Treatise on the principal mathematical instruments employed in surveying, levelling, and astronomy : explaining their construction, adjustments, and use. With an appendix, and tables / by Frederick W. Simms.

  • Frederick Walter Simms
Date:
1834
Catalogue details

Licence: Public Domain Mark

Credit: Treatise on the principal mathematical instruments employed in surveying, levelling, and astronomy : explaining their construction, adjustments, and use. With an appendix, and tables / by Frederick W. Simms. Source: Wellcome Collection.

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    In the first figure, the limb is divided to 15', and these divisions are subdivided by the vernier to 15. In the second figure, the limb is divided to 10', and subdivided by the vernier to 10. In the third, the limb is divided to 20', and sub¬ divided by the vernier to 30 5 and in the fourth, the limb is divided to 20', and subdivided by the vernier to 20. E, on each figure, is placed where a division on the vernier coincides with one on the limb. In the first, the reading is 45° 46' 30; in the second, 60° 21' 20; in the third, 21° 23' 30; and in the fourth, it is 17° 2', and between O ' and 20, and as the 2' line is about as much in advance of the one on the limb near to it, as the 20 line is behind the one near to it, the reading may be taken as 17° 2' 10. The fifth figure represents the scale of a barometer, reading 30.435 inches, and is drawn much larger than the reality, to render it more intelligible. 30 THE THEODOLITE. As an angular instrument, the theodolite has from time to time received such improvements that it may now be considered as the most valuable instrument employed in surveying. Instruments of this kind, of the best construction, may to a certain extent be used as altitude and azimuth instruments ; and several astrono¬ mical operations, such as those required for determining the time, the latitude of place, &c. may be performed by them, and to a degree of accuracy sufficient for most of the purposes that occur in the ordinary practice of a surveyor. There are various modes of constructing theodolites to suit the convenience or the views of purchasers; but we shall confine our¬ selves to a description of one of the most perfect, as a person acquainted with the details of its adjustments and use, will find no difficulty in comprehending those of others. Description of the Theodolite. This instrument (as represented in the next page) consists of two circular plates, A and B, called the horizontal limb, the upper, or vernier plate. A, turning freely upon the lower, and both have a horizontal motion by means of the vertical axis, C : this axis consists of two parts, external and internal, the former secured to the gra¬ duated limb, B, and the latter to the vernier plate, A. Their form is conical, nicely fitted and ground into each other, having an easy and a very steady motion; the external centre also fits into a ball at D, and the parts are held together by a screw at the lower end of the internal axis.
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