CHAPTER I.

The bases of modern astronomy. Their late formation 18 Instruments and measures used by ancient astronomers 19 Weights and measures sought out by modern astronomers 20 Means employed to discover the density of the earth. Measuring by means of plummets not sufficiently exact 20 Measurements with torsion and chemical balances more accurate 21 Sir George B. Airy's theory, and experiments at the Harton colliery 22 Results of experiments not reliable. Theory contrary to the Law of Attraction 23 Proof by arithmetical calculation of its error 24 Difficulties in comparing beats of pendulums at top and bottom of a mine 26 The theory upheld by text-books without proper examination 27 Of a particle of matter within the shell of a hollow sphere. Not exempt from the law of Attraction 28 A particle so situated confronted with the law of the inverse square ofdistance from an attracting body. Remarks thereon 29 It is not true that the attraction of a spherical shell is "zero" for a particle of matter within it 31