LET. I.

OF ASTRONOMY.

9

It is to the heavens alone that we must lookfor a certain and invariable standard. Thosemagnificent bodies, that are placed at suchimmense distances from our globe, move withan order and regularity which is not to befound in any other part of the creation. Fromthem we have obtained all our knowledge ofthe seasons, and the art of reckoning by certainperiods of time. Had their motions been asvarious and mutable as those which we observeupon the earth, we should have had no idea ofthe regular lapse of time or the extent of dura-tion. This is still the state of the uncivilizedand solitary barbarian, who computes his timeby the falls of snow, or the progress of vegeta-tion, and is utterly ignorant of the more refinedwants of society.

The interval between the rising and setting ofthe sun, is a measure of time which is called aday, and is what as pointed out to us by natureherself; but as we are frequently in want of amuch longer period, we are obliged to haverecourse to other observations than those whichdepend upon the appearance and disappearanceof the sun. Some nations compute their timeby the revolutions of the moon, or by months ;some by the revolutions of the fun, or by years;and others again both by months and years.But this requires an exact knowledge of thecelestial motions; and those tvho make use of

both