account of the steel point, upon which the needle swings, being liable to wear, and so prevent freedom of movement, all surveying instruments are fitted with a means of lifting the needle off its point.

In order that the survey may be laid down on paper, or plotted, so that it is possible to determine its position with regard to the points of the compass, it is necessary to take the direction of the first line of the survey by a compass. The instrument used for this purpose is the prismatic compass. Fig. 22 shows a type of instrument made by Messrs. Thornton, Manchester.

Its Use. To obtain the direction in which the first line lies proceed in the following manner. Stand at the end of the line with the prismatic compass, holding it exactly over the first station (or if it is mounted on a tripod, which is better, set the latter up

FIG. 22.

exactly over the station); look
through the slit on the top of the
prism P, align the sight-vane S on
to the end station, and when the
needle steadies read the number of
degrees from the card.
This can

be done by means of the reflecting
prism at the same time as the
sight-vane is observed on the end
station, without moving the eye
from the slit. Care should be taken
to hold the instrument in a horizontal
plane so that the needle may swing
freely.

R is a mirror which is supplied with the instrument. It slides on and off the sight-vane S, and is for the purpose of reflecting the images of objects above or below the horizontal plane of the observer's eye. If the magnetic azimuth of the sun be required the dark glasses N must be interposed between the sun's image and the eye.

When not in use the sight-vane, which is hinged, is shut down on the glass and the prism turned down, so that the instrument can be packed into a leather case. (Cost about two guineas.)

6

The direction of the first line of the survey as obtained by the prismatic compass is called its 'bearing,' and the angle read is always so many degrees East of North. The bearing of a line, then, is the angle it makes with the magnetic meridian, and is measured from the meridian to the line in the direction of the hands of a clock. Meridian may be defined as a great circle supposed to be drawn so as to pass through the poles of the earth, and the zenith and nadir of any given place intersecting the equator at right angles.' The azimuth of a straight line is the angle which it makes with the great circle of the meridian passing through a point in the same; or, in plainer language, is an angle taken horizontally with the

meridian as opposed to an angle in elevation. It will be understood, from what has been written, that the prismatic compass gives 'bearings' and not angles.' The horizontal angles between any distant objects are obtained by taking the difference of their observed bearings.

To test the accuracy of a Prismatic Compass.-Stand on a straight line and observe the bearings of this line in both directions: these should be the opposite points of the compass. Repeat the experiment on another straight line nearly at right angles to the first. This will test the card as to accurate centring.

It is not necessary to have a Prismatic Compass to obtain the bearing of the first line of a survey, though it is the proper instrument for the purpose; it may be taken with an ordinary compass thus: Hold the compass exactly over the station, wait until the needle steadies and points to the magnetic north, then read the number of degrees contained by the angle formed by the needle and the survey line. It is as well to take this reading from either end of the chain line as a check. Note in the field book whether the bearing is east or west of north in this case.

Surveying with the Prismatic Compass.There are two methods, one is known as Traversing, to be described later, and the other as the Triangulation method.

C

GL

FIG. 22A.

Measure a base line in the centre of the area so that the majority of important points around may be visible from the ends of it. Take bearings from the two ends of the base line to all the objects required, and check bearings from each of these objects on to the ends of the base line, and on to one another.

Bearings taken from both ends of a line should differ by 180°. The positions of objects are thus fixed by the intersection of lines drawn from the points of observation. Any two of the extreme points of the first survey thus fixed will act as a fresh base for extending the survey in a similar manner in other directions. A large area of country can thus be surveyed in pieces, but the accuracy of the work will only be approximate unless checked by fitting in with the known points of some trigonometrical survey. Fig. 22A shows the base line AB of a prismatic compass survey, with the points C, D, E, F, &c., fixed by intersection from either end.

Magnetic Variation.-The compass needle does not point to the true north, i.e. to the North Pole. Its direction is continually and regularly varying, the needle at present (1914) at Greenwich pointing about 15° 10' to the west of the north. It will continue to change annually, approaching nearer to the north at the rate of about seven to nine minutes per annum, and then it will continue to change in the same direction for some distance to the east of north.

Afterwards it will swing back again, the maximum east or west of north being about 25°. The number of degrees given by the compass is the number of degrees which we enter in the field book, the allowance for variation being made only when the work is plotted. In plans of land we generally show the direction of the magnetic needle with the north point as in Fig. 23.

This deviation of the needle is called the Declination of the Needle.' It varies at the same place yearly, at different times of the year, and to a smaller extent diurnally. The diurnal variation is said to change continually at the rate of one or two minutes per hour, being greater in summer than in winter.

