164: ANNUAL OF SCIENTIFIC DISCOVERY. 



a certain gravitating action as the result of that previous condition when the 

 earth is in place, I can conceive consistently, I think, with the conservation 

 of force ; and I think the case is that which Newton looked at in gravity, and 

 is in philosophical respects the same as that admitted by all in regard to light, 

 heat, and radiant phenomena ; and in a more general sense is forced upon 

 our attention by the phenomena of electricity and magnetism, because of their 

 dependence on dual forms of power. 



PENDULUM EXPERIMENTS FOE THE DETERMINATION OF THE 



EARTH'S DENSITY. 



Professor Airy, the Astronomer Royal, recently delivered a lecture before 

 the Royal Institution, " On the Pendulum Experiments lately made in the 

 Harton Colliery, for ascertaining the Mean Density of the Earth." He com- 

 menced by explaining the importance in astronomical investigations of acquir- 

 ing a correct knowledge of the mass of the earth, as the means of determining 

 the force of gravitation and the density of other planetary bodies, .and then 

 noticed the different plans which had been adopted to ascertain the mean 

 density of the earth, inciting the attraction of a plummet to the side of the 

 mountain Schechalliau, and known as the Schechallian experiment; Mr. 

 Cavendish's cabinet experiment, in which he had attempted to arrive at the 

 same result by the attractions of large spheres of lead on light bodies ; and the 

 pendulum experiments which, with the assistance of Dr. "Whewell, Professor 

 Airy had made in 1826, and again in 1828, in the Dolcoath Mine, in Corn wall. 

 Both those experiments were failures, for in each case, after many days of 

 persevering toil and observation, unforeseen accidents injured the instruments, 

 and prevented the attainment of any satisfactory results. Professor Airy 

 minutely explained the principle on which the vibration of a pendulum de- 

 pends, and on which the pendulum experiments were contrived to determine 

 the density of the earth. As the rapidity of the vibration of the pendulum 

 increases with the increase of the force that attracts it downward, it was ex- 

 pected that by noting accurately the number of vibrations in a given time on 

 the surface and at the bottom of a deep mine, the difference would afford a 

 measure of the relative attractions of gravitation at the two points. In those 

 early experiments the great difficulty experienced was in ascertaining pre- 

 cisely the correspondence or difference in the vibrations of the pendulums 

 placed at the top and at the bottom of the mine, which was attempted to be 

 done by means of a chronometer. The recent application of voltaic electricity 

 in the Observatory at Greenwich had suggested a ready mode of noting the 

 variations of the pendulums, and the Astronomer Royal, therefore, with this 

 new power at command, determined to repeat the experiments. With the 

 assistance of a numerous staff of observers, drawn from the different observ- 

 atories in the kingdom, he commenced operations in the Harton Colliery, near 

 Newcastle, which is 1,600 feet deep. The Electric Telegraph Company gave 

 their aid in laying down insulated wires, and, by means of a clock, to make and 

 break contact with the voltaic battery, galvanometers at the oottom and at the 

 top of the pit made simultaneous deflections every fifteen seconds. Several ob- 



