200 Things not generally Known. 



INFLUENCE OF PENDULUMS ON EACH OTHEE. 



About a century since it became known, that when two 

 clocks are in action upon the same shelf, they will disturb each 

 other : that the pendulum of the one will stop that of the other ; 

 and that the pendulum that was stopped will after a while re- 

 sume its vibrations, and in its turn stop that of the other clock. 

 When two clocks are placed near one another in cases .very 

 slightly fixed, or when they stand on the boards of a floor, they 

 will affect a little each other's pendulum. Mr. ElHcote observed 

 that two clocks resting against the same rail, which agreed to a 

 second for several days, varied one minute thirty-six seconds in 

 twenty-four hours when separated. The slower, having a longer 

 pendulum, set the other in motion in 16 J minutes, and stopped 

 itself in 36 minutes. 



WEIGHT OF THE EARTH ASCERTAINED BY THE PENDULUM. 



By a series of comparisons with Pendulums placed at the 

 surface and the interior of the Earth, the Astronomer- Royal has 

 ascertained the variation of gravity in descending to the bottom 

 of a deep mine, as the Harton coal-pit, near South Shields. By 

 calculations from these experiments, he has found the mean 

 density of the earth to be 6-566, the specific gravity of water 

 being represented by unity. In other words, it has been ascer- 

 tained by these experiments that if the earth's mass possessed 

 every where its average density, it would weigh, bulk for bulk, 

 6*566 times as much as water. It is curious to note the dif- 

 ferent values of the earth's mean density which have been 

 obtained by different methods. The Schehallien experiment 

 indicated a mean density equal to about 4 ; the Cavendish 

 apparatus, repeated by Baily and Reich, about 5^ ; and Pro- 

 fessor Airy's pendulum experiment furnishes a value amounting 

 to about 65. 



The immediate result of the computations of the Astro- 

 nomer-Royal is : supposing a clock adjusted to go true time at 

 the top of the mine, it would gain 2J seconds per day at the 

 bottom. Or it may be stated thus : that gravity is greater at 

 the bottom of a mine than at the top by TST^yth part. Letter to 

 James Mather, Esq., South Shields. See also Professor Airy's Lec- 

 ture, 1854. 



ORIGIN OF TERRESTRIAL MAGNETISM. 



The earliest view of Terrestrial Magnetism supposed the ex- 

 istence of a magnet at the earth's centre. As this does not ac- 

 cord with the observations on declination, inclination, and in- 

 tensity, Tobias Meyer gave this fictitious magnet an eccentric 

 position, placing it one-seventh part of the earth's radius from 

 the centre. Hansteen imagined that there were two such mag- 



