LET. III. FIRMAMENT OF THE FIXED STARS. 47

and by continually exhibiting that appearancewhich we call the scintillation, or twinkling ofthe stars. This, probably, arises from their ap-pearing so extremely small, that the interpositionof any very minute substance, of which thereare many constantly floating in the atmosphere,deprives us of the sight of them : but as theinterposed body soon changes its place, we againfee the star ; and this succession being perpetual,occasions the twinkling.

But a more remarkable property of the fixedstars, and that from which they obtained theirname, is their never changing their situationwith regard to each other, as the planets do*For though the revolution of the earth upon itsaxis occasions an apparent daily motion of thewhole frame of the heavens, in a contrary direc-tion ; yet any two fixed stars being observed,at several distant intervals of time, will alwaysbe found to preserve the same relative positionduring the whole of this revolution.

It is not to be imagined, that the stars areplaced in one concave surface, so as to be allequally distant from us ; but that they are dis-persed through unlimited space, in such amanner, that there may be as great a distancebetween any two neighbouring stars ? as thereis between our fun and those which are thenearest to him. So that an observer, who wasplaced near any fixed star, would consider it

alone