NOVEMBKB 16, 1917] 



SCIENCE 



489 



But in the footnote at the bottom of page 609, 

 by the term " Lake ore " the writer really 

 means " Lake copper " and his statement thai 

 " the term has now lost its original meaning " 

 is hardly justifiable, since in the first place for 

 " ore " one should read " copper," and in the 

 second place, that western copper should have 

 been almost fraudulently sold as Lake copper 

 does not signify that the term has lost its 

 meaning; otherwise there would have been no 

 object in the trick. In fact the difference in 

 selling price between Lake copper and electro- 

 lytic copper has been unusually great at times 

 during the last three years. 



Although of course, the book is primarily a 

 text-book, yet the summaries of different theo' 

 ries as to ore deposits (see, for instance, the 

 disciission of Mississipi zinc), often largely 

 based upon original studies, are so valuable 

 that no one interested in its field can afford to 

 be without the book. Alfred C. Lane 



Tufts College 



SPECIAL ARTICLES 



EXPERIMENTS WITH A FOCAULT PENDULUM 



In the issue of Science for March 16, last 

 Dr. Carl Barus, under the above title, de- 

 scribed certain measurements of the rotation 

 of the plane of oscillation of a Focault pen- 

 dulum. The present note gives, for the same 

 determination, another method that is simple, 

 direct and of fair accuracy. 



Fig. 1. 



If in Fig. 1 the point A represent an arc 

 lamp that, through the slit B, illuminates a 

 portion of the scale D; and if PQ represent 

 the plane of vibration of a Focault pendulum 

 at a given time, it is evident that the dif- 

 fraction pattern of the wire will travel up and 

 down the scale as the pendulum oscillates. 

 Further, as thq_ plane of the vibration rotates 

 about the center at C, the amplitude of the 

 motion of the shadow on D will decrease, and 



will become zero at the instant when the os- 

 cillation plane includes the line DO A. This 

 amplitude of the shadow's motion will increase 

 again as the plane of vibration continues its 

 rotation towards the position RS. If the 

 position on the scale of one edge of the central 

 band be taken at each successive elongation 

 of the pendulum; and if these readings be 

 plotted against the time (in terms of the 

 period of the pendulum) two approximately 



"* 'LLLjL'V^ ^ r 4' -"h > }i J}ii }i-W)^ jrhh ii k 



straight lines wiU be obtained. The coor- 

 dinates of the intersection of these lines will 

 give (1) the point on the scale where it is cut 

 by the vertical plane that includes the line 

 AC ; and (2) the time (in terms of the period 

 of the pendulum) of the coincidence of the 

 plane of vibration with the vertical plane 

 defined in (1) (see Fig. 2, a and 6). 



mi\ 



's^ is Si ij >r 39 -i' -ii « U -k ii' Ui] ■irlf^o 

 Fig. 26. 



If, next, the lamp be moved to a position 

 indicated in Fig. 1 hy A' b. similar set of 

 observations will determine a second vertical 

 plane and the time of passage of the plane of 

 vibration through it. The number of oscilla- 

 tions that elapse between a given observation 

 of the first set and a given observation of the 



