greater or less degree to almost every object that the vision can embrace. It is only by the application of levelling in combination with the law of gravitation that the equilibrium of any construction can possibly be maintained and as it is only by this application that the laws which govern construction and mechanical motion can possibly be understood, it must for all time hold its own, and of necessity demand the atten- ion of the student. Levelling, as applied to actual practice is, on the one hand, the determination of the rpparent or optical level, which is simply the earths tangent extended in space, or, in other words, a line drawnat right angles from its point of contact with the earths circumference to a line drawn from the poirit of such contact to the centre of the earth, and which may be expressed with greater clearness as the earths radius or direct line of equal gravity on the other hand, it is the determination of the true level which may be described by the arc of a circle radiating from the centre of the earth it also comprises a comparative determination of all the elevations and depressions which form the natural inequalities of the earths surface. Fig. 1 is illustrative of the line of apparent or optical lzvel - A being the point of contact, B the centre of the earth, and C C-the earths mean circumference. Fig. 2 is illustrative of the line of true level-A being the centre of the earth, b b b being an arc of the earths mean circumference. ...