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iaris the radius, or Index bar, moveable along the aro 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 0 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.
« 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.
Thb 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
3. The Horizon-Glass should be parallel with the Index-Glass, when 0 on the
Vernier coincides with O on the Arc of the Instrument.
4. The Lino 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 Op 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 sum 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 0° 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?
A. Find the index error.
Q. 8. How would you find the index error by the horizon?
A. Set 0° on the vernier to 0° on the arc, hold the sextant vertically, and look at the horizon through the horizon-glass: if the horizon and its image are not in one line, move the tangent screw till they are so: the reading is the error.
Note.—The reading may be on the arc proper, or on the arc of excess: it is on the latter when 0° on the vernier is to the right of 0° on the limb, and the reading is then said to be off the arc; when 0° on the vernier is to the left of 0° on the limb, the reading is said to be on the arc. The error is really the difference between 0° on the limb and 0° on the vernier.
Q. 9. How is it to be applied?
A. To be added if off the arc, to be subtracted if on.
Note.—If 0° on the vernier falls to the right of 0° on the limb, i.e. off the arc proper but on the arc of excess, every measurement will be too small, therefore index error is additive: if 0° on the vernier falls to the left of 0° on the limb, i.e. on the arc proper, every measurement will be too great, therefore the index error is subtractive.
10. Place the index at error of minutes to be added, clamp
it, and leave it.
Note.—The Examiner will see that it is correct; this is a reading off the are, i.e. on tht. arc of excess.
11. The Examiner will then place the zero of the vernier on the arc, not near any of the marked divisions, and the candidate will read it.
Note.—In all cases the applicants will name or otherwise point out the screws used in the various adjustments.
Q. 12. How do you find the index error by the sun?
A. Hold the sextant vertically, the index being near 30' on the arc; then look at the sun and move the index till the edges of the sun and its image just touch; take this reading, which is on the arc. Then place the index near 30' off the arc, and again make the edges of the sun and its image touch; take this reading off the arc: half the difference of the two readings is the index error.
Note.—Use the telescope; also clamp the limb and use the tangent screw in perfecting the contact of the sun and its reflected image: take several sights off and on.
Q. 13. How is the same applied?
A. Added, if the greater reading is off the arc; subtracted, if the greater reading is on the arc.
Ex.—Aug. 5th, Beading on — 34' Reading on — Sff
„ off + 29 „ off + 33
2) 5 = diff. 2) 3 = diff.
On greater than off, Ind. Err. — 2'30* Off greater than on, Ind. Err. + V 30*
Q. 14. What proof have you that those measurements or angles have been taken with tolerable accuracy?
A. The sum of the two readings, if correctly taken, divided by 4, should be the sun's semi-diameter for the day as given in the Nautical Almanac.
Ex.—Aug. 5th, Beading on — 34' Beading on — 3C
„ off + 29 „ off + 83
4)63 sum. 4)63 sum.
Sun's semid.... 15' 45' Sun's semid.... 15' 45'
Naut. Aim. 1880, Aug. 5th, gives Sun's Semid. 15' 48'; when there is considerable discrepancy, the admeasurement*) are erroneous.
It remains to explain the fourth adjustment, which is very important, since all observations should be made with a telescope in the eye-piece—and a little practice will soon enable you to use the inverting telescope with facility.
To adjust the line of Collimation.—Put the telescope in the collar; look through it, and turn the telescope until you have two wires parallel with the plane of the instrument; select two distant. objects—stationary ones if possible, otherwise the sun and moon, or two stars of the first magnitude; their distance apart should not be less than 90°: make a contact of the objects as perfect as possible on the wire nearest the plane of the instrument; fix the index to this: then move the sextant so that the objects are brought to the other wire, and if they are still in contact, the axis of the telescope is parallel with the plane of the instrument. If the objects are either apparently separated, or partly cover each other, correct half the error by the screws in the doable collar; then bring the objects again to the wire next the instrument, and. if they are in contact the axis of vision of the telescope is parallel with the plane of the instrument; if not, proceed as at the other wire, and continue till no error remains.
I have explained the true Quadrant and Sextant, but instruments of the first kind are now not unfrequently graduated on the limb to 120°, and of the second kind to 150° or 160°; this arrangement is managed by fixing the index-glass at an angle with the index-bar, and so fixing the horizon-glass that it shall be parallel with the index-glass when 0° on the vernier coincides with 0° on the limb: you can thus read to the limit of the graduated arc.
