Mr. T. G. Bunt on Pendulum Experiments. 558 



that is to say, the pendulum index at evei-y vibration passes 

 ^vithin that distance of the pin in the centre of the cii'cle. When- 

 ever the eUipticity (or semi-conjugate axis) begins to exceed that 

 amount, I consider it time to stop the pendulum and commence 

 afresh. The direction of the elliptic motion is also recorded : 

 when like that of the hand of a watch, it is called ' direct,' and 

 marked + ; when the contrai-y, it is called ' retrograde/ and 

 marked with the sign — . The motion of the line of apsides of 

 'these ellipses (as Mr. Sylvester has lately pointed out) is mixed 

 up with, and considerably affects the apparent amount of, that 

 of the plane of vibration resulting from the rotation of the earth; 

 adding to it when the elliptic movement is direct, and subtract- 

 ing fi-om it when retrograde. 



In beginning an experiment, I draw the pendulum-ball aside 

 until it is nearly over the circumference of the circle, suddenly 

 disengage it, and then usually communicate to it, by a touch on 

 one side, a slight amount of elliptic motion, about equal and con- 

 trary to what was observed at the end of the last experiment. 

 In this manner I at first thought the errors of ellipticity might 

 be neutralized and eliminated; but having since constructed, 

 fi'om observation, a table of these errors for every tenth of an 

 inch of ellipticity + and — , I now introduce into evei-y experi- 

 ment a tabular correction for the ellipticity observed. Seventy- 

 eight experiments, averaging twenty minutes each, thus arranged 

 into eight gi'oups, gave the following results : — 



6g t* ) 



These quantities, thrown into a cui-ve by the usvial method of 

 rectangular coordinates, give a series of points falling nearly in 

 a straight line ; and by paring off slight irregularities, show 

 that 0°-7 per hour is pretty nearly the required correction for 

 every tenth of an inch of ellipticity, on an arc of 7 feet. 



The following summary contains the result of each day's work. 

 [It will be seen that the degrees of the circle have not been very 

 regularly followed : this irregularity was produced by accidental 

 causes merely.] 



Phil. Mag. S. 4. No. 7. Suppl VoX. 1. 2 P 



