open end downwards, into a bason, of the same fluid. The mercury in the tube will then subside, leaving a vacuum in the upper part of the tube; and the height of the column of mercury in the tube, thus sustained by the pressure of the atmosphere on the surface of the mercury in the bason, will be a just measure. of its weight.

It is found by experiment that the height of the column of mercury is not always the same in the same place, but varies generally between 28 and 31 inches, on the surface of the earth. The barometer has been applied with suc cess to the measuring of accessible altitudes. For this purpose let the height of the mercury in a barometer, both at the bottom and top of the eminence or depth to be measured, be observed as nearly as may be at the same time. Also observe the temperature of the air by ther mometers both attached to the barometers, and at a distance from them, in the shade. Then let the column of mercury in the colder barometer be increased by its 9600th part for every degree of difference in the two attached thermometers (Fahr. scale). Subtract the common logarithm of the less column of mercury from that of the greater, and the difference multiplied by 10000 will be the alt. nearly, in fathoms. For a correction apply, by addition or subtraction, one 435th part of the above alt. for every degree of the mean temperature of the two detached thermometers above or below 31 degrees, and the result will be the true alt.

Bissextile, a year consisting of 366 days, by adding a day to the month of February every 4th year. This day was by Julius Cæsar appointed to be the 24th of March (called by the Romans the 6th of the calends) which being reckoned twice, the year was on this account termed bissextile. This year is, on another account, called leap

year.

Calendar, is a table, almanac, or distribution of time, suited to the several uses of society.

Various calendars have been adopted by different nations in different ages of the world. The Roman calendar, as corrected and established by Julius Cæsar, and thence called the Julian calendar, made the year to consist of 3654 days; viz. three years each containing 365, and the 4th 366. But as the solar year actually falls short of the Julian by about 11 minutes, Pope Gregory XIII, in 1582, reformed this calendar, by striking out the surplus days that the seasons had then got a-head of the calendar; (viz. 10 days) and ordering that, in future, 3 days should be stricken out of every 400 years of the Julian account, by calling every centurial year not devisible by 4 (as 1700,

1800, 1900, 2100, &c.) a common year, instead of a leapyear. The year is divided into 12 calendar months, viz. 7 of 31 days, 4 of 30, and 1 of 28 or 29.

Central forces, are those by the influence of which the planets and comets perform revolutions round their centres of motion, and are retained in their orbits. Those forces are of two kinds, viz. the centrifugal, and the centripetal. Centrifugal or projectile force, may be considered as a single impulse, given by the Creator, and which, agreeably to the laws of motion, would carry the body with a uniform velocity, in a rectilineal direction.

Centripetal force, or force of gravity, may be considered as a continually-operating influence, urging the body down towards the centre of motion: and according to the proportion between these two forces the body will describe a circular, or an elliptical orbit.

Chronology, is that science which treats of time, comprehending its remarkable æras or epochs, divisions, subdivisions, and measures.

Circle, is a plane figure bounded by a uniformly-curved line cailed the circumference, every part of which is equally distant from a certain point within the same, called the centre. Diameter is a right line passing through the centre, and terminated on each side by the circumference. Radius, or semidiameter, is the distance from the centre to the circumference.

Circles of the sphere are of two kinds-great, and small. Great circles, are those which divide the sphere into two equal parts; the chief of which are, the equator, the ecliptic, meridians, horizon, azimuth-circles, and circles of celestial longitude. Small circles, are those which divide the sphere into two unequal parts; the chief of which are, parallels of altitude and of depression, parallels of terrestrial, and parallels of celestial latitude.

Circles of celestial longitude, are those great circles of the sphere which cross the ecliptic at right angles. Circum-polar stars, are those which appear to perform daily circuits round the pole, without rising or setting; and such are all those whose polar distance does not exceed the latitude of the place.

Colures, are those two meridians which pass through the equinoctial and solstitial points of the ecliptic, and are hence distinguished into the equinoctial and solstitial colures.

Comets, are certain bodies in the solar system, moving in very excentric orbits, in various planes and directions, and visible but for a short time when near their perihelia; and then generally appearing with a lucid tail or train

of light, on the side of the comet opposite to the sun. Frequently, however, comets are seen without this lucid train; the body or nucleus being surrounded with a bearded or hairy-like atmosphere. The whole list of comets that have been hitherto observed amounts to upwards of 500; of which about 170 have been observed with accuracy, and the elements of their orbits computed. Conjunction, is that aspect in which two celestial bodies, in the zodiac, have the same longitude.

Constellation, this term is applied to any assemblage or number of neighbouring stars in the heavens, which astronomers have classed together under one general name. They are generally designated by the names and figures of some living creatures, and thus delineated on the celestial globe or atlas. The number of constellations, according to the ancients, was 48, viz. 12 near the ecliptic, called the 12 signs of the zodiac, 21 on the north side of the zodiac, and 15 on the south side. Modern astronomers, by forming new constellations out of such stars as were not included in the above, have increased the number to about 70-The several stars in each constellation are distinguished either by letters of the alphabet, or by numbers and some few by proper names; as, Aldebaran, Castor, Pollux, &c. Crepusculum or twilight-circle, is a circle of depression, 18 degrees below the horizon; for, it is found by observation that when the sun crosses this circle, before rising, or after setting, twilight begins or ends. This is occasioned by the rays of light from the sun being refracted and reflected by the earth's atmosphere.

