1 The tropics are two parallels of declination touching the ecliptic. One, touching it at the beginning of cancer, is called the tropic of cancer; and the other, touching it at the beginning of capricorn, is called the tropic of capricorn. The two points, where the tropics cut the ecliptic, are called the solstitial points. Colures are two secondaries to the celestial equator, one passing through the equinoctial points, called the equinoctial colure; and the other passing through the solstitial points, are called the solstitial colure. The times when the sun comes to the solstitial points are called the solstices. Aberration of a star or any heavenly body, is a small apparent motion, occasioned by the progressive velocity of light. This is calculated by means of Tables XXXIX. XLI. or XLII. Nutation is a small apparent motion of the heavenly bodies, occasioned by a real motion of the earth's axis, arising from the attractions of the sun and moon on the spheroidal form of the earth. The effect of this on the right ascension and declination is given in Table XLIII. and on the longitude in Table XL. The correction in this last Table being generally called the equation of the equinoxes in longitude. Precession of the equinoctial points is a small motion of about 504" per year, occasioned by the same cause as the nutation. By this motion the equinoctial points are carried backward from east to west; consequently, the heavenly bodies appear to move forward the same quantity from west to east. The annual variations of the places of the stars from precession, and the secular equations arising from the change of the earth's orbit by the attraction of the planets, are given in Tables VIII. and XXXVII. The arctic and antarctic circles are two parallels of declination, the former about the north, and the latter about the south pole, the distance of which from the two poles is equal to the distance of the tropics from the equator, which is about 23° 28'. These are also called polar circles. The two tropics and two polar circles, when referred to the earth, divide it into five parts, called zones; the two parts within the polar circles are called the frigid zones; the two parts between the polar circles and tropics are called the temperate zones; and the part between the tropics is called the torrid zone. Besides the imaginary divisions of the earth, there are various natural divisions of its surface, formed by nature, such as continents, oceans, islands, seas, rivers, &c. A Continent is a large tract of land, wherein are several empires, kingdoms, and countries conjoined-as Europe, Asia, Africa, and America. An Island is a part of the earth that is environed or encompassed round by the sea, as Long Island, Block Island, &c. A Peninsula is a portion of land almost surrounded with water, save one narrow neck which joins it to the continent, as the Morea. An Isthmus is a narrow neck of land joining a peninsula to the adjacent land, by which the people may pass from one to the other, as the isthmus of Darien. A Promontory is a high part of land stretching itself into the sea, the extremity of which is called a Cape or Headland. A Mountain is a rising part of dry land, over-topping the adjacent country, and appearing first at a distance. An Ocean is a vast collection of water, separating continents from one another, and washing their borders or shores, as the Atlantic and Pacific Oceans. A Sea is part of the ocean, to which we must sail through some strait, as the Mediterranean and Baltic seas. This term is sometimes used for the whole body of salt water on the globe. A Strait is a narrow part of the ocean lying between two shores, and opening a way into some sea, as the Straits of Gibraltar that lead into the Mediterranean Sea. A Creek is a small narrow part of the sea or river, that goes up but a little way in to the land. I A Bay is a great inlet of the land, as the Bay of Biscay, and the Bay of Mexico; otherwise a bay is a station or road for ships to anchor in. A River is a considerable stream of water issuing out of one or various springs, and continually gliding along in one or more channels, till it discharges itself into the ocean: the lesser streams are called rivulets. A Lake is a large collection of waters in an inland place, as the lakes Superior and Huron in America. A Gulf is a part of the ocean or sea, nearly surrounded by the land, except where it communicates with the sea, as the Gulf of Venice. Thus we have given the most useful definitions of Astronomy and Geography, and to assist the learner there is also given Plate V. in which those terms are explained at one view. We may farther observe, that as the latitude of any place upon the earth is counted from the equator upon an arch of the meridian, the difference of latitude between two places, both north, or both south, is found by subtracting the less latitude from the greater; but if one latitude be north, and the other south, the difference is found by adding both latitudes together. 