Dec. 27, 1883] 



NATURE 



the period of swing will bedifierent, and the time taken to make 

 a complete swing will increase or decrea-e as the equntor is 

 approached or receded from. So much for theoretical considei'a- 

 tions. Can they be put to the le»t of e\periment, and an answer 

 obtained from nature herself? The fact is that this idea of 

 Foucault's is so beautifully simple that anybody can make the 

 experiment providing he has the means of using a very long 

 pendulum. This pendulum must be rigidly, but at the same 

 time very independently, supported. 



Beneath the pendulum, in contact with the earth, and therefore 

 showing any movement of rotation which the latter may possess, 

 is a board, on the centre of which the pendulum nearly rests. 

 From the central point of thi-; board lines are described show- 

 ing so many degrees from the central line over which the pendulum 

 bob s\\ ings. "fhese preliminaries being arranged, let the pen- 

 dulum be started. This is done by drawing it out of the vertical 

 and ly.ng it by a thread which is burnt when it is desired to 

 start the experiment. 



Then, in consequence of that quality the exi-tence of which 

 was revealed to us by the rotating disk and which is possessed 

 by this vihratinj pendulum, and in consequence of the precautions 

 which have been taken to prevent its swing being interfered with 

 by the mstion of the earth or other perturbing iiilluences, it 

 should be found, if Foucault's assumption be correct, that the 

 earth is movi -g lieneath the pendnUim. And if all the conditions 

 of the experiment have been complied with it is found that the 

 pendulum moves over the scile as the earth rotates beneath it. 

 That then is one demonstra'i jn of the existence of the earth's 

 rotation. 



The question now arises whether there be any other method 

 of determining the same thing. There i-=, but in an.swering the 

 question in the affirmative it must be said that this second method 

 is neither so simple nor so satisfactory a-' the first. 



We owe it also to the genius of this same man, Foucault. It 

 depends upon the same principles and is connected with the 

 same series of facts as the other. But befoie proceeding to 



Fig. 30. — Transit instrument t 



discuss this second experiment it will be well to consider these 

 two tables, which have been taken from Galbraith and Haughton's 

 "Astronomy," because they show not only what the swinging 

 pendulum should do if it behaves properly, but also what the 

 gyroscope, the instrument used in the second experiment, should 

 do if it behaves properly. 

 The first table is called 



Hourly Motion of Pendulum Plane. 



The second is 



Rotation of Earth deduced from Pendulum. 



The pendulum plane is of course the plane in which the 

 pendulum swings. The first column in Table i gives the place 

 where the pendulum was set swinging, the second the latituue, 



