830

THEORY AND PRACTICE OF ENGINEERING.

Book II.

The meridian may also be found by the plane table,as that shown at MLON; describe on it the arcof a circle M P O, and sink it to the depth of an inch,and then within it describe another arc, as Z, parallelwith the first: at L, place a small piquet or staff.

Place this plane table on a level plane about nineo’clock in the morning, as is done at X, so that itstands on its side NO ; and having OL towards thesun, turn it until the extremity of the staff at L castsits shadow on the circle at Z, and then mark on theground the line RX; and after some space of time,observe another point where it cuts the circle: atthree o’clock place the plane table on its side N O,with O L towards the sun, taking care that onecorner of the plane table touches the line R X, as atthe point R ; and observe as before where the shadowof the staff touches the circle Z, and having remarkedthis point, draw along the foot of the plane table theline IIS; from the point of intersection, It, describethe arc X S, which bisected in T will be the meridianrequired.

The dip of the needle is subject to variations, toaccount for which numerous hypotheses have beensuggested. Halley once supposed the earth a greatmagnet, with four poles or points of attraction, twobeing near each pole; but on reflecting that nomagnet was known to possess more than two poles, he abandoned that idea, and imaginedthe earth to consist of an external shell, which contained the magnetic power, having itstwo fixed poles distant from the poles of rotation, and an inner globe separated from theouter crust by a fluid, also having two poles, and the same axis of diurnal rotation as theshell. The general opinion now is, that the magnetism of the earth is owing to itstemperature ; that the globe itself is a great magnet, and the intensity of its magnetism atany point is inversely as the temperature of that point. Those who are desirous of makingaccurate observations upon the variations to which the needle is subject must have recourseto one of the instruments made expressly for the purpose, as Colonel Beaufoy’s variationinstrument, so generally employed in his numerous magnetical experiments, or Dolland’sdipping needle remarkable for its simple construction, and the adaptation of adjustableagate planes, on which the pivots of the needle rest: for the purpose of proving whetherthese planes are horizontal, there is a contrivance on the under side, by which their levelcan be truly ascertained.

A vessel constructed of iron having caused considerable disturbance of the needle, anumber of experiments were undertaken by Mr. Barlow of the Royal Military Academyat Woolwich, who found that the attracting power of iron is not resident in the mass, buton the surface ; so that a hollow shell of about four pounds weight acts as strongly on theneedle, at the same distance, as a solid iron ball of 200 pounds; this being established, itoccurred at once that a thin iron plate of five or six pounds weight might be made torepresent and counteract the whole amount of the attraction of the vessel, and therebyleave the needle perfectly undisturbed: the action of the ship and that of the plate, asregards their effect on the needle, neutralising each other; upon this principle Barlow’scorrecting plate is made, which, mounted on a tripod stand or fixed pedestal, determinesthe variation of the compass in a ship, as dependent upon the local attraction of the vessel.

Maps drawn by the Compass _The instrument used for this pur-

pose has its rim divided into 360 degrees, and against one of itssides parallel to the meridian is a movable rule or alhidade, the upperedge of which is furnished with a groove to bone the objects eithervertically or otherwise, without disturbing the horizontal position ofthe compass. When a map is to be laid down, two stations are se-lected from whence the whole of the country can be seen: placethe compass, for instance, at the point A, and turn it in such amanner that it points north, and that by boning along the alhidadethe point B is seen; remark then how much the needle declinesfrom the meridian of the compass: turn the north side of the compass towards thenext object to be boned, as C, remarking the degrees indicated, as 22° W. ; write this downon the side of the visual ray, and proceed with the other angles of the position, as that of D,which is 7° W. : then place the compass at the second station B, in such a manner that thenorth side is towards D, and observe the degrees which are indicated by the needle, as 15°W. ; figure this down against the visual ray, from B to D : then turning the north side of the

1039.

Fig. 1040.