Of Surveying Harbours.

two objects to be attained in the survey

the shore along high or low water mark, ngs, to note the points and inlets, and to the places at which rivers and creeks dis- And,

er the channels, their direction, depth, and on of shoals, the depth of water upon them, bottom, and in short, whatever may contrisafe navigation.

the principal points and trace the shore.

rovided a boat and crew, row once or twice ur, mark the more important and prominent , let station-staves with flags upon them be

e a base line, and form a series of triangles, gles at the stations already chosen. Let the triangles be measured with the theodolite, alculated; after which, the high or low water aced along the shore with the compass, as red.

e that Plate 6 is a map of a harbour to be sur

nspecting it, that the upper end of the lake at ion of the harbour at B, the rocks at C, the fisheries at E, and the two bays at Fand G nt points. At these points, therefore, let staaced. Then, measure the distance from A to ne, and let the work be begun at A.

staff at A, and place, by means of a plumb-line, e theodolite over the station. Then, having strument, bring the o of the eyeglass vernier to the o of the limb, and tighten the clamp-screw the body of the instrument until the telescope comes near the base line AB: then tighten the clamp-screw K, an means of the lower tangent-screw L, and the thumb-scre bring the intersection of the spider's lines to coincide the bottom of the staff at B. Then, direct the lower scope to the same point, without moving the limb.

Having thus placed the instrument, examine the opp vernier, and if it stands exactly at 180o, enter the dire from A to B, 00, as in the field notes below.

But if the reading of the opposite vernier exceeds enter half the excess for the direction. If the reading is than 180o, take half of what it falls short, from 360o, enter the remainder for the direction from A to B.

The two verniers are used to avoid any error which m arise from a defective graduation of the limb, or from an perfect centring. A false centring, is when the centre of limb or vernier plate is out of the axis of the instrument, when this is the case, it is a fruitful source of error.

Both verniers should be read at every observation, an mean between the readings taken for the true direction.

Having thus placed the instrument, loosen the clamp-so of the vernier plate, and direct the telescope to station Note the degrees, and take a mean between the reading the two verniers for the minutes, and enter the result opp direction AE, as in the field notes. Do the same for station G, and then enter in a column to the right, the a formed by the lines which join the stations. The angle either be the difference of the readings, or the difference tween 360o and the larger reading, plus the smaller readi

Having sighted to all the stations which can be seen A, remove the instrument and replace the station staff.

Take the theodolite to B, the other extremity of the line. It is now required to place the instrument in su

n from A to B, and place the o of the eye› point so found. Then clamp the vernier direct both the telescopes to station A. It he line of the limb drawn through o and with the base line AB, the o being towards e the theodolite is like placed. d the limb, loosen the clamp-screw of the sight to stations E and C, and enter the

d to all the stations which can be seen from station-staff and remove the instrument to he direction BC=57° 12′ add 180o, and the Having levelled the instrument, place the vernier at 237° 12', and then sight to station of the theodolite will then have the same

as at the stations A and B. enter the directions as below.

- CB

CE

■CD

Station C.

Then sight to

mstrument to E. To the direction CE=18027', he sum will be 360° 27'. Then place the o of 27', and direct the telescope to C. Or, the be placed at E by adding 180° to the direction from A, or to the direction BE, as taken from irecting the telescope to A or B.

the instrument in a similar manner at every Le of the limb passing through o and 180°, conto the base AB, the o being constantly in the ards A. The instrument is thus placed at all nd the following are the results of the measureangles.

The measurements which have been made, enable us calculate the lengths of the lines joining the several static For, commencing with the triangle AEB, we know all angles and the base line AB; we can, therefore, find sides ΕΒ, ΕA. We shall then know one side and all angles of the triangle CEB, and by pursuing the calculati the sides of all the triangles can be readily found.

Since the third angle of a triangle can al vays be fou when two of the angles are known, it may seem unnecess to measure all the angles. But when the three angles measured and their sum found equal to 180°, the worl proved to be right, and this verification should never omitted.

It is not probable that the sum of the three measured gles will be exactly equal to 180°. But they ought not _ of the instrument will be much diminished.

nod of determining points by a series of cons, is called the method by triangulation. It to any number of triangles, and if the three riangle be measured, and the work carefully tep, there is little danger of error. We have nod only in the survey of a harbour, but it h equal advantage in all surveys in which ⚫ be determined, and is, indeed, the only one d on, where great accuracy is required.

The Manner of using the Compass.

mpass is often used in connection with the although a rude instrument, may yet be relied ter lines and smaller parts of a survey. The manner of keeping the field notes.

er into two equal parts, by two parallel lines Cher, and consider each part as a separate leaf h leaf is divided into three spaces, and the generally smaller than either of the others, al.

begin at the bottom of the first page, and run › the top. They then commence again at the next page, and run up to the top; thence to the third page, and thus, for as many pages as require.

compass is used in the way we are about to listances to objects which lie on the right or left , are determined by means of offsets.

ning of every course is designated in the middle and the bearing is entered directly above. The of the middle column, express the distances from g of each course to the offsets, and those in the indicate the lengths of the offsets, or the disects on the right or left of the compass lines. ns, at which the compass is placed, are designated niddle column, and the bearing of each course is