414 Rev, C S. Lyman on the Pendulum Experiment 



m 



which is just half of radius^ that is, to the latitude of 30*^. In 



this way the base of the cone goes on diminishing as compared 



with its height until we reach the equator, where the height of 



it in fact becomes infinite, and the tangents form a cylinder. Of 



course here the pendulum plane has no relative motion. South of 



the equator the conditions are reversed, and as we pass towards 



the south pole, the plane of vibration will be found to revolve in j 



the opposite direction, or from right to left, and with a velocity as 



in the other hemisphere varying with the sine of the latitude. 



7. Modifications of the experiment. — Various analogous meth- 

 ods of rendering sensible the earth's rotary motion have been pro- 

 posed, but none of them, so far as we know, have yet been tested 

 by actual experiment, except that of M. Bravais with a conical 

 pendulum. Many of them have been devised on the mistaken 

 assumption that a heavy body suspended above the earth, will, 

 as the result of its own inertia, be independent of the earth's 

 motioji. M. Baudrimont (Comptes Rendus, No. 8, ISol,) not 

 only proposes such an experiment, (a heavy ball suspended by a 

 string as free from torsion as possible,) but even imagines, as he ^ 



explains in a long article, that the principle may be applied, by 

 having an index attached to the heavy body, to the construction 

 of B. perfect time keeper. But every such device, whether it be a 

 ball, or horizontal rod, or balanced wheel, must fail of showing 

 any results, except the oversight of the contriver, for the simple 

 reason that it contains no provision for divesting the body of the 

 movement which it has in common with the earth's surface, so 

 as to leave it to be governed solely by its own inertia. 



The experiment proposed by M. Poinsot (L'Institut, 1851, No. 

 897,) of a bent spring suspended by its angle and with the two 

 arms tied together by a thread, which thread is to be afterwards 

 burnt and the arms allowed to extend themselves, does obviously 

 contain such a provision, and were it not for the torsion of the , 



suspension wire or thread, might be expected to succeed. 



So the device of M. de Tessan, (L'Institut, No. 901, 1851,) 

 viz., a suspended horizontal bar, which while partaking of the 

 rotary motion of the earth, shall by some means be turned end 

 for end, is doubtless correct in principle, since the momentum of 

 each particle of the rod before reversal, would after passing to 

 the opposite side of the vertical, tend to cause the rod to rotate 

 in the opposite direction. But in practice, this momentum would 

 probably be destroyed in passing the vertical, and the experiment 

 prove unsuccessful. A rod suspended by its center of gravity 

 and suddenly let down from a vertical to a horizontal position, 

 would be more likely to succeed — its inertia causing it to main- 

 tain when horizontal a constant direction, like the plane of the 

 pendulum. An experiment on this principle was suggested by 

 Lieut. E. B. Hunt at the Albany meeting of the American Sci- 

 entific Association. 



