RECENT STUDIES IN GRAVITATION. 



201 



line due to Chimborazo, one of the Andes peaks, by finding the dis- 

 tance of a star on the meridian from the zenith, first at a station on 

 the south side of the mountain, where the vertical was deflected, and 

 then at a station to the west, where the mountain attraction was nearly 

 inconsiderable, so that the actual nearly coincided with the geograph- 

 ical vertical. The difference in zenith distances gave the mountain 

 deflection. It is not surprising that, working in snowstorms at one 

 station and in sand storms at the other, Bouguer obtained a very 

 incorrect result. Hut at 'east he showed the possibility of such work, 

 and since his time many experiments have been carried out on his 

 lines under more favorable conditions. Now, however, I think it is 

 generally recognized that the difficulty of estimating the mass of a 

 mountain from mere surface chips is insurmountable, and it is admitted 

 that the experiment should be turned the other way about and regarded 



Pig. 1. — Cavendish's apparatus. 



as an attempt to measure the mass of the mountains from the density 

 of the earth known by other experiments. 



These other experiments are on the line indicated by Newton in his 

 calculations of the attraction of two spheres. The first was carried 

 out by Cavendish. 



In the apparatus (fig. 1) he used two lead balls, B B, each 2 indies 

 in diameter. These were hung at the end of a horizontal rod 6 feet 

 long, the torsion rod, and this was hung up by a long wire from its 

 middle point. Two large attracting spheres of lead, \Y W, each 1'2 

 inches in diameter, were brought close to the balls on opposite sides, 

 so that their attractions on the balls conspired to twist the torsion rod 

 round the same way, and the angle of twist was measured. The force 

 could be reckoned in terms of this angle by setting the rod vibrating 

 to and fro and finding the time of vibration, and the force came out to 

 less than one three-thousandth of a grain. Knowing M x M 2 and r, the 



