THE ADJUSTMENTS OF THE SEXTANT.

1st. The index-glass, or central mirror, must be perpendicular to the plane of the instrument.

Place the index to about 60°-viz., to near the middle of the arc or limb. Hold the sextant with its face up, the index-glass being placed near the eye, and the limb turned from the observer. Look obliquely down the glass; then, if the part of the arc to the right, and its image in the mirror, appear as one continued arc of a circle, the adjustment is perfect; if the reflection seems to droop from the arc itself, the glass leans back; if it rises upward, the glass leans forward. The position is rectified by the screws at the back.

2nd. The horizon-glass, or fixed mirror, must be perpendicular to the plane of the instrument.

Set o on the index to o on the arc; hold the instrument horizontally -viz., with its face up; look through the sight-vane, or through the socket which receives the telescope, and direct the sight to the horizonglass; give the instrument a small nodding motion; then, if the horizon, as seen through the transparent part of the horizon-glass, and its image, as seen in the silvered part, appear to be in a continued straight line, the adjustment is perfect; if the image be the lower, the glass stoops forward; if it be the higher, the glass leans backward; and the upper adjusting screw, at the back of the horizon-glass, is to be carefully turned till the coincidence of the reflected and real horizons is quite perfect.

3rd. The horizon-glass must be parallel to the index-glass. Set o on the index to o on the arc; screw the tube or telescope into its socket, and turn the screw at the back of the instrument till the line which separates the transparent and silvered parts of the horizon-glass appears in the middle of the tube or telescope. Hold the sextant vertically—that is with its arc or limb downwards—and direct the sight through the tube or telescope to the horizon; then, if the reflected and true horizons do not coincide, turn the tangent screw at the back of the horizon-glass till they are made to appear in the same straight line. Then will the horizon-glass be truly parallel to the index-glass.

4th. The axis of the telescope must be parallel to the plane of the instrument.

Turn the eye-piece of the telescope till two of the parallel wires in its focus appear parallel to the plane of the instrument; then select two objects, as the sun and moon, whose angular distance must not be less than from 100° to 120°, because an error is more easily discovered when

the distance is great; bring the reflected image of the sun exactly in contact with the direct image of the moon, at the wire nearest the plane of the sextant, and fix the index; then, by altering a little the position of the instrument, make the object appear on the other wire; if the contact still remains perfect, no adjustment is required; if they separate, slacken the screw furthest from the instrument in the ring which holds the telescope, and tighten the other, and vice versa if they overlap.

5th. To find the index error.*-Move the index till the horizon, or any distant object, coincide with its image, and the distance of o on the index from o on the limb is the index error; sutractive when o on the index is to the left, and additive when it is to the right of o on the limb.

Example.-The horizon and its image being made to coincide, the reading is 2' on the arc. Then 2' is the index correction to be subtracted from every angle observed. Or measure the sun's horizontal diameter, moving the index forward on the divisions until the images of the true and reflected sun's touch at the edges; read off the measure which will be on the arc; then cause the images to change sides, by moving the index back; take the measure again, and read off; this reading will be off the arc; half the difference of the two readings is the index correction.

When the reading on the arc is the greater, the correction is subtractive; when the lesser, additive.

One-fourth of the sum of the two readings should be equal to the sun's semi-diameter in the Nautical Almanac for the day.

Thus, suppose the observations in example I to be made on September 26th, 1865, here one-fourth of the sum of the two readings is 16′ 0′′, agreeing with the semidiameter as given in the Nautical Almanac for the given day.

This affords a test of the accuracy with which the observation has been made.

NOTE.-If both readings are on the arc, or both off the arc, half their sum is the index correction-subtractive when both on, additive when both of the arc.

*The index error of reflecting astronomical instruments, such as the sextant, is the difference between the zero (0) point of the graduated limb, and where the zero (o) point ought to be, as shown by the index, when the index-glass is parallel to the horizon-glass.

FF

LOG LINE.

THE length of the stray-line should be sufficient to allow the log-chip to be clear of the eddies of the vessel's wake.

The distance between the knots should bear the same proportion to the number of seconds run by the glass intended to be used, as the number of feet in a nautical mile bears to the number of seconds in an hour.

The number of feet in a nautical mile is 6080.

The number of seconds in an hour is 3600.

Therefore, to find the length of a knot corresponding to a 28 seconds glass, we proceed as follows:

3600 6080 :: 28

28

48640 12160

ft. in.

