the upper microscope to bisect the south end of the needle, and read its vernier; raise the Y's, and lower gently; repeat the readings, commencing now with the south end. The mean of the four readings is the inclination: poles direct; face of needle to face of instrument; face of instrument east = a.

3. Turn the vertical circle 180° in azimuth, and repeat the process of No. 2, taking again the mean of the four readings, which will be the inclination: poles direct; face of needle to face of instrument; face of instrument west = a'.

4. Reverse the needle on its bearings, and observe as before: poles direct; face of needle reversed; face of instrument west = a".

5. Turn the vertical circle 180° in azimuth, and observe: poles direct; face of needle reversed; face of instrument east = a". The inclination, poles direct, will then be

6. The poles of the needle must now be reversed by means of the bar magnets, by the following process :-Take the needle off the agates, holding it by the end which in the preceding observations was a South pole, and which is now to be converted into a North pole; place it with the flat side (which is lettered) uppermost in the wooden frame designed to prevent any injury occurring to the axle, being careful that the end to be made a North pole is placed towards that part of the wooden frame which is marked accordingly; secure the needle by the brass centre-piece, and place the frame with one end towards the right hand and the other towards the left. Now take the bar-magnets, one in each hand, and let the North pole of the bar-magnet be lowermost in the hand which is towards the end of the frame in which that end of the needle is placed which is to be made a South pole; and let the South pole of the bar-magnet in the other hand be lowermost. Draw the magnet about ten times along the flat side of the needle: the North pole of one bar-magnet being drawn along the end of the needle which is to be made a South pole; and the South pole of the other bar-magnet being drawn along the end of the needle which is to be made a North pole. The needle must then be turned over in the wooden frame, so that its other flat side may become uppermost, which must also be rubbed by the magnets ten times in the manner already described.

The bar-magnets should be held one in each hand, nearly in a vertical position, the lower ends resting on the needle; and must be drawn along the grooves in the wooden frame from near the centre to beyond the ends of the needle. When the process thus described has been gone through, it will be found, on replacing the needle on the agates, that the end which previously dipped below the horizontal line is now inclined above it.

7. The observations described in Nos. 2, 3, 4, and 5, must now be repeated, which will give four other mean readings, b, b', b", b". The inclination with the poles reversed will then be

and the true Magnetic Inclination of the place of observation will be

a + B
@=
2

8. Two such determinations will generally be found sufficient; but if the results differ from each other more than 3' or 4' it is desirable to repeat the observations.

9. On arriving at a new station it is always desirable to magnetise the needle afresh before the observations are commenced. It is indifferent whether an observation is commenced with the end marked A as a North or as a South pole; but it is convenient to call that state of the needle in which the end A is a South pole, and the end B a North pole, "poles direct," and vice versa.

B.-Total Force.

1. Dr. Lloyd has recently suggested a mode of employing the dipcircle for measuring the variations of the total force independent of changes in the magnetic moments of the needle or needles employed. For this purpose the instrument is furnished with two additional needles, which may be called for distinction Nos. 3 and 4, the poles of which are at no time to be reversed or disturbed; Nos. 1 and 2 being the needles used for observing the inclination in the usual way. No. 3 is an ordinary dipping needle; No. 4 is a similar needle loaded with a small fixed and constant weight, acting in opposition to magnetism. The frame, carrying the microscopes of the circle, is also fitted to receive and to retain No. 4 securely in a constant position, when it is used as a deflector of No. 3.

2. The observations consist of two processes; by the one process the position of equilibrium is observed of No. 3 between the action of the earth's magnetism, and that of No. 4 used as a deflector, having its North pole directed alternately towards the magnetic North and South; and by the other process the position of equilibrium of No. 4 is observed between the action of the earth's magnetism and that of the small constant weight with which it is loaded.

3. The observations for the inclination and total force may be conveniently taken in the following order :

1o. Needle No. 1 is to be placed on the agate planes, and a complete observation of the inclination taken with it in the manner already described.

2o. Needle No. 3 is now to be substituted for No. 1, and No. 4 firmly

attached to its supports between the microscopes, and always in the same position. The inclination of No. 3 to the horizon is then to be observed in one position of the needle and circle. The observation is to be repeated with the north end of No. 4 turned in the opposite direction by the revolution of the moveable arms which carry the microscopes; half the difference of the readings in the two positions is the angle of deflection u'.*

ท,

3o. Needle No. 3 is now to be removed, and the loaded needle, No. 4, substituted; and its inclination to the horizon, », is to be observed in the four positions of the needle and circle. The deviation of this needle from the position due to the earth's magnetic force alone is u = n, the angle being positive (+) when measured at the same side of the horizontal line with, and negative (-) in the contrary

case.

4o. Repeat the observations (20).

5o. Make a complete observation of the inclination with needle No. 2.

4. The value of the total force is given by the formula

R = A

COS n sin u sin u'

where A =

X
COS

sin u sin u'
COS 1

as observed at a base station, where X (the horizontal component) has been determined with the unifilar magnetometer, and the inclination has been also observed.

