NOVEMBEK 16, 1917] 



SCIENCE 



491 



ual motion. When all was perfectly still — as 

 indicated by the absence of movement of the 

 shadow D — the damping system was lowered 

 away and the thread behind the pulley quickly 

 burned through. If the bob were left hang- 

 ing after the removal of the damping system, 

 air currents and the tremors of the building 

 soon set it swinging again — for these observa- 

 tions were made while other operations were 

 being carried on in the same building. After 

 releasing the bob the position of the arc lamp 

 was adjusted so that the amplitude of the 

 shadow's motion was decreasing and was about 

 5 mm. on the scale. Readings were then made 

 of successive elongations until the plane of the 

 pendulum's motion had passed completely 

 through the plane fixed by the slit and the 

 vertical through the point C. Readings were 

 always begun with the outward swing of the 

 pendulum so that no ambiguity resulted from 

 the recording only the millimeters and tenths 

 after the first. The record for the first few 

 points of experiment A (below) for instance 

 was : 



23.42 cm. 



.89 



.47 



.87 



etc. 



Blanks (when the arc sputtered or the eye 

 did not catch the turning point) were indi- 

 cated, both in the record and on the graph, by 

 strokes. 



The determination of the point on the floor 

 directly beneath the center of suspension was 

 effected as follows : A metal plate with a peep- 

 hole (1 mm. diam.) was held in the laboratory 

 stand so that the plumb-bob, hung through the 

 hole, fell just over the edge of one of the feet 

 of the stand, about a meter below. A straight- 

 edge placed on the floor against this foot, when 

 observed through the peep-hole, defined a ver- 

 tical plane. The bob was then set swinging 

 through an arc of amplitude equal to its own 

 radius and the position of the straight-edge 

 was adjusted until at extreme elongations the 

 sphere appeared tangent to the straight-edge 

 on opposite sides successively. A line drawn 

 along the straight-edge must contain a point 



vertically under the center of suspension. 

 In this same manner two other lines, each 

 at about 60° to the first, were determined, 

 and the center of the resulting triangle (about 

 1 mm. altitude) was taken as the point re- 

 quired. 



Trouble was found at first at the suspension 

 point itself, but this was finally overcome by 

 boring a 5-mm. hole half way through a stout 

 piece of brass and finishing it through with a 

 half millimeter drill. The wire was then in- 

 serted, the larger hole being in the lower side 

 of the bar. The hole was then filled with 

 solder, sufficient being used to leave the sur- 

 face slightly convex. This excess was scraped 

 away with a knife, leaving a plane surface 

 from which the pendulum could swing. The 

 bar was then clamped into place against the 

 roof joist. 



The details of a set of five consecutive read- 

 ings taken on the fifteenth of May, 1917, are 

 as follows : 



Latitude of Kingston 44° 13'. 



Period of Pendulum T = 7.50 sec. 



Distance to scale from center of oscillation 



Coordinates of intersections of lines on 

 graph fj, <j, Kj, X,. 



Angular velocity of plane of vibration 



X2 — Xl 



(«s - ti)T200 



10.9 r 



19.5 T 

 10.5 2* 



13.3 r 



14.4 2 



39.3 T 



64.5 r 

 44.1 T 

 53.9 r 



48.6 r 



zi (cm.) 



23.65 

 21.58 

 20.90 

 20.94 

 22.64 



X2 (cm.) 



25.81 

 24.92 

 23.39 

 24.04 

 25.30 



Mean . 



Calculated value at Kingston., 



u (Radl&nB 

 per Second) 



5.07x10-5 



4.95 



4.95 



5.09 



5.18 



5.05X10- 



5.08X10-6 



Of these the e.Tperiment of shortest dura- 

 tion was A, which included 28.4 periods or 

 about 3^2 minutes; the longest was B, of 45 

 periods, or about 5I/2 minutes. 



Will C. Baker 

 Physical Laboratory, 

 Queen's University, 

 Kingston, Ont., 

 May 18, 1917 



