PROTRACTOR

, a mathematical instrument, chiefly used in surveying, for laying down angles upon paper, &c.

The simplest, and most natural Protractor consists of a semicircular limb ADB (fig. 7, pl. xix) commonly of metal, divided into 180°, and subtended by a diameter AB; in the middle of which is a small notch C,| called the centre of the Protractor. And for the convenience of reckoning both ways, the degrees are numbered from the left hand towards the right, and from the right hand towards the left.

But this instrument is made much more commodious by transferring the divisions from the circumference to the edge of a ruler, whose side EF is parallel to AB, which is easily done by laying a ruler on the centre C, and over the several divisions on the semicircumference ADB, and marking the intersections of that ruler on the line EF: so that a ruler with these divisions marked on one of its sides as above, and returned down the two ends, and numbered both ways as in the circular Protractor, the fourth or blank side representing the diameter of the circle, is both a more useful form than the circular Protractor, and better adapted for putting into a case.

To make any Angle with the Protractor.—Lay the diameter of the Protractor along the given line which is to be one side of the angle, and its centre at the given angular point; then make a mark opposite the given degree of the angle found on the limb of the instrument, and, removing the Protractor, by a plane ruler laid over that point and the centre, draw a line, which will form the angle sought.

In the same way is any given angle measured, to sind the number of degrees it contains.

This Protractor is also very useful in drawing one line perpendicular to another; which is readily done by laying the Protractor across the given line, so that both its centre and the 90th degree on the opposite edge fall upon the line, also one of the edges passing over the given point, by which then let the perpendicular be drawn.

The Improved Protractor is an instrument much like the former, only furnished with a little more apparatus, by which an angle may be set off to a single minute.

The chief addition is an index attached to the centre, about which it is moveable, so as to play freely and steadily over the limb. Beyond the limb the index is divided, on both edges, into 60 equal parts of the portions of circles, intercepted by two other right lines drawn from the centre, so that each makes an angle of one degree with lines drawn to the assumed points from the centre.

To set off an angle of any number of degrees and minutes with this Protractor, move the index, so that one of the lines drawn on the limb, from one of the forementioned points, may fall upon the number of degrees given; and prick off as many of the equal parts on the proper edge of the index as there are minutes given; then drawing a line from the centre to that point so pricked off, the required angle is thus formed with the given line or diameter of the Protractor.

PROVING of Gunpowder. See Eprouvette, and Gunpowd<*>r.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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PROPORTIONALITY
PROPORTIONALS
PROPOSITION
PROSTHAPHERESIS
PROTRACTING
* PROTRACTOR
PSEUDO Stella
PTOLEMAIC
PTOLEMY
PULLEY
PUMP