22 Pendulum-eotperimeiits at Harton Colliery. [Feb. 2, 



the same signals; and these evidently give comparisons of the 

 clocks, and therefore give the means of comparing their rates. 

 Tlius (by steps previously explained), 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 insure that the pendulum received no injury in the inter- 

 change, 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 

 60 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 2f seconds per day, or that gravity 

 is increased by — ^ 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 

 temperature-reductions are therefore liable to any uncertainty 

 which may remain on the correction for 7^. The reductions em- 

 ployed were those deduced by Sabine from direct experiment, and 

 their uncertainty 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 account the deficiency of matter in the valley of the 

 Tyne, in the hollow of Jarrow Slake, and on the sea-coast. It is 

 also necessary to obtain more precise determinations of the specific 

 gravities of the rocks about Harton colliery than have yet been 

 procured. Measures are in progress for supplying all these de- 

 ficiencies. It seems probable that the resulting number for the 

 earth's density will probably be diminished by these more accurate 

 estimations. 



[G. B. A.] 



