148 GREAT MEN OF SCIENCE 



Newton's researches. But nevertheless, what is required is 

 experimentally proven knowledge, and the proof of the 

 force, which according to the law must be very small, 

 between two not very large masses, and hence had always 

 remained unnoticed, was an important rounding off of our 

 knowledge of gravitation. Conversely, the measurement of 

 such a force in the case of two masses known by weighing, 

 gives us the means of estimating the mass of the earth with 

 accuracy in the same units as that in which the two masses 

 are measured. In Newton's case, on the other hand, the 

 mass of the earth formed the unit in which the masses of 

 the other heavenly bodies were measured; but this unit 

 itself remained unknown, and could only be estimated upon 

 the assumption of a mean density for the earth's material. 

 It now became possible to determine this density with cer- 

 tainty as the quotient of the total mass and total volume of 

 the earth. 



In order to measure this very small force to be expected, 

 Cavendish made use of a simple apparatus, the torsion 

 balance, which had shortly before been made use of by 

 Coulomb for another investigation of equal importance. 

 It consisted of a long thin wire, at the lower end of which 

 hung a light horizontal rod, carrying at each end a lead ball. 

 It was then possible to arrange near the two movable lead 

 balls, two much larger fixed balls of lead, which by their 

 attraction upon the smaller ones would produce a deflection 

 of the rod and twist the suspension wire. The measurement 

 of the very small deflection was difficult; the slightest draught 

 of air made it impossible. The apparatus was therefore 

 observed through an opening in the wall by means of a 

 telescope in the next room; and in other respects also. 

 Cavendish showed the greatest care in this, the first measure- 

 ment ever made of such extraordinarily small forces. His 

 result for the mean density of the earth, namely about 5.5, 

 has been entirely confirmed by much later measurements 



