DIAL

, or Sun-Dial, an instrument for measuring time by means of the sun's shadow. Or, it is a draught or description of certain lines on the surface of a body, so that the shadow of a style, or ray of the sun through a hole, should touch certain marks at certain hours.

Sun-Dials are doubtless of great antiquity. But the first upon record is, it seems, the dial of Ahaz, who began to reign 400 years before Alexander, and within 12 years of the Building of Rome: it is mentioned in Isaiah, chap. 38, ver. 8.

Several of the antients are spoken of, as makers of dials; as Anaximenes Milesius, Thales. Vitruvius mentions one made by the ancient Chaldee historian Berosus, on a reclining plane, almost parallel to the equator. Aristarchus Samius invented the hemispherical dial. And there were at the same time some spherical ones, with a needle for a gnomon. The discus of Aristarchus was an horizontal dial, with its rim raised up all around, to prevent the shadow from stretching too far.

It was late before the Romans became acquainted with dials. The first sun-dial at Rome was set up by Papyrius Cursor, about the 460th year of the city; before which time, Pliny says there is no mention of any account of time but by the sun's rifing and setting: the first dial was set up near the temple of Quirinus; but being inaccurate, about 30 years after, another was brought out of Sicily by the consul M. Valerius Messala, which he placed on a pillar near the Rostrum; but neither did this shew time truly, because not made for that latitude; and, after using it 99 years, Martius Philippus set up another more exact.

The diversity of sun-dials arises from the different situation of the planes, and from the different figure of the surfaces upon which they are described; whence they become denominated equinoctial, horizontal, vertical, polar, direct, erect, declining, inclining, reclining, cylindrical, &c. For the general principles of their construction, see Dialling.

Dials are sometimes distinguished into primary and secondary.

Primary Dials are such as are drawn either on the plane of the horizon, and thence called horizontal dials; or perpendicular to it, and called vertical dials; or else drawn on the polar and equinoctial planes, though neither horizontal nor vertical. And

Secondary Dials are all those that are drawn on the planes of other circle<*>, beside those last mentioned; or those which either decline, incline, recline, or deincline.

Each of these again is divided into several others, as follow:

Equinoctial Dial, is that which is described on an equinoctial plane, or one parallel to it.

Horizontal Dial, is described on an horizontal plane, or a plane parallel to the horizon.—This dial shews the hours from sun-rise to sun-set.

South Dial, or an Erect, direct South Dial, is that described on the surface of the prime vertical circle looking towards the south.—This dial shews the time from 6 in the morning till 6 at night.

North Dial, or an Erect, direct North Dial, is that which is described on the surface of the prime vertical looking northward. This dial only shews the hours before 6 in the morning, and after 6 in the evening.

East Dial, or Erect, direct East Dial, is that drawn on the plane of the meridian, looking to the east.—This can only shew the hours till 12 o'clock.

West Dial, or Erect, direct West Dial, is that described on the western side of the meridian.—This can only shew the hours after noon. Consequently this, and the last preceding one, will shew all the hours of the day between them.

Polar Dial, is that which is described on a plane passing through the poles of the world, and the east and west points of the horizon. It is of two kinds; the first looking up towards the zenith, and called the upper; the latter, down towards the nadir, called the lower. The polar dial therefore is inclined to the horizon in an angle equal to the elevation of the pole.—The upper polar dial shews the hours from 6 in the morning till 6 at night, and the lower one shews the hours before 6 in the morning, and after 6 in the evening, viz, from sun-rise and till sun-set.

Declining Dials, are erect or vertical dials which decline from any of the cardinal points; or they are such as cut either the plane of the prime vertical, or of the horizon, at oblique angles.

Declining dials are of very frequent use; as the walls of houses, on which dials are mostly drawn, commonly deviate from the cardinal points.

Of declining dials there are several kinds, which are denominated from the cardinal points which they are nearest to; as decliners from the south, and from the north, and even from the zenith.

Inclined Dials, are such as are drawn on planes not erect, but inclining, or leaning forward towards the south, or southern side of the horizon, in an angle, either greater or less than the equinoctial plane.

Reclining Dials, are those drawn on planes not erect, but reclined, or leaning backwards from the zenith towards the north, in an angle greater or less than the polar plane.

Deinclined Dials, are such as both decline and incline, or recline.—These last three sorts of dials are very rare. |

Dials without Centres, are those whose hour lines converge so slowly, that the centre, or point of their concourse, cannot be expressed on the given plane.

