X - Queries to the Opticks [1721]

Summary

Query 21

Is not this medium much rarer within the dense bodies of the Sun, stars, planets and comets, than in the empty celestial spaces between them? And in passing from them to great distances, does it not grow denser and denser perpetually, and thereby cause the gravity of those great bodies towards one another, and of their parts towards the bodies; every body endeavouring to go from the denser parts of the medium towards the rarer? For if this medium be rarer within the Sun’s body than at its surface, and rarer there than at the hundredth part of an inch from its body, and rarer there than at the fiftieth part of an inch from its body, and rarer there than at the orbit of Saturn; I see no reason why the increase of density should stop anywhere, and not rather be continued through all distances from the Sun to Saturn, and beyond. And though this increase of density may at great distances be exceeding slow, yet if the elastic force of this medium be exceeding great, it may suffice to impel bodies from the denser parts of the medium towards the rarer, with all that power which we call gravity. And that the elastic force of this medium is exceeding great, may be gathered from the swiftness of its vibrations. Sounds move about 1140 English feet in a second minute of time, and in seven or eight minutes of time they move about one hundred English miles. Light moves from the Sun to us in about seven or eight minutes of time, which distance is about 70000000 English miles, supposing the horizontal parallax of the Sun to be about 12”. And the vibrations or pulses of this medium, that they may cause the alternate fits of easy transmission and easy reflection, must be swifter than light, and by consequence above 700000 times swifter than sounds. And therefore the elastic force of this medium, in proportion to its density, must be above 700000 x 700000 (that is, above 490000000000) times greater than the elastic force of the air is in proportion to its density.