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OF THE MOTION, REFRACTION,

PI. IV.fig. 13.

Globose, and every magnitude of stars,

And sow’d with stars the heav’n thick as a field.

First in his east the glorious lamp was seen,

Regent of day, and all th’ horizon roundInvested with bright rays, jocund to runHis longitude through heav’n's high road ; the grayDawn, and the Pleiades before him danc’d,

Shedding sweet influence.”

Milton.

This is the language of poetry, sublime andenergetic, but not sti'ictly conformable to truth;for it is now certain that light is a material sub-stance, which flows directly from the fun; andwe are also able to ascertain the velocity withwhich it moves. M. Roemer, a Danish philo-sopher, was the first who showed, that it employsabout eight minutes in its passage from the funto the earth; and as this singular doctrine willnaturally excite your attention, I shall give youhis explication of it, in as easy and familiar away as possible. The idea was first suggested tohim by observing the eclipses of Jupiter’s moons,and the conclusion was deduced as follows.

Let A and B be the earth in two differentpoints of its orbit, whose distance from eachother is equal to the earth’s distance from thefun S ; it is then plain, that if the motion oflight were instantaneous, the satellite 1 wouldappear, to a spectator at A, to enter into Jupiter’sfliadow S S, at the fame moment of time, as toanother spectator at B. But from a great num-ber of observations it was found, that when the

earth