4. Description of Gunter's Quadrant.
This quadrant, invented by Edmund Gunter, Profes sor of Astronomy in Gresham College, has been in use since 1618. It is commonly made of wood, with its limb divided into and two sight vanes placed in one of the radii next the division of 90°. It has like wise a stereographical projection of the sphere on the plane of the equinoctial, and a calendar of the months close to the divisions on the limb. The indications on the limb arc obtained by a plummet-line with a move able bead. The places of five stars are also laid down on the quadrant, so that a series of astronomical pro blems can be performed with the quadrant in the same manner as with a celestial globe. Thus, 1st, if the thread is laid to the day of the month, it will cut on the limb the degree of the sun's meridian altitude, and vice versa. 2d, If the bead is slid on the thread to the sun's place in the ecliptic, and if the thread be made to cut on the limb the sun's meridian altitude, as ob served with the quadrant, the bead will fall upon the hour of the day. 3d, If the bead is first laid to the sun's place, and then on the given hour of the day, the thread will cut the sun's altitude on the limb. 4th, If the bead is set to the sun's place, and the thread mov ed to the line of declination, the bead will cut the sun's declination. 5th, The bead being rectified to the hour of the day, as in art. 2, and the sun's altitude ob served, bring the thread to the complement of the alti tude, and the bead will point out the sun's azimuth among the azimuth lines. 6th, The thread being laid upon the sun's place in the ecliptic, it will point out on the limb his right ascension, and vice versa. 7th, In order to find the hour of the night from any of the five stars laid down on the limb, put the bead to the star to be observed, and, by art 2, find how many hours it is from the meridian. From the star's right ascension substract the sun's right ascension in time, and add this difference to the observed hour or the star from the meridian, the sum is the hour of the 5. Descripticn of Sutton's or Collins' Quadrant.
This quadrant, see Fig. 11, fitted to the latitude of London, contains a stereographic projection of one quar ter of the sphere between the tropics, the eye being in its north pole. The lines which run from right to left
are parallels of altitude, and those which cross them are azimuths. The lesser of the two circles which bound the projection is one-fourth of the tropic of Capricorn, and the greater is one-fourth of that of Cancer. From a point on the left edge of the quadrant are drawn the two ecliptics, having the signs of the zodiac, and from the same point are drawn the two horizons. The limb is graduated both in degrees and hours, and, from the sun's altitude, the hour or the day may be found to a minute. The divided quadrants nearest the centre con tain the calendar of months, and beneath them is the sun's declination. Several of the most remarkable stars between the tropics are laid down on the projection, and immediately below the projection is the quadrant and line of shadows. The method of using this quadrant is nearly the same as that of Gunter's.
6. Description of the Horodictical Quadrant.
This little instrument derives its name from its pro perty of telling the hour of the day, and is made as fol lows The quadrant CAB Plate CCCCLXXVI. 12. has its limb AB divided into 90°, and round its cen tre C are described seven concentric circles, having the signs of the zodiac added to them. A ruler is now ap plied to the centre C and the limb AB, and the several parallels are marked, the degrees corresponding to the altitudes of the sun when in those degrees, for the given hours. The points belonging to the same hour arc then connected by a curve line, and the number of the hour added. A pair of sights are now fitted to the radius CA, and a thread furnished with a plummet, and a sliding bead, is attached to the centre C. By bringing the bead to the parallel on which the sun is, and directing the quadrant to the sun till a solar ray passes through the sights, the bead will point at the hour of the day, as the plummet-line cuts all the parallels in the degrees corresponding to the sun's altitude. As the bead is in the parallel which the sun then describes, the bead must show the present hour, even though hour-lines pass through the degrees of altitude to which the sun rises every hour.