473.—With larger, what are termed geodetic instruments, to be described in the next chapter, constructive errors are not permissible; but these instruments are observed under altogether different conditions, which are suitable to the precision demanded. A large theodolite is generally fixed upon solid rock, or masonry with good foundation, or upon a very firm solid framed stand, and is protected from wind, sun and rain. Where it is necessary on level ground to elevate the instrument for more extensive view, a proper structure is built, in which the theodolite is isolated from the outer walls or enclosure carrying the stage upon which the observer works, so that no vibration or deflection of this, caused by the wind or the weight of the body, affects the instrument. Under such conditions angles are read on various points of the circle by micrometer microscopes so as to obtain a sufficient number of means, that personal and instrumental errors may be reduced to a minimum.
CHAPTER X.
LARGE THEODOLITES USED ONLY FOR GEODETIC SURVEYS—STANLEY'S 10- AND 12-INCH—14-INCH ALTAZIMUTH—COLONEL STRANGE'S 36-INCH THEODOLITE.
474.—Large theodolites employed upon geodetic surveys. Where the complete survey of a country has to be made, a system of large triangles is formed over the country from convenient positions which are naturally or artificially elevated so as to obtain distant views with the telescope. These triangles are correctly measured by angles subtended from a very carefully measured base or bases set out upon approximately level planes, which are generally of a mile or more in length. Where measurements are derived from such bases by constant intersection of angular positions extended therefrom to large triangles or other polygons, it becomes important that the theodolite employed should measure such angles with great accuracy. In this case the vernier reading does not possess sufficient refinement, and the divisions representing the degrees have to be magnified to appear wider apart, so that they can be more finely subdivided for the reading to be taken by means of a micrometer microscope capable of subdividing the divisions made upon the instrument even to single seconds of arc. The theodolites used for the superior triangulation of Great Britain were Ramsden's 36-inch and 18-inch, which, although constructed in the last century, remain excellent instruments.
475.—The construction of large theodolites is varied very considerably according to the conditions present in the country to be surveyed. This subject if carried into detail would extend much beyond the intended limits of this work. This chapter will therefore be limited to the description of a 10- or 12-inch instrument, and to two historical instruments which have been used successfully for geodetic work.
Fig. 196.—Stanley's 10-inch transit theodolite.
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