moreover, increases as it becomes colder, to the utmost limit to which refrigeration can be carried.

At a white heat, iron appears to be totally susceptible of any magnetic impression whatever, and a white-hot ball of iron may be brought close to a delicate magnetic needle without at all disturbing its state of repose. Nickel loses its magnetic influence at a much lower temperature, and becomes totally indifferent when heated to 630° Fahrenheit.(74)

(74) The most important magnetic instrument is the Compass. Of this there are two varieties, the Mariner's and the Surveyor's. The essential part of both is a magnetized bar or needle, supported, as in figure 72, on a vertical point. The needle is made heavier at one end than at the other, in order to counteract the dip; and, when it is to be used in succession in different parts of the world, proper poise is effected by a moveable weight.

The form of needle which is now preferred is a modification of that proposed by Kater. His experiments showed a superiority in those made of four bars, united in the form of the sides of a rhomb. The same advantages exist in the modified form represented beneath, which is constructed by rounding the obtuse angles of the rhomb.

In the Mariner's Compass, the needle is attached to a circular card, which revolves or traverses with it. The outer circumference of the card is divided into 360 degrees. Within this is a circle divided into 32 equal parts, called points of the compass. Those which correspond with the two ends of the needle are called North and South. Those at right angles to them, East and West. These four are called cardinal points. The mode of naming the other points will be best understood by inspection of the figure. See opposite.

The axis on which the needle hangs rises from the bottom of a deep box, which is loaded with a heavy weight. This axis terminates in a steel point, which rests in an agate cup, cc, Beneath this cup is a ring, a a, which is moveable, and can, by means of the handle, rr, which is produced to the outside of the compass box, raise the needle from the pivot and prevent it from traversing. At two opposite points, near the upper surface

a

a

n

a

of the box, are two pivots, which rest on a cirular ring. This ring has two pivots at right angles to the first two, which rest on an outer ring which is fixed. The weight of the box, thus hanging from two sets of pivots at right angles to each other, will keep the pivot on which the needle rests vertical. The arrangement will be understood from a reference to the first figure on the next page.

In the Surveyor's Compass the divisions are cut on a ring which surrounds the needle, and forms a part of the box which encloses it. The degrees are divided into four quadrants, which are numbered to the right and left from the N. and S. points.

When a compass is fitted for the purpose of ascertaining the variation of the needle, it is called an Azimuth Compass, from an astronomic term which is used to denote the horizontal angle contained between the position of a body and the N. and S. point. If this angle be referred to the meridian, it is the true azimuth; but if to the N. and S. points of the needle, it is the magnetic azimuth: and the difference between these two angles is the variation. By placing sights on the upper surface of the

Dox which contains the angle, as represented in the preceding figure, the magnetic azimuth is measured by directing these sights to a heavenly body; and, if the hour of the day is known, the true azimuth can be calculated. A more convenient azimuth compass was invented by Kater. In this the divisions of the

E

H

A

card are reflected by a surface placed at an angle of 45o, in a horizontal direction, so that the eye may see at the same time an object by direct vision, and a reflected view of the division immediately between the eye and the object. The divisions on the card are carried on from 0° to 360°, and are marked, the N point of the needle being marked s. Smalcalder's modification of this is more convenient than Kater's original form; and, in an

instrument recently constructed in France by Chevalier, the divisions are cut on the outside of a ring which surrounds the

card, and the reflection is rendered unnecessary. These forms are not only better than the old azimuth compass, but are superior for the use of surveyor's to their ancient instrument. When the variation is to be determined with great accuracy, instruments called Variation Transits are employed. One of the most convenient of these is that constructed by Dollond, of London. This has a telescope instead of ordinary sights, and this is adapted to a vertical angle, by which astronomic observations for determining the time may be made simultaneously with those of azimuth. (See figure on page 271.)

The best form of an instrument for observing the dip is represented in the annexed figure.

The construction of the astatic needle is founded on the principle that a body which is moveable round an axis cannot receive any motion from a force which acts parallel to this axis.

The astatic needle is represented in the annexed figure, where ns is a magnetic needle, moveable round the axis a b. If this axis is placed in the direction in which terrestrial magnetism acts, the needle will rest in any position. This effect is easily produced by two motions perpendicular to each other, one of which is obtained by the milled head S', which, by an endless screw on its axis, works in the teeth of the wheel C, and the