314 Royal Institution. 



swings is found, there is yet another thing required : — namely, the 

 proportion of upper-clock-pendulum-swings to lower-clock-pendu- 

 lum-swings in the same time ; or, in other words, the proportion of 

 the clock rates. It was for this that the galvanic signals were re- 

 quired. A galvanometer was attached to each clock, and an appa- 

 ratus was provided in a small auxiliary clock, which completed a cir- 

 cuit at every 15 seconds nearly. The wire of this circuit, passing 

 from a small battery through the auxiliary clock, then went through 

 the upper galvanometer, then passed down the shaft of the mine to 

 the lower galvanometer, and then returned to the battery. At each 

 galvanometer there was a small apparatus for breaking circuit. At 

 times previously arranged, the circuit was completed by this appa- 

 ratus at both stations, and then it was the duty of the observers at 

 both stations to note the clock times of the same signals ; and these 

 evidently give comparisons of the clocks, and therefore give the 

 means of comparing their rates. Thus (by steps previously ex- 

 plained) the number of swings -made by the upper pendulum is 

 compared with the number of swings made in the same time by the 

 lower pendulum. 



Still the result is not complete, because it may be influenced by 

 the peculiarities of each pendulum. In order to overcome these, 

 after pendulum A had been used above and pendulum B below, they 

 were reversed ; pendulum B being observed above, and A below ; 

 and this, theoretically, completes the operation. But in order to 

 ensure that the pendulum received no injury in the interchange, it 

 is desirable again to repeat the experiments with A above and B 

 below, and again with B above and A below. 



In this manner the pendulums were observed with 104 hours of 

 incessant observations, simultaneous at both stations, A above and 

 B below ; then with 104 hours, B above and A below ; then with 

 CO hours, A above and B below ; then with 60 hours, B above and 

 A below. And 2454 effective signals were observed at each station. 



The result is, that the pendulums suffered no injury in their 

 changes ; and that the acceleration of the pendulum on being 

 carried down 1260 feet is 2^ seconds per day, or that gravity i3 in- 

 creased by — j^j part. 



It does not appear likely that this determination can be sensibly 

 in error. The circumstances of experiment were, in all respects, 

 extremely favourable ; the only element of constant error seems to 

 be that (in consequence of the advanced season of the year) the 

 upper station was cooler by 7° than the lower station, and the tem- 

 perature-reductions are therefore liable to any uncertainty which 

 may remain on the correction for 7°. The reductions employed 

 were those deduced by Sabine from direct experiment, and their un- 

 certainty must be very small. 



If a calculation of the earth's mean density were based upon the 

 determination just given, using the simple theory to which allusion 

 is made above, it would be found to be between six times and seven 

 times the density of water. But it is necessary yet to take into 



