223. The first part of the preceding Table shews how much of the celestial Equator passes over the Meridian in any given part of a mean solar day, and is to be understood the same way as the Table in the 220th article. The latter part, intitled, _Accelerations of the fixed Stars_, affords us an easy method of knowing whether or no our clocks and watches go true: For if, through a small hole in a window-shutter, or in a thin plate of metal fixed to a window, we observe at what time any Star disappears behind a chimney, or corner of a house, at a little distance; and if the same Star disappears the next night 3 minutes 56 seconds sooner by the clock or watch; and on the second night, 7 minutes 52 seconds sooner; the third night 11 minutes 48 seconds sooner; and so on, every night, as in the Table, which shews this difference for 30 natural days, it is an infallible Sign that the machine goes true; otherwise it does not go true; and must be regulated accordingly: and as the disappearing of a Star is instantaneous, we may depend on this information to half a second. [Illustration: Pl. VI. _J. Ferguson inv. et delin._ _J. Mynde Sc._] CHAP. XIII. _Of the Equation of Time._ [Sidenote: The Sun and Clocks equal only on four days of the year.] 224. The Earth’s motion on it’s Axis being perfectly uniform, and equal at all times of the year, the sidereal days are always precisely of the same length; and so would the solar or natural days be, if the Earth’s orbit were a perfect Circle, and it’s Axis perpendicular to it’s orbit. But the Earth’s diurnal motion on an inclined Axis, and it’s annual motion in an elliptic orbit, cause the Sun’s apparent motion in the Heavens to be unequal: for sometimes he revolves from the Meridian to the Meridian again in somewhat less than 24 hours, shewn by a well regulated clock; and at other times in somewhat more: so that the time shewn by an equal going clock and a true Sun-dial is never the same but on the 15th of _April_, the 16th of _June_, the 31st of _August_, and the 24th of _December_. The clock, if it goes equally and true all the year round, will be before the Sun from the 24th of _December_ till the 15th of _April_; from that time till the 16th of _June_ the Sun will be before the clock; from the 16th of _June_ till the 31st of _August_ the clock will be again before the Sun; and from thence to the 24th of _December_ the Sun will be faster than the clock. [Sidenote: Use of the Equation Table.]