Rk. rock; Sh.

=

shoal; Sp. = springs; Vis.

visible;+ Obs. Spot = Place where observation was made. Anchorage is indicated by an anchor, and the depth is generally close by.

Soundings are reduced to mean Low Water of ordinary Spring Tides, and are expressed in Fathoms and fractions of a fathom, or in Feet and fractions of a foot.

The Velocity of Tide is expressed in Knots and fractions of a knot.

A number with a line and dot above signifies no bottom at the depth given, thus 123 no bottom at 123 fathoms.

=

All Heights are in feet above High Water ordinary Springs, and where there is no tide, above the mean level of the sea.

The Rise of Tide is measured from the mear Low Water level of ordinary Springs; the Range of Tide from the Low Water of one tide to the High Water of the following tide.

All distances are in Nautical Miles; a Cable's length is the tenth part of the nautical mile.

A fathom

=

8674 nautical mile. 13 nautical miles 15 statute miles, very

nearly.

SOUNDING is the operation of trying the depth of the water, and the quality of the sea bottom, by means of a plummet (lead) sunk from a ship.

There are two plummets used for this purpose-the hand-lead and the deepsea lead-which are attached, by means of a stop, to a long line called the

LEAD-LINE.

Both leads are cylindrical in form, and tapered, being widest across at the bottom end (the heel), which is hollowed out for the reception of a lump of tallow; this tallow is called the arming, and its purpose is to bring up some of the bottom it touches, so that the quality or nature of the ground you strike soundings on may be compared with the instructions on the chart or in the sailing directions, and the ship's position approximately known thereby. The narrow upper end of the lead has a hole in it, through which is rove a gromet well served over to keep it from chafing. In the end of the lead-line there is a long eye spliced, which is also served over. The eye is rove through the gromet and taken over the lead, being thus secured.

The HAND-LEAD, of which there are two-one 7 pounds weight, and the other 9 pounds, hence readily thrown by the hand-is used in shallow water, when in the vicinity of land, and for sounding in narrow channels, rivers, and harbours, to show how much water the ship is in, as well as the bottom if necessary.

The HAND-LEAD-LINE, which is usually 25 fathoms in length, is marked at every 2 or 3 fathoms, so that the depth of water may be ascertained either in the day or night; it is said to have nine marks and eleven deeps-the latter not being indicated by marks; they are as follows::

Calico, bunting, and serge are preferred as distinctive marks, because a man can tell the difference in the dark by the feel.

The line is always marked from the heel of the lead, and should be well stretched before marking.

Sounding with the hand-lead, which is called taking a cast of the lead, as well as heaving the lead, the leadsman stands in the chains to windward. Having the line all ready to run out, without interruption, he holds it nearly at the distance of a fathom from the lead, and having swung the latter backwards and forwards three or four times, in order to acquire the greater velocity, he swings it round his head, and thence, as far forward as is necessary; so that, by the lead's sinking while the ship advances, the line may be almost perpendicular when it reaches the bottom. The leadsman then makes known the depth of water in a kind of song:-thus, if the mark of 5 fathoms is close to the surface of the water, he calls, "By the mark five;" as there are no marks at 4, 6, 8, &c., he estimates those numbers, and calls, "By the deep four," &c.; if he judges it to be a quarter or a half more than any particular number, he calls, "and a quarter five,' "and a half four," and so forth: if he conceives the depth to be three-quarters more than a particular number, he calls it a quarter less than the next, thus at 4 fathoms he calls, "a quarter less five;" thus the only fractions of a fathom spoken of are a quarter and a half.

Note.-An up and down cast is the only true one; and the error in soundings is generally in excess. Deep is a corruption of dip, the leadsman having to haul up and then dip the lead line when ascertaining the unmarked fathoms.

The DEEP SEA LEAD is of larger size than the hand lead, varying from 28 to

35 pounds in weight, and is attached to a much longer line, to find bottom in 100 or more fathoms.

The DEEP-SEA LEAD-LINE is a strong and water-laid line, marked as the hand lead-line to 20 fathoms; and then a strand with three knots indicates 30, four knots 40 fathoms, and so on, with an additional knot for every ten ; the intermediate fives are marked by small strands with a single knot, or by a piece of leather. At 100 fathoms is a piece of bunting, and then re-commence the knots. To use this lead effectively it is usual to bring-to the ship; the lead is then thrown as far as possible from the ship on the line of her drift, so that, as it sinks, the ship drives more perpendicularly over it. There is a suitable reel for this line.

Note.-To heave the lead properly requires practice, and nerve; there is a knack and sensitiveness to the lead taking the ground which can only be acquired early in life. The principal Patents are- -MASSEY'S Frictionless Shield Sounder, and WALKER'S Harpoon Sounding Machine.

THE SEXTANT AND QUADRANT.

The SEXTANT derives its name from the extent of its limb, which is the sixth part of a circle, or 60°, but being an instrument of double reflexion it is divided into 120°. It is used for measuring angles,-as the altitudes of, and distance between, heavenly bodies,—as well as angles between terrestrial objects.

The QUADBANT is properly an octant, as the limb is only the eighth part of a circle, or 45°, but, like the sextant, being an instrument of double reflexion, it is divided into 90°.

