1. An instrument for making angular measurements.
So called from its embracing an are of 90° or somewhat more.
Formerly much employed in making astronomical observations.
When fixed to a wall it was called a
mural quadrant; this is now superseded by the
mural circle. See mural circle; mural quadrant.
The quadrant appears to have been used for solar observations about 290 B. C.; and we learn that the Arabian astronomers, about 995 A. D., had a quadrant of 21 feet 8 inches and a sextant of 57 feet 9 inches radius.
Hipparchus and Ptolemy used
diopters, in which the distance of a star from the zenith was observed by looking through two sights fixed in a rule, annexed to a second rule, which was kept vertical by a plumb-line, the angular distance between the two rules being measured.
Among the
Alexandrian astronomers it was noticed that it was not necessary to use a whole circle (see armil), and Ptolemy says that he found it more convenient to observe altitudes by means of a square flat piece of stone or wood, with a
quadrant of a circle inscribed on one of its flat faces, about a center near one of the angles.
The extreme radii of the quadrant represented the horizon and the zenith lines, and, a peg being placed at the center, the observation was made by reference to the point of the are upon which the shadow of the peg fell.
The quadrantal form was adopted, as it secured the maximum length of radius in an instrument which could measure the greatest altitude of any celestial body.
The
Chaldeans asserted that a man walking without stopping might go round the circuit of the earth in a year.
The
Alexandrians recognized the rotundity of the earth, and
Eratosthenes attempted to determine its size by observing the difference between the altitude of the sun at
Alexandria and at
Syene on a given day.
Syene was found to be on the tropic, for a vertical gnomon at noon on the summer solstice cast no shadow.
Alexandria was observed to be distant from the zenith by a fiftieth part of the circumference, and the measured distance between these two cities, which were in the same longitude, was found to be 5,000 stadia.
This gave a circumference of 250,000 stadia to the earth, and a radius of about 40,000.
This differed considerably from the
opinion of Aristotle, and slightly from that of Posidonius, the friend of
Cicero.
The length of the
stadium used was 202 3/4 yards. See armil; astronomical instruments.
Under the Khalif al Maimun, a measurement of two degrees of the meridian was made in the plain of Sind-jar, in Mesopotamia (elsewhere stated to have been on the shore of the
Red Sea). The Arabian astronomers divided themselves into two bands, one proceeding north and the other south, applying their measuring-rods to the ground till each reached a point distant one degree from the starting-point.
The respective measurements were 56 miles and 56 2/3 miles, of 4,000 cubits each.
The cubit is stated to have been the “black cubit” = 27 inches, each inch “the thickness of six grains of barley.”
The
dhoneys of the
Coromandel coast are navigated by the natives with the assistance of a little instrument for ascertaining the latitude.
It consists of a little square board, with a string fast to the center, at the other end of which are certain knots.
The upper edge of the board is held by one hand, so as to touch the north star, and the lower edge the horizon.
Then the string is brought with the other hand to touch the tip of the nose, and the knot which comes in contact with the nose tells the latitude.
A quadrant was constructed by Copernicus at
Thorn in 1510.
In 1590,
Davis dispensed with the plumb and adapted the quadrant for use at sea.
Previous to this the astrolabe and mariner's cross had been universally employed by seamen for determining the latitude; the longitude was derived from dead reckoning or guess-work.
The telescope was adapted to the astronomical telescope by
Picard.
All these old forms were superseded for nautical purposes by the reflecting quadrant, invented by
Sir Isaac Newton, 1670.
He communicated the invention to
Dr. Halley, who failed to give it publicity, and it was reinvented by
Godfrey of
Philadelphia, and also by
Hadley in
England.
An instrument constructed on
Hadley's plan was submitted to the Royal Society in 1742, and from having been first made known by
Hadley, the reflecting quadrant has been generally called
Hadley's.
This and all similar reflecting instruments are based on the fact that the angle between the first and last directions of a ray which has undergone two reflections in the same plane is equal to twice the inclination of the reflecting surfaces to each other.
Such instruments, therefore, measure angles double the extent of their are, the degrees and subdivisions being halved; thus, the quadrant, having an arc of 45° only, measures angles up to 90°, the reflecting circle measures two circumferences, and the sextant, which has now largely superseded the quadrant at sea, having a limb of 60° arc, will measure angles of 120°. See sextant.
All measure either vertical or horizontal angles; the quadrant is, however, seldom employed except for taking altitudes of the sun for latitude or local time.
The image of an object reflected from a mirror fixed on a pivoted arm is reflected to the other mirror on a fixed arm, where it is again reflected, and by moving the pivoted arm the image is brought into apparent coincidence with the sea horizon, which is viewed through a part of the fixed mirror left unsilvered for that purpose.
The movable limb is provided with a vernier, which indicates the degrees and minutes of elevation on the graduated fixed limb.
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Gunner's quadrant. |
Graham,
Bird,
Ramsden, and Troughton are the great names in the history of this instrument, their celebrity being especially due to their great successive improvements in graduating Instru-ment (which see).
2. An instrument used by gunners for giving a cannon or mortar the angle of elevation necessary to attain the desired range.
It has a graduated arc and a plumb-line, which indicates the angle of elevation upon the arc when one arm is placed within the bore or the other against the face of the piece in a perpendicular position.
In a more finished and accurate form, a spiritlevel is substituted for the plumb, and one of the branches of the instrument is pivoted and slides over the face of the arc so as to show the elevation.
