THE ADJUSTMENT OF THE SEXTANT. 1.-The Index Glass should be perpendicular to the plane of the Instrument. To determine if it be so, bring the vernier to the middle of the arc, and with the limb turned from the observer, look obliquely into the mirror, then if the reflected and true arcs appear as one continued arc of a circle, the index glass is in perfect adjustment. 2.-The Horizon Glass should be perpendicular to the plane of the Instrument. With O on the vernier coinciding with O on the arc, hold the sextant horizontally, and looking at the horizon, observe if the reflected and true horizons are in one line; or, the instrument being held perpendicularly, look at any convenient object, as the sun, sweep the index glass along the limb, and if the reflected image pass exactly over the direct image without any lateral projection, the horizon glass is perpendicular. 3.-The Horizon Glass should be parallel with the Index Glass, when O on the vernier exactly coincides with O on the arc. To ascertain this, hold the instrument vertically, and direct the sight, through the telescope or sight vane, to the horizon, and if the reflected and true horizons form one continous line, the horizon glass is parallel with the index glass. 4.-To adjust the line of Collimation, or to set the axis of the telescope parallel to the plane of the sextant, Fix the telescope in its place, taking care that two wires are parallel with the plane of the instrument; select two objects, as the sun and moon, or moon and star, which are more than 90° distant from each other, bring them into contact on the wire nearest to the instrument; then by slightly moving the sextant, see how they appear on the other wire; if they are still in contact the Line of Collimation is in adjustment; but if the bodies separate when brought to the far wire, the object end of the telescope inclines towards the plane of the sextant; if they overlay, it declines from the plane. 5.—To determine the Index Error, measure the sun's diameter on the arc of the instrument, and on the arc of excess, which is done by holding the sextant perpendicularly, and bringing the true and reflected suns in exact contact on each side of 0; half the difference of the two readings will be the index error, which is additive when the reading on the arc of excess is the greater, but subtractive when the reading on the arc of excess is the less of the two. MERCATOR'S CHART. Mercator's Chart, which has been compared to a cylinder unrolled, is a convenient and ingenious method of representing the surface of the globe as if it were a plane. The lines drawn from the top (North) to the bottom (South) of the chart are meridians; those from left (West) to right (East) are parallels; the top and bottom are graduated parallels; the extreme right and left are graduated meridians. The latitude of any place is measured on a graduated meridian, and its longitude on a graduated parallel. To find the course between two places, which is represented on this chart by a straight line, lay the edge of a parallel rule on the places, then slide it down until it comes exactly on the centre of one of the compasses, and the course can be read off. To find the distance between two places, one general rule will apply; Take half of the distance between them; the point midway between the two places indicates the latitude,to which one leg of the compass is to be referred on the graduated meridian; carrying the other leg first North and then South of that latitude, the degrees and minutes intercepted between the extreme points, will be the approximate distance when the two places are on the same parallel, or when they lie obliquely, and the true distance when they are on the same meridian. It must be remembered that when the true course between two places is known, Easterly varation allowed to the left and Westerly variation to the right of this, will give the compass course. To ensure accuracy in determining the place of a ship by cross bearings, the difference of the bearings should be as near as possible 90°. THE LOG-LINE AND GLASSES. The speed of a vessel is acertained by means of the Logline, and a sand glass running a given number of seconds. To determine the length of a knot on the log line, we have the following rule: The length of the knot (in feet) must bear the same proportion to a geographical mile (in feet), that the seconds of the glass used at the time of heaving the log, baer to the seconds contained in an hour; from which it follows, that the number of knots and parts of a knot run during the interval indicated by the glass, will give the number of miles and parts of a mile the ship has sailed in an hour, supposing the rate of sailing to be uniform. The geographical mile being about 6080 feet, we have for glasses running 28 the following proportions : For glasses running 303 we have, 3600s 30s :: 6080 ft.: 50·666 ft. 50 ft 8 in. : = the required lengths of the knot, but if 80 feet be rejected from the geographical mile, and the first and third terms of the proportion reduced by dividing them by 600, the statements become hence the method very commonly adopted to ascertain the length of the knot; viz. annex a cipher to the number of seconds run by the glass, and then divide by 6. The log-line is about 120 fathoms in length, and must always have a sufficient quantity of what is termed "stray line," marked off by a bit of bunting, or black leather, from about 10 to 20 fathoms from the log-ship, in excess of the admeasured portion, in order to allow the log to get clear of the eddy of the ship's wake; this length must be determined by the size of the vessel. For fuller information on this subjeet, see the Epitome.] |