Both instruments are constructed on the same principles, and have similar parts; but the Quadrant is generally the rougher and commoner instrument of the two, sufficient however for taking altitudes at sea. From a mechanical point, however, both instruments can be made equally accurate and perfect. will be sufficient to describe the Sextant.

It

Accuracy of the Instrument.-When the joints of the framework are close and tight, and the various screws fit closely and act well,-when the centering is perfect, when the graduation of the limb and the vernier is accurate in every part, -when each of the reflectors or mirrors has its two faces parallel, and the glass perfectly clear, and when the shades have clear glasses, the two faces of each glass parallel, and the set work with all their faces parallel,-the instrument may be considered perfect, as regards the optical and mechanical principles of its construction, and without any sensible error, but what the adjusting screws can rectify.

DESCRIPTION OF THE PARTS OF THE SEXTANT.-The form of the Sextant is that of the figure, p. 56, and the flat upper surface is the plane of the instrument; the following are the principal parts:

A A is the Arc (or limb) which is graduated from right to left, from 0 (the zero point) to 120° or 150°; this is the arc proper, and the subdivisions are by 10', 15', or 20', according to the size of the instrument. To the right of O, from left to right, there is also a small space of graduation which is called the arc of

excess.

I RV is the radius, or Index bar, moveable along the arc and round a centre, and having a dividing scale (called the Vernier) close to the arc, by which the subdivisions of the arc are read off.

I is the Index Glass, a reflector which moves with the index-bar, and is fixed on it in such manner that its plane is over the centre of motion of the index, and perpendicular to the plane of the instrument.

H is the Horizon Glass, one half of which is a reflector, and the other half plain to admit of objects being seen through it; it is fixed perpendicularly to the plane of the instrument, and parallel with the index-glass when O on the vernier stands at O on the arc.

s are the index Shades, of coloured glass, to be turned down between the index glass and horizon glass, as the sight requires, to moderate the brightness of an object.

s are the horizon Shades, of coloured glass, to be turned up before the open part of the horizon glass, when required.

T is the Telescope, to be inserted in the collar r, which is a double ring. Also, the place of the sight vane.

M is the Microscope (moveable) for reading the arc and vernier.

t is a Tangent screw for giving a small motion to the index after it has been partially fixed by a clamping screw, which is at the back.

THE ADJUSTMENTS OF THE SEXTANT OR QUADRANT.

To describe the adjustments of one instrument is to describe those of the other-and before either is adapted for use

1. The Index-Glass should be perpendicular to the Plane of the Instrument. 2. The Horizon-Glass should be perpendicular to the Plane of the Instru

ment.

3. The Horizon-Glass should be parallel with the Index-Glass, when O on the Vernier coincides with O on the Arc of the Instrument.

4. The Line of Collimation (or in other words, the Axis of the Telescope) should be parallel with the Plane of the Instrument.

For the first three of these adjustments, together with the method of finding the Index Error of the Instrument, it will be sufficient to give answers to the questions set in the Board of Trade paper

ADJUSTMENTS OF THE SEXTANT.

The Applicant for the Yachting Certificate of Competency will answer in writing, on a sheet of paper which will be given him by the Examiner, all the following questions, numbering his answers with the numbers corresponding to the questions.

Q. 1.

What is the first adjustment of the sextant?

A. To set the index-glass perpendicular to the plane of the sextant.

Q. 2. How do you make that adjustment?

A. By placing the index near the middle of the arc and looking obliquely into the glass to see if the arc and its image are continuous; if they are not, slacken or tighten (as required) the outermost of the three adjusting screws. Note.-The sextant is to be turned face upwards, and the arc away from you: the image is the reflected part, and if it appears lower than the true arc, tighten the adjusting screw. The most modern instruments have an adjusting screw at the top of the index-glass, and the screws at the back should not be meddled with,

Q. 3. What is the second adjustment?

A. To set the horizon-glass perpendicular to the plane of the sextant.

Q. 4. Describe how you make that adjustment?

A. Set 0° on the vernier to 0° on the arc; hold the sextant obliquely, looking through the horizon-glass to the horizon; if the horizon and its image form one line, the glass is perpendicular; if not, make them so by the screw at the top.

Note.-Screw in the telescope; hold the sextant in a position between horizontal and vertical, giving it a slight motion: the reflected horizon should appear neither above nor below the real one, when in adjustment. But the sun can also be used, by looking directly at it, using the shades; then if, on moving the vernier backward and forward, the reflected sun does not pass directly over the true sun, turn the screw at the top of the glass until it does so pass,

Q. 5. What is the third adjustment?

A. To set the horizon-glass parallel to the index-glass, with O° on the vernier set to 0° on the arc.

Q. 6. How do you make the third adjustment?

A. By holding the sexant vertically, and looking through the horizon-glass to the horizon: if the horizon and its image form one line the horizon-glass is parallel to the index-glass, if not make them so by the screw at the bottom. Note. Still use the telescope: the line of the true horizon in the unsilvered part of the glass should be perfectly continuous with the reflected horizon: the screw for the adjustment is at the back and at the lower part of the glass.

Q. 7. In the absence of a screw how would you proceed?
Find the index error.

A.

Q. 8.

How would you find the index error by the horizon?

A. Set O° on the vernier to 0° on the arc, hold the sextant vertically, and