O. T. Sherman — A Pendulum Study. 



Art. XX.— A Pendulum Study; by O. T. Sherman. 



" Le mode d'installation du pendule, la nature du support 

 sur lequel il est place, en raison de 1'elasticite de la matiere 

 dont il est compose ou de sa masse relativernent peu consider- 

 able, peut exercer une influence sur le mouvement du plan de 

 suspension. Cette influence est tres-sensible pour un support 

 en bois ; . . . . elle est encore appreciable entre un pilier en 

 pierre . . . . et le pilier tres-massif sur lequel les observations 

 ont ete faite .... a Geneve."— E. Plantamoub. 



The above describes pretty completely the only result which 

 it is my fortune to draw from a series of pendulum observa- 

 tions taken during a northern journey. But, what is the mode 

 of action of the disturbance? It affects the mean of the obser- 

 vations; can its correction be drawn from the observations 

 themselves? 



The stand with which the expedition was furnished was formed 

 by three beams of about five inches square, and fastened by 

 bolt and screw mutually at right angles. These are braced in 

 each of the three coordinate planes. The pendulum case, of 

 light wood, rested on a bracket and was secured to the upright 

 by bolt and screw. The agate planes were fastened to a 

 w ighb solid o-u ). metal < isting, and it in turn by screws to 

 the back of the case; the whole forming a support as stable 

 as the material would allow. While studying the subject since 

 returning, the case has been fastened to the brick wall of the 

 clock room of the Observatory of Yale College. The connec- 

 tion is made through wood. The results show, if any, very 

 little improvement over the portable stand. • The pendulum 

 ; made to beat in three-quarters of a second 

 at Washington, D. C. It was customary in the field to meas- 

 ure the amplitude of the arc by making the cross wire of the 

 observing theodolite coincide with the extremities of the arc, 

 and noting the angle subtended. The observations were not 

 frequent and no discrepancy was noticed. When the instru- 

 ment again reached my hands it was provided with an arc 

 divided into degree and ten-minute spaces. The scale was sub- 

 jected to a critical study and the errors of the graduations tab- 

 ulated. We then endeavored to trace the decrement of the arc 

 by observations every five minutes. After making every cor- 

 rection it was found" impossible to represent the observations 

 by any smooth curve. The observations for the larger ares 

 might be fairly represented by a somewhat sinuous, the 

 smaller arcs by a very jagged, fine. Thoroughly dissatisfied 

 with the action of this scale it was replaced by a net formed 

 by a number of silk threads passed over two similar screws. 



