the retardation was observed to be the greatest when the earth was in that part of its orbit most remote from him. The diameter of the orbit of the earth being a hundred and ninety millions of miles, we are more remote from Jupiter, by the whole of that distance, at one time than at another; as, when the earth is in its orbit at _a_, its distance is greater from _c_ than when at _b_ by the interval between the two points. But notwithstanding this immense addition of space, or any conceivable increase, an eclipse would be observed to occur no later at the one than at the other, if light were propagated instantaneously. Roëmer found, however, a difference of eleven minutes to exist, which he afterward estimated at fourteen, but which the precision of modern astronomy has fixed at sixteen minutes and a quarter. This determines the progressive motion of light, and the rate of its velocity. It requires time for its transmission; and flying over the diameter of the earth’s orbit in sixteen and a quarter minutes gives it a velocity of twelve millions of miles a minute, or upward of a hundred and ninety thousand miles a second. Thus, in the eighth part of a second, it accomplishes the passage of a space equal to the equatorial circumference of our globe: yet so vast is the system to which we belong, that this swift-winged messenger, which requires no more than two hours to travel from the central sun to the farthest planet, could not dart through the intervening solitudes between us and the nearest of the stars under a period of five years. Notwithstanding the velocity of the rays of light, which travel more than fifteen hundred thousand times faster than a cannon ball, experiment has not yet been able to detect that they have any impulsive power. The surmise has, however, been thrown out—and it is not improbable—that the attrition of the solar beams with the terrestrial surface may have some connection with the phenomena of heat.