l£T. XII. ARTIFICIAL DIVISIONS OF TIME. 2SZ

ittore of another revolution as is equivalent tothe space flie has advanced in her orbit duringthat time; which, at a mean rate, is the threehundred and sixty-fifth part of a circle. So th^tin three hundred and sixty-6ve days, the earthwill have turned three hundred and sixty-sixtimes round her axis j and, therefore, as onecomplete rotation makes a sidereal day, therewill be one sidereal day in a year more than thereare solar days, be that number what it will,either on the earth, or on any other planet.

This regular return of the fixed stars to themeridian, affords us an easy method of determin-ing whether our clocks and watches go true.For if, through a small hole in a window shutter,or in a thin plate of metal properly fixed forthat purpose, we observe at what time any stardisappears behind a chimney, or the corner of abuilding, at a small distance ; then, if the famestar disappears, the next night, three minutesand fifty-six seconds sooner, by the clock orwatch, than it did the night before; on the se-cond night seven minutes, fifty-two secondssooner; on the third night eleven minutes, forty-eight seconds sooner; and so on ; it is a certainsign that the machine goes right; but if it doesnot observe this rule, it is evidently not true,and must therefore be regulated accordingly :and as the disappearing of a star is instantaneous,

we