GEODESY, the science of the measurement of the earth's surface, and of great por tions of it. The reader will find under EARTH the principal results of geodetical meas urements, and under TRIANGULATION, an account of some of the methods of obtaining them. Geodesy has many physical difficulties to contend against. In measuring a par ticular length with a view to obtaining a base line for calculating other lines by trigon ometrical observations, there is first a difficulty arising in the use of the unit of length, whatever it may be, whether rod or chain. In the use of rods, it is difficult to lay them all precisely in the same direction, and to prevent error arising from intervals between the rods. In the use of chains, again, the greatest care is needed to keep all the links stretched, while the difficulty of avoiding error through not preserving the line of direc tion is but little diminished. Further, in all cases, the tendency of the units to change magnitude with changes of temperature, and the unevenness.of the earth's surface, are
pregnant sources of error. After all these difficulties have been overcome, and a suffi cient base line obtained, a new class of difficulties are encountered. In taking trigono metrical observations of distant objects, it is found that the three angles of any triangle which we may form are together in excess of two right angles; the angles are, in fact, more of the nature of spherical than plane angles. For this, in using the angles as plane angles (for greater simplicity), a correction has to be made. Further, a correction is required for the effect of horizontal refraction on the results of observations on C.is taut objects—a most fluctuating source of error—to evade which, as far as possible, it is usual to make observations when the atmosphere has been for some time undisturbed. See Puissant's INork on Geodesy.