Makch 16, 1917] 



SCIENCE 



267 



pendulum bob (4 kg.), swung from a tliin 

 brass wire (diameter .7 mm.). Measurements 

 were continued tbroughout 35 minutes. 



Pendulum Observation 

 Stationary lens, focal distance 60 cm.; diameter 

 15 cm. Bob 4 kg. on brass wire. (1) Experi- 

 mentally 9i = l° equivalent to a = 3.8 cm. for 

 D = 25 cm. (2) Experimentally iDB2 = l cm. 

 equivalent to x = 9 cm. for 19 :^ 25 cm. ; K^ = 



^ = -§§- = .1144 « = 15° X sin = 10.01 e"'-" in 



V 603 



Providence. 



9i from direct measurement of 6 and x. 



6, from direct measurement of magnification: 

 s aX u and x at v. 6' + 6" by the general equation 

 (3) §3. By equation (5), e = 10.4'' and 9.9°, re- 

 spectively. 



The results for 9^ (computed from the direct 

 evaluation of 6, §4) and for 4 (computed from 

 direct measurement of magnification §4) are 

 practically identical. These data for 6 de- 

 crease in the lapse of time, definitely. In part 

 this may be ascribed to an insufficiently accu- 

 rate estimate of the arc D of the pendulum, 

 for which a value derived from the logarithmic 

 decrement might with advantage have been 

 substituted. The high initial value is in part 

 to be associated with an incorrect initial zero. 

 But it is also probable that some secondary 

 disturbance is developing and superimposed 

 on the data for the earth's rotation. 



The value of 6 found from the equation (4) 

 is given in the second part of Table I. with the 

 mean data used, for the first four observations 

 taken in pairs. It is of about the same order 

 as the others and also gives promise of de- 

 creasing. 



6. The Vibrating Lens Pendulum. — To in- 

 crease the magnification indefinitely, i. e., to 

 exhibit the rotation in shorter time, it will 

 be necessary to use the lens L, Fig. 3, as the 

 bob of a pendulum, swung doubly bifilarly, or 

 in some similar manner, but in such a way as 

 to have the same period as the Foucault pendu- 

 linn, B. As the bifilar suspension is still liable 

 to vibrate laterally it is unsuitable for this and 

 other reasons. It was therefore replaced by a 

 massive compound pendulum LT, Fig. 4, about 

 a meter long, weighted above with 1.5 or 2 kilo- 

 grams to secure as long period as that of the 

 Foucault pendxilum (4 seconds). The steel 

 knife edge at K should rest on a horizontal flat 

 brass fork P, as it will be necessary to rotate 

 the pendulum slightly around its longitudinal 

 axis LT in the adjustments. The weights W 

 are between screw bolts to regulate the period. 

 The lens L used was an ordinary photographic 

 bullseye lens, 10 cm. in diameter, quite thick 

 and with a focal distance of about 10 cm. 

 The magnification was between 62 and 65. 



As the distance between B and L, Figs. 3 

 and 4, is but 10 cm. the weights W interfere 

 with the string for large arcs of vibration, D. 

 This would have to be modified in a lecture 

 apparatus, for instance by doubling the lens 

 (condenser doublet) or by forking the weights. 

 Furthermore the vibrations of L die down 

 more rapidly than those of B. Since however 

 the pendulum L is weighted above, there is no 

 difficulty in accelerating the lens L cautiously 

 with the fingers when necessary before obser- 

 vation. 



In adjusting the apparatus, B must first be 

 quite at rest. The pendulum L is then started, 

 and if the image of the wire of B vibrates on 

 the screen, the lens L is to be rotated on its 

 longitudinal axis, by successive trials, until the 

 image is stationary. Hence the arc traced by 

 the optical center of L passes through the wire 

 of the Foucault pendulum. B is now to be de- 

 flected as above and held until the image of 

 the wire is still iixed in the same place, after 

 which B is released with the two pendulums in 

 step. These operations succeed much easier 

 than would be expected. 