Besides taking altitudes and measuring distances of heavenly bodies, it is of the first importance that you should make yourself familiar with the method of taking angles of terrestrial objects; by doing so, and with a good compass bearing, you can, when coasting, plot down your position on a chart with great accuracy.
To Read The Sextant.
I shall now endeavour to instruct yon how to read off the Sextant by means of the vernier; look at an instrument while you read the description; first, note that the starting point of the vernier, on the right, is sometimes 0°, and sometimes an arrow head or other device. The limb (arc) of the instrument is cut (graduated) to degrees, and parts of a degree; the degrees are indicated at intervals by numerals, the intermediate long strokes are also degrees, and the shortest divisional strokes are parts (i.e. a certain number of minutes) of a degree. You will also see long and short strokes on the vernier,—the long ones are minutes of a degree, and the shorter ones seconds of a minute. Both are read from right towards left.
If, on the limb, from degree to degree, there are five short strokes, and consequently six spaces, the sextant is cut to 10', and the short strokes on the vernier from minute to minute are each 10", so that you can measure an angle by such an instrument to 10" of arc; and when you get accustomed to its use you can read to half that quantity, viz. 5".
If, on the limb, from degree to degree, there are three short strokes, and consequently four spaces, the sextant is cut to 15', and then the short strokes on the vernier from minute to minute are each 15"; and you can measure an angle to 15".
If, on the limb, from degree to degree, there are two short strokes, and consequently three spaces, the sextant is cut to 20', and then the short strokes on the vernier from minute to minute are each 20"; and you can measure an angle to 20".
These are the most common graduations of the sextant.
We will read the arc or limb to minutes of a degree, and when you understand this much, you will soon be able to read to seconds. First, examine well the sequence of degrees and minutes.
I. Take a sextant cut to 10'; then, on any part of the arc, the first short stroke is 10', the second 20', the third 30', the fourth 40', and the fifth 50';—if the O of the vernier exactly coincides with a long stroke of the arc, the reading is degrees, and no minutes; it may be 10, 14, 20, &c.—any number. Now put O of the vernier to coincide exactly with the fifth short stroke to the left of 20° on the arc, and the reading will be 20° 50', since each short stroke of the five beyond 20° represents 10'. Lastly, fix your eye on the space between 42° and 43°; now put O of the vernier to stand somewhere between the third and fourth short strokes to the left of 42°; in the first place the reading will be 42° 30', but it must be something more because the vernier indicates minutes between 30' and 40'; now look along the line of the vernier and see which minute stroke on it coincides with any stroke on the arc of the sextant; let us say that it is the seventh minute stroke; then the reading will be 42° 37'. When, after this manner, you place the vernier at any part of the instrument and can read off the degrees and minutes, you will soon acquire a knowledge of the seconds, though for ordinary practical purposes it is sufficient to read off to the nearest minute.
II. Take a sextant cut to 15'; then, on any part of the arc, the first short stroke is 15', the second 30', and the third 45';—if O of the vernier exactly coincides with a long stroke of the arc, the reading is degrees, and no minutes: now put O of the vernier to coincide with the third short stroke to the left of 40° on the arc, and the reading will be 40° 45', since each short stroke of the three beyond 40° represents 15'. Lastly, look to the strokes between 57° and 58°; now put O of the vernier to stand somewhere between the first and second short stroke to the left of 57°; in the first place the reading will be 57° 15', but it must be something more because the vernier indicates minutes between 15' and 30' ; next look along the line of the vernier, and see which minute stroke on it coincides with any stroke on the arc of the sextant; let us say it is the eleventh minute stroke, then the reading will be 57° 15' and 11' to add to it, making 57° 26'; and so on for any other indication, thus, if O of the vernier stands between 48° 45' and 49°, let us say on looking along the vernier the ninth minute stroke on it coincides with a stroke on the arc, then the reading is 48° 54'.
III. Take a sextant cut to 20'; then, on any part of the arc, the first short stroke is 20', and the second 40':—if O on the vernier exactly coincides with a long stroke of the arc, the reading is degrees, and no minutes: now put O of the vernier to coincide with the second short stroke to the left of 41° on the arc, and the reading will be 41° 40'. Lastly, look to the strokes between 36° and 37°; now put O of the vernier to stand somewhere between the first and second short stroke to the left of 36°; in the first place the reading will be 36° 20' and something more; next Lok along the vernier and see which minute stroke on it coincides with any stroke on the arc of the sextant; let us say it is the fourteenth stroke, then the reading will be 36° 20' and 14' to add to it, making 36° 34'.