March 16, 1917] 



SCIENCE 



265 



in Alaska. Juneau is to become a regular 

 Weather Bureau station and climatological 

 section center. ITot only will the climate of 

 Alaska become more fully known but also it is 

 thought that the general weather and storm 

 forecasts for the United States will be helped. 



Another $10,000 is to be used in extending 

 the river and flood and the frost-warning 

 services. 



The Weather Bureau has recently announced 

 a new civil service examination designed prin- 

 cipally for college graduates who are compe- 

 tent to carry on scientific investigation. The 

 initial salary is $1,260 a year. 



Charles F. Brooks 

 Yale College 



SPECIAL ARTICLES 



EXPERIMENTS WITH THE FOUCAULT 

 PENDULUM 



1. Introductory. — In view of the relatively 

 large angular velocity of the earth, it should 

 be possible to exhibit this rotation by aid of 

 the Poucault pendulum in a few minutes, and 

 this in such a way that reasonably accurate 

 quantitative results may appear. As the 

 pendulum partakes of the rotation of the earth 

 it is not feasible to attach mirrors to the bob, 

 even if this were useful. It is equally clear 

 that the combination of a horizontal pendulum 

 and a Foucault pendulum at its end, or of a 

 large pivoted balance beam with two identical 

 pendulums at its ends will lead to no solution 

 of the problem. In the following note I shall 

 give the results of an optic and of an electric 

 method which I recently had occasion to test 

 and which may interest the reader. A few 

 remarks will also be made on an earth inductor 

 pendulum. 



2. Apparatus. The question is obviously 

 solved if the swing of the pendulum is re- 

 garded with a distant telescope with an ocular 

 micrometer, sighting in the plane of vibration. 

 The equivalent objective result may be ob- 

 tained if as in Fig. 1, a lens L (not too strong) 

 is placed near the pendulum. The string at 

 rest C is to be at the conjugate focal distance 

 u to the distance v of the screen 8 from the 

 lens. The string must be strongly illuminated 



by a Nernst burner N, or sunlight, or the like, 

 and the arc of vibration ah or cd = D must 

 not be so large as to seriously throw the image 

 m of the string at 8 out of focus. A lens of 

 focal distance of about 60 cm., for a swing D 

 (double amplitude) not larger than 30 cm., 

 does very well. If 8 is about at 6 meters u 

 will be somewhat short of 70 cm. The pen- 

 dulum bob should obviously be heavy (3-6 kg.) 

 and the string long (4r-5 meters) so that 

 vibration may be slow (period 4 seconds or 

 more), air currents ineffective and observa- 

 tion at 8 easy. 



The vibration is started with the arc ah in 

 the direction of the optical center of L, or 

 otherwise the lens is so placed. In this case 

 the image of the string is stationary at m on 

 the screen. Of course lateral vibration and 

 rotation of the bob around the string as an 

 axis must be scrupulously avoided. This is 

 easily done by letting the bob fall from a 

 lateral hitching cord with one hand after all 

 vibration has been checked by the loose fingers 

 of the other hand, and the image is at m. 



The image m soon begins to vibrate right 

 and left more or more fully on the screen 8 

 and after the earth has rotated over the angle 

 B, the point c is replaced by the elongations 

 dd' and the point m has expanded into the 

 elongations at a distance x apart. With a 

 swing of D = 36 cm. originally, the distance x 

 increases to nearly 5 cm. in 5 minutes, or about 

 1 cm. per minute with the dimension of pen- 

 dulum and lens given above. The rate falls 

 off because the arc D diminishes. 



3. Equation. — Fig. 1 shows that if 6 is the 

 angle of rotation, for the distance x between 

 the elongations at the screen 8 and the double 

 swing of pendulum cd = D, and if the con- 

 stant h = u/v, approximately 



(1) e'=^^-^^=(fc-D/2«).r/Z), 



remembering that the angles ff at c remain 

 small and are initially nearly the same as B at 

 the center. Furthermore with the same 

 approximation 



(2) S = fl'(l + D/2u) = fc(l - D2/4m=)(i/Z)). 



Hence after reduction if the rates per hour 

 be dotted 



