446 UNIVERSITY OF VIRGINIA PUBLICATIONS 



or finally any irregularity in the atmospheric pressure on the two sides of 

 the vib-plane, will cause this motion which, with the amplitude usually 

 employed, will produce a deviation quite different both in amount and 

 sometimes in direction from Foucault's. The experiment requires first 

 a fixed point of suspension; next, avoidance of lateral disturbance in start- 

 ing and lastly, a very small amplitude of vibration. It is by no means easy 

 to secure all these conditions in ordinary lecture rooms. 



It is noticeable that M. Foucault appears soon to have abandoned the 

 pendulum for the gyroscope in this problem, and that in the memorable 

 sesqui-centennial repetition of his pendulum-experiment in the Pantheon 

 by M. M. Berget and Flammarion February 26, 1902, the latter astronomer 

 speaks of seeing elliptical movement of the needle. In neither the original 

 trial nor its repetition, can the reader more than guess at the amplitude 

 of the vibrations. The radius of the sand-arc is noted, but for the starting 

 distance we are left to conjecture. 



Aftler 1851 attempts were made here, year by year, as doubtless in many 

 other ecture rooms, to exhibit Foucault's pendulum experiment to the 

 class in Physics. There were several difficulties met with. The pendulum 

 was of necessity not a long one, and with ordinary amplitudes it involved 

 a considerable maximum speed of the bob, encountering an air resistance 

 easily becoming unsymmetrical. Then the length of time required was 

 inconvenient, a deviation of 5° caUing here for more than half an hour. 

 This duration made it impossible to insure the needed fixedness of the sus- 

 pension point in a room occupied by a hundred spectators, especially as 

 the result could only be seen by those near the lecture-table and must be 

 taken on faith by the majority. The same troubles must have been fatally 

 common in England, for Dr. Greenhill, of Woolwich, in a laughing fling 

 at the physicist, publicly advised each one to hide a pouch of condensed 

 air in his sleeve, and by a suitable unseen tube force the swinging pendulum 

 to do its duty. 



Then matters went on here imsatisf actorily from year to year until 

 1890. In the summer of 1889, I was engaged with Mr. James Miller, then 

 my Assistant, in a repetition of Bessel's Konigsberg experiment to deter- 

 mine the gravity acceleration 'g' by means of a differential pendulum. The 

 pendulum was swung in the tower of the then unfinished chapel. It had a 

 vibration-time of 4 seconds, and its length enabled us to see amplitudes 

 less than l°.t The experiment required groups of swings, each about 20 



t Foucault's original Pantheon Experiment used an amplitude certainly greater 

 than 5°; and its repetition in 1902 must have involved amplitude greater than 

 6f ° as required by the size of the circles of sand. 



