Note e — BOOK I.

Mr. Romer’s ingenious Hypothesis about the Velocity of Light, hath been established by the Royal Academy, and in the Observatory for eight Years, as our Phil. Trans. Nᵒ. 136. observe from the Journ. des Scavans; our most eminent Astronomers also in England admit it: But Dr. Hook thinks with Monsieur Cartes, the Motion of Light Instantaneous, Hook Post. Works, pag. 77. And this he endeavours to explain, pag. 130, &c.

What Mr. Romer’s Hypothesis is, may be seen in the Phil. Transact. before-cited: As also in the before commended Sir Isaac Newton’s Opticks: Light is propagated from luminous Bodies in time, and spends about seven or eight Minutes of an Hour in passing from the Sun to the Earth. This was first observed by Romer, and then by others, by means of the Eclipses of the Satellites of Jupiter. For these Eclipses, when the Earth is between the Sun and Jupiter, happen about seven or eight Minutes sooner than they ought to do by the Tables; and when the Earth is beyond the ☉, they happen about seven or eight Minutes later than they ought to do: The reason being, that the Light of the Satellites hath farther to go in the latter Case than in the former, by the Diameter of the Earth’s Orbit. Newt. Opt. L. 2. Part. 3. Prop. 11.

Now forasmuch as the Distance between the Sun and the Earth (according to the Computations in my Astro-Theology, B. 1. ch. 3. Note 2.) is 86051398 English Miles; therefore, at the rate of 7½ Minutes, or 450 Seconds in passing from the Sun, Light will be found to fly above 191225 Miles in one Second of Time.