360,0)17024,0(473

1440

2624
2520

104

12

360)1248(3

1080

We have for glasses running 30 seconds and 32 seconds the following proportions:

IN nautical phrase the lead line has "nine marks and eleven deeps." At two fathoms, the mark is leather; at three fathoms, leather; at five, white rag; at seven, red rag; at ten, a piece of leather with a hole in it; at thirteen, blue rag; fifteen, white rag; seventeen the same as at seven; at twenty fathoms, a piece of cord with two knots.

Deep-sea lead lines are marked the same as far as twenty fathoms; then add a piece of cord with an additional knot for every ten fathoms, and a strip of leather for every five fathoms,

LIGHTS AND FOG SIGNALS.

THE following is the Admiralty notice respecting lights and fog signals to be carried and used by sea-going vessels, to prevent collision.— Merchant Shipping Act Amendment Act, June 1st, 1863.

STEAM VESSELS.-All sea-going steam-vessels, when under steam, shall, between sunset and sunrise, exhibit the following lights :A bright White Light at the foremast head.

I.

A Green Light on the starboard side.

A Red Light on the port side.

The Mast-head Light shall be so constructed as to be visible on a dark night, with a clear atmosphere, at a distance of at least 5 miles, and shall show an uniform and unbroken light over an arc of the horizon of 20 points of the compass; and it shall be so fixed as to throw the light 10 points on each side of the ship-viz., from right ahead to 2 points abaft the beam on either side.

2. The Green Light on the starboard side, and the Red Light on the port side, shall be so constructed as to be visible on a dark night, with a clear atmosphere, at a distance of at least 2 miles, and show an uniform and unbroken light over an arc of the horizon of 10 points of the compass; and they shall be so fixed as to throw the light from right ahead to 2 points abaft the beam on the starboard and on the port sides respectively.

3. The side lights are to be fitted with inboard screens projecting at least 3 feet forward from the light, so as to prevent the lights from being seen across the bow.

4. Steam-vessels under sail only, are not to carry their Mast-head Light.

STEAM SHIPS When towing other ships, shall carry Two Bright White Mast-head Lights vertically, in addition to their side lights, so as to distinguish them from other steam ships. Each of these mast-head lights shall be of the same construction and character as the mast-head lights which other steam ships are required to carry.

SAILING SHIPS under way, or being towed, shall carry the same lights as steam ships under way, with the exception of the White Mast-head Lights, which they shall never carry.

Whenever, as in the case of small vessels during bad weather, the Green and Red Lights cannot be fixed, these lights shall be kept on deck, on their respective sides of the vessel, ready for instant exhibition; and shall, on the approach of or to other vessels, be exhibited on their respective sides in sufficient time to prevent collision, in such manner as

to make them most visible, and so that the Green Light shall not be seen on the port side, nor the Red Light on the starboard side. To make the use of these portable lights more certain and easy, the lanterns containing them shall each be painted outside with the colour of the light they respectively contain, and shall be provided with suitable screens.

SHIPS AT ANCHOR.-Ships, whether steam ships or sailing ships, when at anchor in roadsteads or fairways, shall exhibit, where it can best be seen, but at a height not exceeding 20 feet above the hull, a White Light, in a globular lantern of 8 inches in diameter, and so constructed as to show a clear, uniform, and unbroken light visible all round the horizon, and at a distance of at least I mile.

SAILING PILOT VESSELS shall not carry the lights required for other sailing vessels, but shall carry a White Light at the mast-head, visible all round the horizon,-and shall also exhibit a Flare-up Light every 15 minutes.

OPEN FISHING BOATS and other open boats shall not be required to carry the side lights required for other vessels; but shall, if they do not carry such lights, carry a lantern having a Green Slide on the one side and a Red Slide on the other side; and on the approach of or to other vessels, such lantern shall be exhibited in sufficient time to prevent collision, so that the Green Light shall not be seen on the port side, nor the Red Light on the starboard side.

Fishing vessels and open boats when at anchor, or attached to their nets and stationary, shall exhibit a bright White Light; they shall, however, not be prevented from using a Flare-up in addition, if considered expedient.

Fog Signals. Whenever there is a fog, whether by day or night, the fog signals described below shall be carried and used, and shall be sounded at least every 5 minutes, viz. :

(a.) Steam ships under way shall use a Steam Whistle placed before the funnel, not less than 8 feet from the deck.

(b.) Sailing ships under way shall use a Fog Horn.

(c.) Steam ships and sailing ships when not under way shall use a Bell.

RULE OF THE ROAD.

Two Sailing Ships meeting.-If two sailing ships are meeting end on or nearly end on so as to involve risk of collision, the helms of both shall be put to Port, so that each may pass on the Port Side of the other.

Two Sailing Ships crossing.-When two sailing ships are crossing so