A chart is published which gives the declination of the needle at different points of the earth's surface, and lines drawn through those points at which the variation is the same are called Isogonic Lines.' The line along which there is no variation is called the Agonic Line.'

ने

FIG. 23.

The arrow represents the magnetic north and the star the true north.

Besides this variation the needle is subject to violent and irregular disturbances which are known as magnetic storms. The needle in these circumstances oscillates rapidly. Earthquakes, volcanic eruptions, and an Aurora Borealis will cause these occasional vibrations.

Local attractions, such as iron pipes in the ground, lamp posts, railings, even steel about the person, may cause trouble.

It will, consequently, be seen that the compass is not an instrument of precision, and a survey conducted with it can probably not be relied upon to a greater degree of accuracy than . It is, however, a very useful instrument for rapid work and for filling in the details of a survey, especially when there are several points on which the work may be checked. So far we have considered only magnetic needles which are set free to move in a horizontal plane, but, though it does not concern our work with the prismatic compass, it is worth while noting that if needles were so supported as only to move in a vertical plane, it would be found that they would' dip' down from the horizontal.

The dip' varies, just as the declination, at different places, and lines connecting those places where the dip' of the needle is the same are called Isoclinic lines.' The line along which there is no dip is called the Aclinic Line.' At the magnetic poles a dipping needle stands vertical.

As the declination of the needle is different at different places and different times, it is therefore absolutely necessary that a surveyor who uses a compass should be able to determine its variation on the spot by astronomical observation.

To find direction of True North.-1. In our hemisphere the shortest shadow of the sun on any day points to the true north.

2. A north and south line may be found approximately by

pointing the hour hand towards the sun and bisecting the angle made between it and the XII on the dial.

3. Obtain the magnetic north by the prismatic compass and find its variation by reference to Whitaker's Almanac, under the heading Magnetic Elements.'

4. Select a convenient piece of ground, which is level, and erect a pole (this might be done on the base line) perpendicularly. About an hour or so before mid-day make a mark on the ground at the extreme end of the shadow and put in a peg. Now, with the bottom of the pole as centre, and the distance from it to the peg as radius describe the arc of a circle upon the ground in the same direction in which the shadow will move. After mid-day, when the sun is sinking, watch carefully until the end of the shadow again coincides with a point in the arc described, and put in a second peg. Bisect the distance between these pegs and put in a third peg. A line ranged through the bottom of the pole and

the peg marking the bisection will be in the direction of true north. (See Fig. 24.)

Nos. 3 and 4 are sufficiently accurate for ordinary chain surveys, but in the case of some important trigonometrical survey an astronomical observation would be necessary, as follows :—

5. Take the bearings of the sun at sunrise and sunset, when its lower limb (i.e. the lower edge of its circle) is a semidiameter above the horizon. Bisect the angle formed and this will be the direction of the true meridian.

6. Observe a star an hour or so before it crosses the meridian (the time of this can be ascertained from the 'Nautical Almanac '), and note on the theodolite the horizontal and vertical angles. When the star is observed again at the same altitude after passing the meridian read the horizontal angle. If the horizontal angle between the first and second observations of the star be bisected, the direction of the true meridian is obtained.

Nos. 5 and 6 require a knowledge of the use of the theodolite, which is explained later in the chapter on Theodolite Surveying.

SURVEYING PAST OBSTACLES

To continue a chain line which would pass through a building

(Fig. 25).—Let ABCD be the chain line, the building being as shown. It is required to find the distance BC and to continue the chain line CD in a straight line with AB.

Take any point F in AB. Erect perpendiculars FE and BK. The lengths of the perpendiculars must be sufficient to clear the building and be equal. The lengths of FB and CD should never be less than three times that of the perpendiculars FE and CH. Now put in poles at E and K and continue the chain line from E through K. When the building is passed put in poles at H and J and erect perpendiculars similar in all respects to those at F and B. Put in poles at C and D. The chain line may now be continued from C and will be in a straight line with AB. The distance from B to C is equal to the distance measured from K to H.

To continue a chain line across a pond (Fig. 26).—Let ABCD be

the chain line, the pond intervening as shown. Set up poles at B and C. Measure back from B towards A a distance BE equal to 50 links. Fix one end of the chain at B and the other at E. Now take hold of the fiftieth link and pull it out taut to F. EBF is then an equilateral triangle. Put in poles at E and F and chain a line from E through F to K until the point K is rather more than midway between B and C. Now make another equilateral triangle on EK —namely, HKJ-as before described. Put in poles at K and J and chain a line from K through J to L, making KL equal to EK. Then EL equals EK or KL.