553 Ml*. T. G. Bunt on Pendulum Experiments. 



In homogeneous light we obtain a great number of rings, even 

 when the radius is only a few feet long ; and with such glasses, 

 by means of the gyreidometer, the properties of the colours can 

 be observed with sufficient accuracy. Were the bottom of the 

 case a, and of the slide b broken through beneath the place 

 where the lens rests^ the instrument would be suitable to the 

 observation of the rings formed by trammitted light. By it also 

 can be measured the rings formed between glass and metal, or 

 any other solid opake body ; it being merely necessary to form 

 a lens from the body, and to set it in the place which the glass 

 lens has been supposed to occupy in the foregoing desci'iption. 



LXXX. Pendulum Experiments. By Thomas G. Bunt. 

 To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, Bristol, May 1!), 1851. 



DURING the last three or four weeks I have been devoting 

 much time and attention to a series of pendulum experi- 

 ments, similar to those of Foucault in Paris, and carefully re- 

 cording the particvdars of each, in order to obtain from actual 

 observation the true amount of angular motion of the plane of a 

 pendulum's vibration in this latitude (51° 27'), and compare it 

 with the results which mathematicians are endeavouring to obtain 

 from rigorous analysis. 



The lofty spire of St. Nicholas in this city, which rises to a 

 height of about 190 feet from the ground, was the building 

 selected for these experiments ; and its admirable adaptation to 

 my purpose has been one of my chief inducements to pursue 

 them. The octagonal spire, which rests on a square tower, is 

 divided by a floor into two compartments or chambers of about 

 equal height ; in the lower of these, measuring 194^ feet in dia- 

 meter below, and 9|^ feet above, I suspended a pendulum of 53 

 feet in length. A steel wire g^th of an inch in diameter was 

 made to pass tightly through a hole in a steel plate fixed to the 

 upper floor, and firmly fastened to a support above. From this 

 is suspended a leaden ball weighing 53| lbs., a solid of revolu- 

 tion, and nearly a sphere. From the bottom of the ball projects 

 a pointed wire index ; and about ^ of an inch under this is a pin 

 driven into the floor, around which, as a centre, is described and 

 carefully divided into degrees, a circle of about 9^ feet diameter. 

 The zero and 180° of this circle are in the true meridian. 



The slight elliptic motion which the pendulum after a short 

 time acquires, and which, I believe, no precautions will entirely 

 prevent, has been throughout these experiments most carefully 

 watched and recorded ; and during the latter portion of them, I 

 have been accustomed not to permit it to exceed \ of an inch ; 



