as I have already remarked, sometimes materially affect the needle.

In order to be able to resume an incomplete survey, a mark must be made at the last station by cutting a cross in the sod, or driving a strong peg into the ground. And it is necessary, both on beginning and ending a day's work, to take angles between the back or forward stations, and any two fixed points that may be visible.

TO LAY DOWN OR PLOT THE SURVEY UPON PAPER.

It will readily be perceived that angles, measured accurately to minutes of a degree with the theodolite, will avail us little, unless we have the means of plotting them with equal accuracy; and this we are enabled to do by using a protractor, furnished with a vernier, which may be either of a circular or semicircular form. Let us suppose the learner to be provided with one of the latter shape, such as he will find described in a subsequent section.*

Draw a line, N A, upon the paper, to represent the meridian of A (see plate VIII.); mark a point upon it for the first station, A. Apply the protractor, so that its diameter shall lie along the meridian line, having its centre at the station point, and set the vernier to the first bearing entered in

* For plotting traversing surveys on a small scale, a circular pasteboard protractor has long been in use amongst the Ordnance surveyors: it is made by describing a circle with a radius of five inches, and dividing the circumference to one-half or one-fourth of a degree. The zero of this protractor being made to agree with the meridian on paper, to which all the angles of a survey have reference, a parallel ruler is stretched across its diameter to the opposite corresponding angle, and then moved forward to the point from whence the bearing is to be drawn. For surveys on a very large scale, however, the semicircular brass protractor, with a vernier, is necessarily more accurate. I am informed that pasteboard protractors, printed from a plate for the use of the Ordnance surveyors, may be purchased at the Ordnance Map Dépôt, Southampton.

the field-book, namely, 50° 4′-or this may be done previous to placing the protractor: draw a fine line on the paper along the arm of the protractor from the first station, which line defines the direction of station 2 (B), lying 50° 4′ N.E. of the meridian.* Next, from any scale of equal parts selected for the survey, take in the compasses 480, being the whole length measured to 2,† which distance, taken along the line just drawn, fixes 2 (B). Through this point, draw a line, N B, parallel to N A, for the meridian of 2. Now set the protractor to 121°20', apply it to the line, N B, and draw an indefinite line in a N.W. direction, being the check bearing to a windmill: which done, set the protractor to 10°, and draw the line, B C, measure 450 from your scale, and fix the point, C, 3. Make NC parallel to N B, and having set the protractor to 74° 50′, the check bearing on the windmill, draw a line which will intersect the check line from B, and fix the position of the windmill. The protractor is then set to 165°, applied to the line N C, and the distance to D being laid off, a line parallel to C N is drawn through D; when, if the check bearing 31° 25' is found to meet at the windmill, a proof is afforded of the accuracy both of the surveying and plotting thus far.

The station lines of the survey having been thus laid down, the off-sets are then to be plotted; and now a plotting scale, or slip of paper divided to a scale, is extremely useful. Beginning at O A, lay off 200 and 400 along the

* The learner will bear in mind that the meridian here spoken of is not necessarily the magnetic meridian, but may be any line assumed as a working meridian.

+ A good method is to divide a slip of drawing paper very accurately, according to the intended scale, and apply it to the chain line: this is an expeditious mode, but less exact than taking distances from a diagonal scale with a pair of compasses.

The ivory protractor, with lines for adjusting to a meridian, such as we use for plotting in the field, makes a good plotting scale.

D

line A B, then turn your slip of paper, so as to be at right angles to the line, A B, and at the point, A, set off 40 and 30, as seen in the plan; then at 200, again use the slip scale, and mark off 50 on the left, and 20 on the right, of the line A B; at 400, 60 and 10. Join these points of the cross measurements, or off-sets, and the boundaries of the road will be correctly shown.

The road in plate VIII. being accurately drawn to a scale of half an inch to 100 feet-namely, the large diagonal scale on the ivory protractor-a comparison of it with the description given, will, it is hoped, render the whole process perfectly intelligible.

It may here be observed, that when a survey on a large scale is made, it is usual to note the kind of fence which borders a road, and gates, large trees, &c., are put in; for these, and various other objects noticed in a plan, certain conventional signs are used, for which, see section on plandrawing.

EXAMPLE OF TRAVERSING.

The following example of traversing is taken from Mr. Simms's treatise on instruments; and I recommend it to the student's attention, as highly useful in showing how a check is maintained over a road survey, so as to preclude a possibility of error.

Let the diagram in page 36 represent a survey of roads to be performed with a theodolite and chain. Commencing on a conspicuous spot, a, near the place where two roads meet, the theodolite must be set up and levelled, the upper and lower horizontal plates clamped at zero, and the whole instrument turned about until the magnetic needle steadily points to the N S line of the compass-box, and then fixed in that position by tightening the clamping screw

A (see plate VII.). Now release the upper plate, and direct the telescope to any distant conspicuous object within or near the limits of the survey, such as a pole purposely erected in an accessible situation; that it may be measured to, and the instrument placed upon the same spot at a subsequent part of the operation, as A and B, and after bisecting it with the cross wires, read both the verniers of the horizontal circle, and enter the two readings in the fieldbook; likewise, in the same manner, take bearings, or angles, to all such remarkable objects as are likely to be seen from other stations, as the tree situated on a hill; and lastly, take the angle to your forward station, b, where an assistant must hold a staff for the purpose, on a picket driven into the ground,* in such a situation as will enable you to take the longest possible sight down each of the roads that meet there. In going through the above process, at this and every subsequent station, great caution must be used to prevent the lower horizontal plate from having the least motion after being clamped in its position by the screw, A.

Next measure the distance from a to b, and set up the instrument at b; release the clamp-screw A only, not suffering the upper plate to be in the least disturbed from the reading it had when directed at a to the forward station b; with the instrument reading this forward angle, turn it bodily round, till the telescope is directed to the station a (which is now the back station), where an assistant must hold a staff, tighten the clamp-screw, A, and by the slowmotion screw, B, bisect the staff as near the ground as possible, and, having examined the reading, to see that no disturbance has taken place, release the upper plate, and, setting it at zero, see if the magnetic needle coincides, as in the

A picket should always be left in the ground at every station, in order to recognise the precise spot, should it afterwards be found necessary to return to it again.

first instance, with the N S line of the compass-box; if it does, all is right, if not, an error must have been committed in taking the last forward angle, or else the upper plate must have moved from its position before the back station had been bisected; when this is the case, it is necessary to return and examine the work at the last station. If this is done every time the instrument is set up, a constant check is kept upon the progress of the work; and this indeed is the most important use of the compass. Having thus proved the accuracy of the last forward angle, release the upper plate, and measure the angles to the stations m and r, and, as before, to whatever objects you may consider will be conspicuous from other places; and lastly, observe the forward angle to the station c, where the theodolite must next be set up, and measure the distance bc.

At c, and at every succeeding station, a similar operation must be performed, bisecting the back station with the instrument reading the last forward angle; then take bearings to every conspicuous object, as the tree on the hill, the station A, &c., which will fix their relative situations on the plan, and they afterwards serve as fixed points to prove the accuracy of the position of such other stations as may have