THE LAST RIDDLE 451 



to the finer aether below, which cannot be without the bodies 

 descending to make room above for it to go into." 



In the 2ist of his celebrated queries, in the Opticks, Newton 

 elaborates his ideas still further, endeavouring even to con- 

 ceive what the properties of the ather must be in order that 

 it might serve as an explanation of this inexplicable power. It 

 runs : 



" 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, doth it not grow denser and denser perpetually, and 

 therefore 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 Orb of 

 Saturn ; I see no reason why the Increase of density should 

 stop anywhere, and not rather be continued through all dis- 

 tances from the Sun to Saturn, and beyond. And though this 

 Increase of density may at great distances be exceeding slow, 

 yet if the elastick 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 elastick 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 70,000,000 

 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 Reflexion, must be swifter than Light, and by con- 

 sequence above 700,000 times swifter than Sounds. And there- 

 fore the elastick force of this Medium, in proportion to its 

 density, must be above 700,000 x 700,000 (that is, above 

 490,000,000,000) times greater than the elastick force of the 

 Air is in proportion to its density. For the Velocities of the 

 Pulses of elastick Mediums are in subduplicate Ratio of the 



