850

THEORY AND PRACTICE OF ENGINEERING.

Rook II.

darker colour, when they are projected against the sky. All this being arranged, the opera-tions for the survey may commence at sea. There are three methods adopted for deter-mining, by reference to fixed points on shore, the locality of any station at sea. The firstconsists in observing by means of the compass the bearing of two ormore points on shore, by which means the position of the observer isdetermined when the position and bearings of the points on shore aregiven with respect to each other. Suppose A B to be two objects onshore, and S the position of the observer, from which the angles andmeridian is observed. In the triangle ASN the angles at S and Nare given, consequently the angle at A is known; in the same manner theangle at B is known. Then on the side A B, and the adjacent anglesA and B being given, the point S may be found : with the compass,however, the angles cannot be determined nearer than within one ortwo degrees; when two compasses are employed, one at a height of2 or 3 feet above the other, the mean between the two observationsmade will perhaps approach the truth.

The second method is by observing at the same period of time fromtwo or more stations on shore the bearing of the observer at sea, and this is only to be doneby means of well preconcerted signals.

The third method consists in measuring from the boat by means of the sextant the anglessubtended by three or more objects on shore, the positions of which are given; from thesedata the position of the observer is determined.

An instrument called a Station Pointer is used to insure greater accuracy in takingangles; this is formed of three rules which revolve on a common centre, in such amanner that two triangles can be set out with it; the middle rule is double, and has afine wire stretched along its openings ; the others have also a fine wire, which is stretchedfrom end to end, and so adjusted that all the three wires tend to the centre of the in-strument ; in the centre is an opening through which a steel pin may be passed;attached to the middle limb are two verniers, with arcs of about 100° each, and when allthe limbs are closed, the verniers mark zero, and as the limbs are opened the verniers markon the corresponding arcs the angles they respectively form: the angles subtended bythree stations on the shore at the place of the observer at sea are the measures to whichthe verniers are to be set; and when so set the instrument is laid on the plan, and movedtill the three wires pass through the three stations ; the centre of the instrument ihenoccupies the relative place of the observer, and a dot marked by the steel pin determinesthe point on the plan: a graduated circle on paper or on glass may serve this purposeas well; by drawing on the upper surface lines diverging from the centre at the givenangles, the circle being moved until these radii pass through the stations, the centre otthe circle will give the point required. Excepting in a case where the observer is in thecircumference of the circle passing through the three stations, the measure of two anglesis sufficient to determine his position ; it is as well to observe as many angles as possible,and where accuracy is required, the observations of two angles only should never be con-sidered sufficient. The angles arc taken with the sextant or the reflecting circle, and

measured in the plane of the objects: should this plane be inclined to the horizon, theangles of elevation of each station above the horizon should be observed, to give thenecessary data for reducing the hypotenusal to the horizontal angle: when the differenceof elevation is great, an ideal vertical line may be drawn from the higher object downwardsuntil it apparently meets the base, and the results will besufficiently near the truth; as in the height of a mountain, theline B C may be dropped. It is not usual to apply a telescopeto the sextant, as the objects are brought readily into the fieldof the mirrors by the unassisted eye; and time is a great object,and perhaps the most important of all in making the obser-vations. Sea-water destroying the silver on glass, metallic re-flectors are generally substituted.

Sounding Lines, to ascertain the depths, are formed of strongpliable cord or lead line, divided into feet by different colouredpieces of cloth or other marks; the lead fastened at the extremity is like the frustum of acone, with its base so hollowed out that grease may be introduced into it, which serves,when let down upon sand or mud, to show the nature of the bed: lines of different lengthsand strength are used, and leads differing in weight according to the depth of the water tobe sounded; these lines are liable to sudden changes, and must be constantly compared withsome known standard; deep soundings must be taken when the boat is still, and the depthmeasured in a vertical position: in shallow water sounding rods are substituted, and wherehard rocks occur they are perhaps more convenient. Sunken rocks, reefs, and shallows, re-quire great accuracy in the survey, and it is necessary to cast anchor, in order to get theangles and their measurement with more certainty : when the shoal or reef is so far outat sea, that only two objects on shore can be seen, an assistant boat must be moored

Fig. 1074.

Fig.1073.