Quadrantal Dial. See Horodictical Quadrant.

Reflecting Dial. See Reflecting Dial.

Cylindrical Dial, is one drawn on the curve surface of a cylinder. This may first be drawn on a paper plane, and then pasted round a cylinder of wood, &c. It will shew the time of the day, the sun's place in the ecliptic, and his altitude at any time of observation.

There are also Portable Dials, or on a Card, and Universal Dials on a Plain Cross, &c.

Refracted Dials, are such as shew the hour by means of some refracting transparent duid.

Ring Dial, is a small portable dial, consisting of a brass ring or rim, about 2 inches in diameter, and onethird of an inch in breadth. In a point of this rim there is a hole, through which the sun beams pass, and form a bright speck in the concavity of the opposite semi-circle, which gives the hour of the day in the divisions marked within it.

When the hole is fixed, the dial only shews true about the time of the equinox. But to have it perform throughout the whole year, the hole is made moveable, the signs of the zodiac, or the days of the month, being marked on the convex side of the ring; hence, in using it, the moveable hole is set to the day of the month, or the degree of the zodiac the sun is in; then suspending the dial by the little ring, turn it towards the sun, and his rays through the hole will shew the hour on the divisions within side.

Universal, or Astronomical Ring Dial, is a ring dial which shews the hour of the day in any part of the earth; whereas the former is confined to a certain latitude. Its figure see represented below.

It consists of two rings or flat circles, from 2 to 6 inches in diameter; and of a proportionable breadth &c. The outward ring A represents the meridian of any place you are at, and contains two divisions of 90 degrees each, diametrically opposite to one another, the one serving from the equator to the north pole, the other to the south pole. The inner ring represents the equator, and turns exactly within the outer, by means of two pivots in each ring at the hour of 12.

Across the two circles goes a thin reglet or bridge, with a cursor C, sliding along the middle of the bridge, and having a small hole for the sun to shine through. The middle of this bridge is conceived as the axis of the world, and the extremities as the poles: on the one side are drawn the signs of the zodiac, and on the other the days of the month. On the edge of the meridian slides a piece, to which is fitted a small ring to suspend the instrument by.

In this dial, the divisions on the axis are the tangents of the angles of the sun's declination, adapted to the semidiameter of the equator as radius, and placed on either side of the centre: but instead of laying them down from a line of tangents, a scale of equal parts may be made, of which 1000 shall answer exactly to the length of the semi-axis, from the centre to the inside of the equinoctial ring; and then 434 of these parts may be laid down from the centre towards each end, which will limit all divisions on the axis, because 434 is the natural tangent of 23° 28′. And thus, by a nonius fixed to the sliding piece, and taking the sun's declination from an ephemeris, and the tangent of that declination from the table of natural tangents, the slider might be always set true within 2 minutes of a degree. This scale of 434 equal parts might be placed right against the 23° 28′ of the sun's declination, on the axis, instead of the sun's place, which is there of little use. For then the slider might be set in the usual way, to the day of the month, for common use; or to the natural tangent of the declination, when great accuracy is required.

To use this Dial: Place the line a (on the middle of the sliding piece) over the degree of latitude of the place, as for instance 51 1/2 degrees for London: put the line which crosses the hole of the cursor to the degree of the sign, or day of the month. Open the instrument so as that the two rings be at right angles to each other, and suspend it by the ring H, that the axis of the dial, represented by the middle of the bridge, may be parallel to the axis of the world. Then turn the flat side of the bridge towards the sun, so that his rays, striking through the small hole in the middle of the cursor, may fall exactly on a line drawn round the middle of the concave surface of the inner ring; in which case the bright spot shews the hour of the day in the said concave surface of the ring.

Nocturnal or Night-Dial, is that which shews the hour of the night, by the light, or shadow projected from the moon or stars.

Lunar or Moon Dials may be either purposely described and adapted to the moon's motion; or the hour may be found on a sun-dial by the moon shining upon it, thus: Observe the hour which the shadow of the index points at by moon light; find the days of the moon's age in the calendar, and take 3-4ths of that number, for the hours to be added to the hour shewn by the shadow, to give the hour of the night. The reason of which is, that the moon comes to the same horary circle later than the sun by about three quarters of an hour every day; and at the time of new moon the solar and lunar hour coincide. |

Dial Planes, are the plane superficies upon which the hour lines of dials are drawn.

Tide Dial. See Tide Dial.