808

THEORY AND PRACTICE OK ENGINEERING

Ecok II.

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meridian line already drawn. Either the time or the steps taken may be counted fromone position or station to another ; the writer found Sir "\\ illiam Gill s Itinerary of Greece a sure guide, although it contained only the bearings of the places, and the time occupiedin riding or walking; when the traveller has no better map, he must find his course by suchgeneral directions, which have often proved his only security.

Of Demicirdes and their Construction -They °

are usually made either of copper or of wood,and are 12 inches in length, 8 inches wide, andA an inch in thickness; usually a sheet of whitepaper is glued on their surface, upon whichthe divisions are marked. To graduate thedemieircle, divide its diameter A B into twoequal parts in the point C; from this point,with the radius C A, describe the semicircleA D 15, and from the point C elevate the per-pendicular C I on the diameter A B, which willcut the demieircle A D B in two equal partsin D.

To graduate the semicircle A D B, open thecompasses to the extent of the radius C A,and carry the opening three times on the semi-circle A I) B, viz. from A to E, from E to E,and from F to B ; also carry the same extentAC, on the semicircle, from D to G and II :then the summit A 1) B will be divided intosix equal parts. To have the ten degrees,divide each of these six parts into three other equal parts, as A G is divided in thepoints K, L, and G, and each of these must again be divided, and so on till the whole isdivided into 180 degrees.

Demicirdes with sights (a pinnules ) used formeasuring angles, distances, &c., are usuallyabout 15 inches in diameter; the degrees arenumbered from each extremity of their dia-meter, where a sight is attached by a screw, asat C and D. At the centre E is a movablerule with a sight at each of its extremities;in the middle of the semicircle is a compassto show the cardinal points.

A telescope is frequently added to them,by which the angles of objects at a con-siderable distance may be very accuratelytaken, and the whole is placed on a stand :with such an instrument it is possible todetermine the position of the several head-lands and principal objects on a coast, tocomplete a maritime survey, and afterwardsto lay them accurately down upon* paper.

Suppose two boats are anchored at a knowndistance from each other, and the bearingsof the several objects taken from each ; thenif the measured distance between the boatsbe drawn out to serve as a scale, and thevarious lines laid down according to the ob-served angles, their points of intersection woulddenote the positions of the objects that havebeen noticed; or the same might be done onland, by selecting a plain on which a baseline could be measured, and then fixing stationstaves at the extremities; from these stationstaves the angles which the prominent featuresof the country make with each other may betaken, and the whole may be laid down toany scale, by adopting a similar process to thatpreviously described.

The instrument called the station pointer docs not materially differ from a demieircle,over the centre of which moves a number of arms that can be directed to any object; ge-nerally three rulers, connected by a common centre, are so arranged that they can be turned

Fig. 905.

Fig. 957.