WEIGHING THE WORLDS 



by the British astronomer royal Maskelyne about 

 the year 1774, and results were obtained which gave 

 at least a general notion of the mass of the earth. 

 But, as has been said, all experiments of this type, 

 are lacking in accuracy because it can never be pos- 

 sible to determine with strict reliability the mass of 

 the mountain. 



THE CAVENDISH TEST 



'A test of a far more accurate type was contem- 

 plated more than a century ago by the Rev. John 

 Michell, an Englishman, who constructed an appara- 

 tus which he did not live to operate but which after- 

 ward came into the hands of the famous Cavendish 

 and by him was used to effect what may be consid- 

 ered the first fairly accurate weighing of the earth 

 in the years 1797-98. The apparatus consisted essen- 

 tially of two lead balls each two inches in diameter 

 placed at the ends of a rod suspended at its center by 

 a long wire. Proper caution being taken to shield 

 the apparatus from draughts of air, it was provided 

 that two other leaden balls, each twelve inches in 

 diameter, might be brought near the small suspended 

 balls, on opposite sides. If, then, the attraction suf- 

 ficed, the small balls would be drawn toward the 

 larger ones by a movement which could be measured 

 by the swing of the connecting rod. The exact dis- 

 tance between the centers of the large and small 

 balls being measured, and the mass of the balls be- 

 ing accurately determined, the observed deflection, 

 measurable in terms of the torsional stress of the 

 suspending wire, gave the third term of a proportion, 



63 