1. Consequently, if a ship in north latitude sails northerly, or in south latitude southerly, she increases her latitude; but in north latitude sailing southerly, or in south latitude sailing northerly, she decreases her latitude, because she' sails nearer to the equator, from whence the latitude is reckoned. 2. Wherefore, in north latitude sailing northerly, or in south latitude sailing southerly, the difference of latitude, added to the latitude left, gives the latitude in. 3. In north latitude sailing southerly, or in south latitude sailing northerly, the difference of latitude, subtracted from the latitude left, gives the latitude in 4. When the latitude decreases, and the difference of latitude is greater than the latitude sailed from, subtract the latitude left from the difference of latitude, and the remainder will be the latitude in, but of a different name, for it is evident in this case, that the ship has crossed the equator. 5. The difference of longitude between two places, being both east or west, is found by subtracting the less longitude from the greater; but if one be in east longitude and the other in west, their sum is the difference of longitude, when it does not exceed 180°, but if it exceeds 180°, that sum must be subtracted from 360°, and the remainder will be the difference of longitude. 6. Therefore in east longitude sailing easterly, or in west longitude sailing westerly, the difference of longitude added to the longitude left, gives the longitude in, when that sum does not exceed 180°; but if it exceeds 180°, the sum, subtracted from 360°, leaves the longitude in, but of a different name from that left. 7. In east longitude sailing westerly, or in west longitude sailing easterly, the difference of longitude, subtracted from the longitude left, gives the longitude in; but when the difference of longitude is greatest, the longitude left must be subtracted from that difference, and the remainder will be the longitude in, but of a different name from the longitude left. What has been said will be rendered familiar to the learner by the following examples. * In this rule it is supposed, that the sum of the longitude left, and the difference of longit de, is fess than 360°, which is always the case when the difference of longitude is less than 1800, which we In the last example it is evident that as the difference of latitude is more than the latitude left, the ship must have crossed the equator, and consequently come into south latitude. NOTE. When one of the places has no latitude, or is on the equator, the latitude of the other place is their difference of latitude. EXAMPLE V. EXAMPLE VI. What is the difference of longitude between Cape A ship from Cape Charles, in Virginia, sails east- ward till her difference of longitude is 400 miles, 70° 34' W. what longitude is she in? Lisbon's long. Diff. of long. In miles 9 9 W. Cape Charles' long. Diff. of long. 400 miles = 76° 04' W. 6 40 E. 69 24 W. EXAMPLE VIII. 3685 EXAMPLE VII, A ship from 15° 40′ E. long. sails westward till her diff. of long. is 270 15', what long. is she in? What is the difference of longitude between Bar- Long. left Ditf. of Long. EXAMPLE IX. 150 40' E. 27 15 W. 11 35 W. EXAMPLE X. A ship from longitude 160° 20' W sails westward until she differs her longitude 41° 20′; what longitude is she in? 160° 20′ W. What is the difference of longitude between Ma nilla and New-York light house? Long. left 1210 02′ E. Diff. of long. Maniila's long. New-York light-house Sum exceeds 1800 Subtract it from Diff. of long. 74 01 W. 195 03 Long. in 164 57 41 20 W. 201 40 360 00 158 20 E In the last example the ship has crossed the opposite meridian, and therefore has come into a longitude of a different name, PLANE SAILING. LANE SAILING is the art of navigating a ship upon principles de PLANE duced from the supposition of the earth's being an extended plane, on which the meridians are all parallal to each other.* A map of the several parts of the earth, constructed upon these principles, is called a PLANE CHART. When the parts of the earth are thus delineated on a plane, it is easy to see the track by which a ship may go from one place to another, and also what angle this track makes with the meridian.t. Ships at sea are kept in this track by means of an instrument called the mariner's compass. The MARINER'S COMPASS is an artificial representation of the horizon of any place. It consists of a circular piece of paper (see Plate VI. fig. 1) called a card, divided (like the horizon) into 360 degrees or 32 points. This is fixed on a piece of steel, called a needle, to which the magnetic virtue has been communicated by means of a loadstone, which has the property of pointing steadily towards the north, and carrying the card with it, when turning freely on a pivot or any thing to support it. Thus all the points of The explanations of Plane Sailing, and the definitions of this page (and in the former editions of this work) are nearly the same as those given by Moore, in his Practical Navigator; by Robertson in his Elements of Navigation, and by most writers on Navigation. The method of calculating this angle on the true principles of sailing on the spherical surface of the earth, will be given hereafter. |