other. Such flagstaffs then constitute the principal stations in the survey, the lines which they make being base-lines.

The several base-lines are denominated first, second, third, &c The numbers run from the point where the survey commences, and each base-line should embrace as great a length of the projected railway as is consistent with details. In the figure there are only two base-lines in the survey, AC being the first base-line and BD the second base-line.

1

To lay down the base-lines, and draw the projected railway upon an ordnance or other map.

Having fixed flagstaffs at the principal stations A, C, B, and D, the next work is to find out the points to represent those stations upon an ordnance map (or any other map thought preferable). This done, the projected line is then drawn upon the map with a steady hand, its direction being subject to any deviation which the details of the survey may suggest.

To range the secondary and minor lines.

Before placing the station poles that determine the secondary and minor lines, and that enable the surveyor to take bearings, he must first make himself thoroughly acquainted with the boundaries of estates, counties, and parishes, in order to ascertain how far the projected line will interfere with private and public interests. This is a work often surrounded with much difficulty, interest being not unfrequently concealed at this stage, and when doubts exist an increased number of station poles may be advisable so as to get fences, roads, and other objects faithfully delineated, purposely to avoid objections to the accuracy of the plan.

The ranging of the secondary and minor lines on each side of the base-lines, and the setting of the station poles, are similar in railway surveying to what they are in an ordinary survey of several adjoining estates. For the details of the work, we may, therefore, refer back to previous sections. For such data, ordnance maps are seldom or never to be relied upon. The bearings of old church steeples and like prominent objects may be correct, but it is for the most part otherwise with the areas and positions of fields, and the innumerable little things upon which interested parties place a value-things that require to be shown upon the plan-some of which may not have existed at the time of the ordnance survey; consequently all details should be carefully laid down from an actual survey. In the example, the different stations on each side of the base-lines, and the secondary and minor lines which they make, will be seen

upon the drawing or plan, and the plotting data they involve understood.

The distance on either side of the projected railway that requires to be surveyed is invariably specified in the order or agreement. It generally considerably exceeds the parliamentary requirements subquently given and illustrated in Plate XIII., by "limits of deviation," and may extend from 30 chains to a mile. Thus in the figure it extends to Lynch Church on the one side, and to Linton Church on the other, both of which serve as stations.

The reason of this extra breadth, as compared with the parliamentary limits, is to make ample provision for deviation, and to include the interests of all parties that may be affected, in order to induce them to come forward, and either support or oppose the projected railway as circumstances may direct.

To take bearings and measure the first base-line AC.

When the surveying staff employed is large, it may be divided as in land-surveying; one party or division taking the theodolite, and the other the chain. But when it is small, the taking of the bearings and measuring with the chain proceed together, an extra labourer being employed to carry the theodolite when the latter work is being prosecuted between stations. This conjunct method is that which the present example illustrates, the chain and theodolite going together, the staff being undivided.

The work commences at station A, and in a complete survey of the whole length of a projected railway, the bearings of the first baseline should be taken from a meridian, as in land-surveying. But when a portion of such a survey is only called for, the bearing of the first base-line may be taken from that of the base-line immediately preceding it on that side, provided station A is in it. If the contrary is the case, as it generally is, two or three other methods may be adopted. First, the base-line AC may be produced to the auxiliary station a, and the bearings at station A taken from it. Second, the bearings at A may be taken from c. Third, the bearings at A may be taken from station 1, the line y 1 thus serving as a meridian.

The first and second of these methods are identically the same, although differing in the details indicated. In point of fact, the line Caa is only a portion of a base-line which the learner may presume has either been ranged from a meridian or from another base-line for had the plan represented the survey of the whole length of the projected railway, then the line CA would have extended beyond A in the direction Aa until its divergence from the railway called for

another base-line; and we may further observe that, in a similar manner, the base BD would have proceeded till its deviation called for a base-line on that side, which may be taken as the third base-line, to range from station 15, in the same manner as the second base-line ranges from B on the first base-line.

From these observations, the learner will readily perceive how the total number of base-lines in a complete survey of the whole length of a projected railway may be ranged from a meridian, and their positions accurately delineated upon a plan. He will also be able to comprehend the proper relation that exists between the survey of a part and of the whole.

According to the above data, the range of the base-line AC with the meridian may be considered as having been previously taken. The bearings at A, therefore, commence either from the auxiliary station a, by the first method, or +1 by the third method. The former is the more accurate of the two methods, as it gives the whole of the details required for rectilinear and angular proof; for if the reading at c is 180°, the line CAa is correct, and if the sum of the angles at A is 360, the whole of the bearings are true, as will be seen below. The readings of the bearings may be entered in the

field-book thus:—

From +a in AC produced

Begin

The several angles at A are now found and read thus:

The bearings at A being taken, the work of measuring with the chain commences from that station to station 3. At +3 the instrument is then placed, and the bearings from A to +1, from +1 to +4, at the north-eastern boundary of Lynch Wood, from +4 to +c, and from +C to + Lynch Church, are each taken and entered in the field-book, as were the bearings at +A. The part of the base-line between +3 and +5 is next driven, the River Ouse being crossed by the bridge on the road from Hurst to Linton, that portion of the line across the river being measured according to Problem XII., Part III. At +5 the bearings are taken from +3 to +6

to get the southern boundary of Lynch Wood, the northern boundary of Linton Mere, and the boundary between the two parishes shown by dotted lines, from 6 to Linton Church, from Linton Church toc, and from + c to Lynch Church; also from Lynch Church to + 3 as a check upon the last angle, so as to determine accurately the position of the church. When the ground is undulating, checks of this kind should never be omitted. The measurement now proceeds to +7, where an angle is taken to +8, the line passing over that from +5 to Linton Church, a minor station pole being placed at the crossing of the two lines. The remaining portion of the first base-line is then driven, viz., from +7 to +C, the distance across the River Ouse being measured by any of the methods given under Problem I. Part VIII. At c the bearings to Linton Church, and to stations 9 and 10, are taken, which conclude the work on the first base-line.

To take bearings and measure the second base-line BD.

The staff now go back to B, and from that station take the bearing from + c to + D, which determines the range of BD. The first portion of the line, viz., from +B to +11, is then measured, the distance across the River Ouse being found, as before, where the first base-line crosses near Linton. The measuring of the remaining portions of the line, and the taking of the bearings at the several stations upon it, may advantageously be left for the learner to conclude as an exercise.

Details of the survey.

The measuring of the secondary and minor lines, the taking of the offsets, and the whole of the remaining portion of the work, with the exception of levelling, and reporting on the probable amount of damages along the projected line, is so exactly similar to land-surveying, that the more advisable course in an educational work of this kind is to leave it to be performed by the student. Such, when taken in detail, forms very suitable exercises, and by this time he must be considered qualified to go through the whole successfully.

With regard to levelling and reporting, all that is generally demanded of the surveyor, relative to the former, in a preliminary survey, is to take some points in the projected railway presumed to be on a level, or in the same horizontal plane with the surface of the ground, or nearly so, the difference between water-level lines and horizontal being nominal, and from these stations to take the angles of elevation and depression, so as to be able to give an approximate estimate of how much land it will occupy. On the