5. The method now described is, however, only applicable to a limited portion of the globe, being especially useful in the higher magnetic latitudes, and cannot be applied (without a readjustment of the loaded needle at a fresh base station) to the opposite hemisphere. If, how ever, the instrument is furnished with a needle such as those employed in Mr. Fox's apparatus (described in Appendix No. 3), in which the weight is attached to a fine thread passing round a light pulley, whose centre is on the axis of the cylindrical axle of the needle, the method becomes universally applicable. The above formulæ then become

1

Ꭱ
= A

and A =

sin u sin u

sin u sin u'.

6. By this means the absolute inclination and the total force relatively to its value at the base station where the constant A was determined, may both be ascertained by the dip-circle alone, without displacement or alteration of its adjustment.

* When the circle (as is usually the case) is divided in quadrants, care must be taken in observations of deflection that when the needle is deflected beyond the vertical the difference of the observed reading from 180° must be taken as the true reading. When it is deflected beyond the horizontal, the observed circle reading is to be entered with the negative sign prefixed, in which latter case the mean deflection will be half the arithmetical sum of the observed readings.

APPENDIX No. 3.

DIRECTIONS FOR USING MR. Fox's APPARATUS FOR OBSERVING THE MAGNETIC INCLINATION AND FORCE.

I. GENERAL REMARKS.

IN fixing the gimball table, it is convenient that it should be so arranged that, when the apparatus is placed on it, the zero divisions of the horizontal circle should coincide with the fore and aft midship-line of the ship.

In preparing for an observation at sea, the circle should be turned in azimuth until the vernier of the horizontal circle shows an angle with its zero corresponding with the difference between the magnetic meridian and the course which the ship is steering. The plane of the circle will then coincide with the magnetic meridian, when the ship is steadily steered. When, from circumstances of weather, &c., the steerage is difficult, an assistant is required to indicate to the observer the times when the ship is steady on her course.

The apparatus is usually furnished with three or four needles, one of which is intended to be used on shore for the determination of the true inclination (when no special instrument is provided for the purpose) by the process previously described, Appendix 2, in which the poles are reversed. The other needles, which are intended for the intensity, are never to have their poles reversed, and care is to be taken not to place them inadvertently near other magnets or iron. Besides the needles, two other magnets are supplied to be used as deflectors. In replacing the needles and deflectors in the travelling box, care should always be taken that the poles of each occupy the places marked for them in the box.

It is desirable to use always the same needle at sea, and to keep it always mounted, clamping it before it is put away for the day; but in case of its undergoing any considerable deterioration from use or accident, one of the other intensity needles may be substituted for it.

When changing the needles at a land station, be very careful not to injure the jewels, or the terminations of the axles of the needles; when a needle is changed it is desirable to hold it chiefly by the grooved wheel; the pivot should first be put into the outer jewelled hole, and the opposite pivot should be carefully guided into the hole at the back whilst the bracket is screwed up.

With respect to the constant weights, it is desirable that the smallest angle of deflection produced by any of the weights employed should not be less than 30°. On account of possible instrumental irregularities it is usual to employ more constant weights than one, with differences between each of half a grain (as for example, 2 grains, 24 grains, 3 grains, &c.). Great care is taken that all the weights which have the

same nominal value should be equiponderant, but it is desirable, if possible, to preserve the same identical weights throughout the whole observations of the same relative series.

II. OBSERVATIONS AT SEA.

A.-Inclination.

1. Direct Observation.-The instrument having been placed on the gimball stand and levelled, and the plane of the circle made parallel to the magnetic meridian, with the face of the circle towards the East, release the needle, which will immediately take approximately the direction of the inclination; rub gently the centre pin at the back with the ivory disk, and read off successively the divisions of the limb indicated by the two ends of the needle; note the readings, which will be +, or positive, when the North pole of the needle dips, and —, or negative, when the South pole of the needle dips: repeat the observation four times, turning the bracket which supports the needle a small quantity before each observation, and being careful to rub the centre pin at the back with the ivory disk whilst reading off. The bracket is turned by means of the screw-heads at the back of the circle, and the object of turning it is to cause the ends of the axle of the needle to have different points of bearing on the jewels in each observation. It is desirable, when four observations are taken, to turn the bracket (say) to the right before each of the first and third observations, and in the opposite direction before each of the second and fourth.

In reading the divisions on the limb, be careful always to bring the division nearest to the needle to coincide with the corresponding division of the second graduated circle immediately behind it, by which means parallax is avoided.

The mean of the four observations or eight readings above described is the apparent inclination by direct observation with the face East.

2. Observation with Deflectors.-Having made the preceding observa. tion, screw in the deflector N (or the north pole of a second needle used as a deflector), and adjust the circle at the back by means of its verniers, so that the deflector may be 40° on one side of the division which in the preceding process (§ 1) was read off as the direct observation with the face East. The needle will then be repelled, and will settle on the opposite side of the dip. Read off (always whilst rubbing with the ivory disk) the divisions indicated by the two ends of the needle. Repeat the observation four times, altering the bearings of the ends of the axle before each observation as above directed. Turn the back circle through 80°, so that the deflector may be 40° on the other side of the apparent dip. Move the needle by the bracket, so that it may be deflected on the opposite side of the apparent dip to what it was before, and make four observations. The mean of the

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