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 Walter Simms.

  • Simms, Frederick Walter, 1803-1865.
Date:
1844
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 Walter Simms. Source: Wellcome Collection.

    18/176 (page 2)
    The operation of measuring with the chain requires at least two persons, one to lead and the other to follow and direct, the first or leader (taking the ring at one end of the chain upon two fingers of his right hand with one arrow, and the remaining nine in his left,) lays his end of the chain, by direction of the fol- lower, in a straight line with the station to be measured to, and there fixes an arrow, while the latter holds the other end of the chain at the starting point; the leader now proceeds onwards until the follower comes to the arrow first laid down, to which he places his end of the chain, and again directs the leader to place a second arrow in line with the forward station: the leader will now have an opportunity of checking the directions of the follower at every succeeding chain’s length, by observing if the latter is also in a line with the back station, at the time he directs him to place one of his arrows in the direction of the for- ward station: they proceed in this manner till the whole line is measured, or the leader has spent all his ten arrows, which, upon counting, he will find in the possession of the follower, (unless some error has been committed,) who must restore them to the leader, and remark in the field-book that they had made one change, or measured 1000 links; they then proceed onwards as before, the leader taking all the ten arrows, until they are again spent, when a second change must be made and entered in the book, and if the line is measured out before a third change takes place, the follower will have in his hand as many arrows as there have been chains laid out upon the last measured part of the distance ; which, together with the odd links and the former ane changes (or 2000), will make up the entire length of the ine. For the purposes of plotting, &c. it will be necessary to re- duce the measurement of the lines which alternately ascend and descend to the correct horizontal measure, for it is evident that the distance between two points, if measured over uneven ground, will be greater than if measured perfectly straight in a horizontal plane. Some surveyors attempt this correction as they proceed, by holding the lower end of the chain above the ground, as nearly horizontal as they can estimate, and if they aim at considerable accuracy, will have a plumb, which they allow to hang from the hand that holds the chain, over the arrow or mark in the ground. In passing over very steep ground, they frequently take half or even a quarter of a chain’s length to accomplish their measurements with, as the whole length would be too great to be held horizontal, when the inclination is con- siderable. But the most correct method is to take the vertical angles along the undulations of the line after it has been mea- sured, and compute the horizontal distances (by a rule in plain trigonometry,) as the whole line is then supposed to be divided into a number of right-angled triangles, the measured portion