Culmination of a star, is the point of its greatest elevation

above the horizon, or where it crosses the meridian. Cusps, the horns of the moon, or any other planet, when less than half its illuminated part is visible.

Cycle, is any certain period of time in which the same circumstances, to which the cycle has a reference, regularly return. The most noted chronological cycles are

1. The cycle of the sun, a period of 28 years, after which the same day of the month will happen on the same day of the week, as in the same year of a former cycle.

2. The Metonic or lunar cycle, a period of 19 years, after which the change, full, and other phases of the moon, will happen on the same days of the month, as in the same year of a former cycle.

3. The cycle of Indiction, a period of 15 years, instituted by Constantine A. D. 312, probably as a stated period of

levying a certain tax, and afterwards used as a civil epoch among the Romans.

Note, the 1st year of the Christian æra was the 1st after leap-year, the 9th of the solar cycle, the 1st of the lunar cycle, and the 312th of the Christian æra, was the 1st of the Roman Indiction. Hence rules may be easily deduced for computing what year of any of these cycles, corresponds to any given year of the Christian æra. Day, a portion of time measured by the apparent revolution of the sun, moon, or stars, round the earth. The day is variously distinguished and denominated, according to circumstances, as follows:

1. An artificial day, is the interval of time between sunrising and sun-setting; and thus contradistinguished from night which is the interval between sun-setting and sunrising.

2. A natural day, includes both the artificial day and the night.

3. An apparent solar day, is the time in which the sun appears to make one complete revolution round the earth. These days, owing to sundry causes, (see equation of time) are not all of the same length, but continually varying.

4. A mean solar day, is an exact mean of all the apparent solar days in the year-Or it is that measured by a well-regulated time-piece.

5. A Lunar day, is the time in which the moon appears to make one complete revolution round the earth; and exceeds a solar day about of an hour.

6. A sidereal day, is the time in which any fixed star appears to make a complete revolution, and is 3m. 55.9 less than a mean solar day.

The day, in civil reckoning, begins among different nations at different times.

1. Among most of the ancient eastern nations, and some of the modern, it begins at sun-rising.

2. Among the ancient Athenians and Jews, the eastern parts of Europe, and the modern Italians and Chinese, it begins at sun-setting.

3. With the ancient Arabians, and still with astronomers, it begins at noon.

4. Among the ancient Egyptians and Romans, the Americans, and the greater part of Europeans, it begins at midnight.

Declination of a celestial body, is an arch of the meridian passing through the body, and intercepted between it and the equator; and is north or south according as the body is north or south of the equator.

Degree, the 360th part of the circumference of any circle. Or the 90th part of a right angle.

Dial, or sun-dial, is a delineation of the meridians of the sphere, on a plane, in such a manner that the shadow of a gnomon or stile, placed with its edge parallel to the Earth's axis, may point out the hour of the day. Dials are particularly denominated from the planes on which they are drawn; as horizontal, equatorial, &c. Digit, the 12th part of the apparent diameter of the sun or moon. The quantity of an eclipse is generally estimated by the digits of the luminary's diameter eclipsed. Dip, the depression of the visible, below the true horizon, which will be more or less according to the height of the eye. The dip corresponding to any given height of the eye may be very readily, and very accurately, found by the following theorem.

dyhvh + 1"; in which h= height of eye in feet, and d the dip in minutes and parts, of a degree: thus for 16 feet the dip, per rule 4'-.2′ 1′′= 3′49′′. Direct motion of a planet, in its orbit, is that by which it appears to the observer to move according to the order of the signs. To a spectator in the sun, the planetary motions would always appear direct. To a spectator in the earth, the motions of Mercury and Venus will appear direct when they are in the superior or opposite parts of their orbits; and the motions of the other planets will appear direct when the earth is in the opposite part of its orbit with respect to them.

Disc, the body or face of the sun or moon as it

appears to a spectator on the earth; or of the earth, as it would appear to a spectator in the moon. Dominical letter. In the Roman calendar, it was customary to prefix the first 7 letters of the alphabet to the several days of the week throughout the year, always beginning the year with the letter A. The letter, then, that was prefixed to the Sundays (Dominici dies) throughout the year, was called the Dominical letter. This may be found for any year of the Christian æra, by the following rule.

Divide the centuries by 4, subtract twice the remainder from 6, and to what remains add the odd years and their 4th part, rejecting fractions; divide the sum by 7, and then the remainder taken from 7 will leave the number of the Dominical letter in the alphabet. Thus for the year 1806 the Dominical letter will come out 5

E.