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.

  • Simms, Frederick Walter, 1803-1865.
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.

    12/126 (page 2)
    THE SURVEYING CROSS AND OPTICAL SQUARE. The instrument formerly employed for laying out perpendi¬ cular lines, was the cross-staff, of which there were various con¬ structions 3 but that in most general use consisted of four sights, fixed at right angles upon a brass cross, and adapted to the top of a staff 3 which being thrust into the ground, with two of the sights placed in any given direction, the other two pointed out the per¬ pendicular required. But this instrument has been almost super¬ seded by the optical square, which is much superior to it both for convenience and expedition 3 and it has also the advantage of greater portability, not being larger than a shallow circular snuff¬ box, which it resembles in shape. It is made of brass, and contains the two principal glasses of the sextant, viz. the index and horizon glasses, fixed at an angle of 45° 3 hence, while viewing an object by direct vision, any other, forming a right angle with it, at the place of the observer, will be referred by reflection, so as to coincide with the object viewed. Thus a line may be laid out perpendicular to a station-line, and from any point on it, by simply standing with the instrument over the given point, and looking through it along the line, having a person to go with a mark or station-staff in the direction the perpendicular is required, and signing to him by hand to move to the right or the left, until his staff is seen by reflection to coincide with some object on the line along which the observer is looking 3 and the place of the staff will be in a perpen¬ dicular to the station-line at the place of the observer. If it be required to find on a line the place of a perpendicular from a fixed object, as a house, &c. the observer himself must move along the line until the image of the object appears, as before, in the direction of the line, and the place where he then stands, will be the spot where such perpendicular would fall. THE PRISMATIC COMPASS. The use of this little instrument is to measure horizontal angles © only, and from its portability it is particularly adapted for military surveying, or where but little more than a sjtetch map of the country is required. It is also very useful in filling in the detail of a map, where all the principal points haye I>een correctly fixed by means of the theodolite 3 and for this purpose it has been extensively em¬ ployed by the gentlemen engaged on the Ordnance survey. It may likewise be used for determining approximately the direction of the true meridian, the variation being determined by comparing the observed azimuth of a celestial object, with its true azimuth deduced from an observation made for the purpose. In the annexed figure, A represents the compass-box, and B the card, which being attached to the magnetic needle, moves as it moves, round the agate centre, a, on which it is suspended. The circum¬ ference of the card is usually divided to 15' of a degree, but it is