side of a drawing or other board, on which the paper should be extended and fixed by pins or otherwise. By holding the head of the square to the straight-edge of the board, at any required place, its blade will determine corresponding parallel lines; and by laying the straight side of the semicircular protractor to the side of the blade, so held, its zero will be adjusted to the fixed line of direction. The motion of the radial arm is not very free, so that it may be adjusted in the hand to the reading for the angle on the graduated arc, and the protractor, after being laid down on the paper, moved about and along the blade of the square to adjust its centre to the angular point, without disturbing the arm from its adjusted position. In this manner the angles may be plotted without producing the reference line.

If the lat. and long. of the angular points, with reference to axes of co-ordinates, be computed, the angular points may be plotted, as already explained, and the verification of the plotted points had by a scale measurement of the plotted lines. This is the only mode of plotting the reference lines of a traverse survey in which a check is obtained on the plotter's work.

PLOTTING THE SECONDARY REFERENCE LINES OF A
DETAIL TRIANGLE.

The reference points of secondary reference lines should be plotted on the primary reference lines, or on one another, from the zero end of the line on which they may be surveyed, as already described for plotting the reference point of the split line. The scale distance between the plotted reference points of a secondary line should be found to agree with the measured distance, with an error uot exceeding 1 in 1000 in fair open

country, and 1 in 1000 in close country. Should these measurements differ by a greater amount than the above limit in the particular case, the straight line pencil trace should not be drawn between the reference points. The line should be entered on an 'office error sheet,' which should state the scale and measured lengths of the line, the number of the 'plot sheet,' the triangle, and the book, page, &c., in which the measurements are entered. Other lines referenced to the reference point or points of the line in error should be plotted, to ascertain if the apparent error be due to erroneous measurements for the reference points. The apparent cause of error should be entered on the error sheet; and, should the lines referred to the same reference point in error be found to plot to another point in the reference-point line, the scale measurement of this point should be also entered on the office error sheet. Further, if the reference point or points in error be not directly surveyed in the survey of their respective lines, the measurements for reference and the arithmetical reductions should be examined. If the error be due to an arithmetical blunder, the correction should be made without obliterating the original figures of the entry or the reduction. In many instances of apparent error, the measurements for the length of the reference line, and for the reference points, will have been accurately made though incorrectly entered. The points ƒ20, ƒ80, &c., are distinguished by the same marks on the chain, so that a careless reading may make the measurement for a point at the former read the latter. Errors of this class should not be corrected in the office, as the error may be due to some other unapparent cause.

In plotting reference lines for the verification of field

measurements, great care should be taken to have the divisions of the scale equally lighted, and to make the scale measurements for reference points from the proper end of the line, as already directed.

In addition to the 'office error sheet,' there should be an error sheet for each book and triangle, to be forwarded with the field-book to the surveyor for corrections. This sheet should give the book, page, and the line in error, together with some indication of the cause of error, without suggesting the probable amount.

[The corrections for errors should be entered on the field-book without erasures or obliterating the original notes, which should be cancelled with a stroke of the pen.] The field-book and error sheet when corrected should be returned to the office, and the points and lines in error replotted from the corrected measurements. The subjoined are examples of office and field error sheets :

The plot for the verification of the secondary reference lines of each triangle should be made on a separate sheet of paper and preserved. The plot of the secondary reference lines on the fair paper for the map or plan should be made with equal care as that for the verification of the measurements of the reference lines. The plot on the separate sheet of paper should be made as soon as may be convenient after the survey of the triangle shall be completed. The plot on the map sheet should not be made until the detail of the triangle is about to be plotted.

PLOTTING DETAIL.

In plotting from the entries of measurements for reference lines or detail, the measurements should be laid down on paper in the order and manner indicated on the field-book. In plotting the reference and other points for perpendiculars and intersected detail, the long or line scale should be adjusted to the line and its extremity, as already described for plotting the line or

reference points on it. Several consecutive points, commencing at the zero, should be plotted in the line. The long scale should be then removed, and the small, or perpendicular scale, applied consecutively to the points, being reference points, in such manner as to make the zero of the scale coincide with the plotted reference point when its edge shall be perpendicular to the reference line. The detail point should be plotted on the perpendicular scale and on the proper side of the reference line. The proper connections for detail, as shown by the field-book, should be made by drawing a neat and firm pencil line along the edge of the short scale adjusted successively to the adjacent detail points. All check measurement should be applied with the scale before drawing the detail in pencil. In this manner the whole of the measured points in the reference line and the detail should be plotted, and the connections clearly and elegantly made. In drawing the detail, the sketching on the field-book should be looked upon simply as a guide for making connections, and not as a guide for features.

Before plotting the detail on a reference line, the plotter should carefully examine for the reference point, which shall be the zero of the measurements with the scale, and afterwards be careful to plot the detail on the proper side of the reference line. It is a cause of much annoyance to find, and of serious error if not found, detail plotted on the wrong side, or from the wrong extremity, of a reference line.

PLOTTING VERTICAL SECTIONS.-ALTITUDES MEASURED WITH THE LEVEL.

On the section paper a fine pencil line should be drawn along a 'straight-edge' to represent the datum line. On