absolutely uniform, that the length of the sidereal day has not varied of a second in 2,000 years. The sidereal day is divided into twenty-four equal portions, which are called sidereal hours, and each of these into sixty portions, termed sidereal minutes, etc.

Astronomical clocks are regulated to keep sidereal time. The day commences when the vernal equinox is on the meridian. Therefore, the time by the sidereal clock does not in any way point out the hour of the ordinary day. It only indicates how long it is since the vernal equinox crossed the meridian, and thus always shows the right ascension of any star which may happen to be on the meridian at that moment. The hours of the clock are easily reduced to degrees (see p. 38). The astronomer always reckons the hours of the day consecutively up to twenty-four.

SOLAR TIME.-A solar day is the interval between two successive passages of the sun across the meridian of any place. If the earth were stationary in its orbit, the solar day would be of the same length as the sidereal; but while the earth is turning around on its axis, it is going forward at the rate o. 360° in a year, or about 1° per day. When the earth has made a complete revolution, it must therefore perform a part of another revolution through this additional degree, in order to bring the same meridian vertically under the sun. One degree of diurnal revolution is about equal to four minutes of time.

Hence the solar day is about four minutes longer than the sidereal day. For the convenience of society, it is customary to call the solar day 24 hours long, and make the sidereal day only 23 hr. 56 min. 4 sec. in length, expressed in mean solar time. A sidereal day being shorter than a solar one, the sidereal hours, minutes, etc., are shorter than the solar; 24 hours of mean solar time being equal to 24 hr. 3 min. 56 sec. of sidereal time.

From what has been said, it follows that the earth makes 366 revolutions around its axis in 365 solar days.

MEAN SOLAR TIME.-The solar days are of unequal length. To obviate this difficulty, astronomers suppose a mean sun moving through the equator of the heavens (which is a circle and not an ellipse) with a perfectly uniform motion. When this mean sun passes the meridian of any place, it is mean noon; and when the true sun is in the same position, it is apparent noon. This day is the average length of all the solar days in the year. The clocks in common use are regulated to keep mean time. When, therefore, it is twelve by the clock, the sun may be either a littleast or a little behind the meridian. The difference between the sun-time (apparent solartime) and the clock-time (mean time), is called the "equation of time." This is the greatest about the first of November, when the sun is sixteen and a quarter minutes in advance of the clock. The sun is the slowest about February 10th, when it is about

fourteen and a half minutes behind mean time. Mean and apparent time coincide four times in the year-namely, April 15th, June 15th, September 1st, and December 24th. On those days the noon-mark on the sun-dial coincides with twelve o'clock. In France, until 1816, apparent time was used; and the confusion was so great, that Arago relates how the town clocks would differ thirty minutes in striking the same hour. As the time varied every day, no watchmaker could regulate a watch or clock to keep it.

THE SUN-DIAL-The apparent time of the dial may be readily changed to mean time, by adding or subtracting the number of minutes given in the almanac for each day in the year, under the heading "sun slow" or "sun fast." As a noon-mark is thus a very convenient method of regulating a timepiece, especially in the country, the following manner of obtaining one without a transit instrument may be useful.

Select a level hard surface which is exposed to the sun from about 9 A. M. to 3 P. M. Upon this carefully describe, with compasses, a circle of eight or ten inches in diameter. Take a piece of heavy wire, six or eight inches in length, one end of which is sharpened. Drive this perpendicularly into the centre of the circle, leaving it just high enough to allow the extreme end of its shadow to fall upon the circle about 9 or 10A.M. Mark this point, and also the place where the shadow touches the circle in the afternoon. Take a point half-way between the two, and drawing

Hence the solar day is about four minutes longer than the sidereal day. For the convenience of society, it is customary to call the solar day 24 hours long, and make the sidereal day only 23 hr. 56 min. 4 sec. in length, expressed in mean solar time. A sidereal day being shorter than a solar one, the sidereal hours, minutes, etc., are shorter than the solar; 24 hours of mean solar time being equal to 24 hr. 3 min. 56 sec. of sidereal time.

From what has been said, it follows that the earth makes 366 revolutions around its axis in 365 solar days.

MEAN SOLAR TIME.-The solar days are of unequal length. To obviate this difficulty, astronomers suppose a mean sun moving through the equator of the heavens (which is a circle and not an ellipse) with a perfectly uniform motion. When this mean sun passes the meridian of any place, it is mean noon; and when the true sun is in the same position, it is apparent noon. This day is the average length of all the solar days in the year. The clocks in common use are regulated to keep mean time. When, therefore, it is twelve by the clock, the sun may be either a littleast or a little behind the meridian. The difference between the sun-time (apparent solartime) and the clock-time (mean time), is called the 'equation of time." This is the greatest about the first of November, when the sun is sixteen and a quarter minutes in advance of the clock. The sun is the slowest about February 10th, when it is about

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fourteen and a half minutes behind mean time. Mean and apparent time coincide four times in the year-namely, April 15th, June 15th, September 1st, and December 24th. On those days the noon-mark on the sun-dial coincides with twelve o'clock. In France, until 1816, apparent time was used; and the confusion was so great, that Arago relates how the town clocks would differ thirty minutes in striking the same hour. As the time varied every day, no watchmaker could regulate a watch or clock to keep it.

THE SUN-DIAL-The apparent time of the dial may be readily changed to mean time, by adding or subtracting the number of minutes given in the almanac for each day in the year, under the heading "sun slow" or "sun fast." As a noon-mark is thus a very convenient method of regulating a timepiece, especially in the country, the following manner of obtaining one without a transit instrument may be useful.

Select a level hard surface which is exposed to the sun from about 9 A. M. to 3 P. M. Upon this carefully describe, with compasses, a circle of eight or ten inches in diameter. Take a piece of heavy wire, six or eight inches in length, one end of which is sharpened. Drive this perpendicularly into the centre of the circle, leaving it just high enough to allow the extreme end of its shadow to fall upon the circle about 9 or 10 A. M. Mark this point, and also the place where the shadow touches the circle in the afternoon. Take a point half-way between the two, and drawing