332 



NA TV RE 



[August 2, 1894 



ratus. B and c repreieiit an accurately turned brass box with a 

 lid L, which can be made to turn round insensibly by the 

 action of the wheels w w. The lid carries two tubular pillars, 

 P P, from the tops of which the balls, M M, hang by phosphor. 

 bronze wires, being definitely held in place by geometrical 

 clamps on the heads of the pillars. The lid also carries two 

 supporting pillars, R R. In the centretube the "beam mirror," 

 N, hangs by means of a quartz fibre from an adjustable torsion 

 head surmounted by a. bell jar, and from the ends of the mirror 

 the two gold balls, m m, hang by separate quartz fibres. Four 

 rings of india-rubber are placed on the base to prevent destruc. 

 tion of the apparatus in case the balls should drop by any 

 accident. Now it is evident that if the lid is turned from the 

 position in which it is shown, that is, with all four balls in one 

 plane, in which position the attractions do not tend to twist the 

 central torsion fibre at all, then these attractions will produce a 

 couple increasing with the angle up to a certain point (65° in 



therefore, is such that a great number of measures which are 

 difficult, and can at the best only be made with a second fjuality 

 degree of accuracy, are of so little consequence that this degree is 

 more than abundant. The final result depends directly upon a 

 few measures which, as I hope to show, can be made with facility 

 and most accurately. These are the horizontal distance from 

 centre to centre of the wires by which the lead balls are su-- 

 pended, the horizontal distance between the centres of the 

 quartz fibres by which the gold balls are suspsnded, the angle 

 through which the mirror is deflected, the masses of the lead 

 but not of the gold balls, and the natural time of oscillation of 

 the mirror when the balls are suspended and when a thin 

 cylinder of small moment of inertia, but of the same weight as 

 the balls, is suspended axially in their stead. 



Before going more into detail and showing how the operations 

 are carried out so that all the quantities may be known with a 

 suflScient degree of accuracy, it will be convenient to project upon 



Fig. 



the particular case), after which the couple falls off again and 

 become* zero when it has turned 180°. 



Since the cflTecl is a maximum at 65°, very great accuracy in 

 the measurement of this angle is of little consequence. By 

 means of a small telescope at a distant tabic, and Ihe divided 

 edge and vernier, I can tell the angle wilh certainly to 1/20 

 degree ; an uncertainty of one-quarter of a degree would be of 

 but little conse(|Ucncc. Again, if the pair ol gold balls twi&t 

 about an axil which is not exactly that round which the lead 

 balK ate carried, if there is any small ecccniricily of the gold or 

 lead ball.i, then eccentricity in the common plane removes the 

 gold balls from a poiilionof minimum effect, eccentricity across 

 the iiLinc removes them from a posiliin of maximum effect, and 

 if the levels of the gold lialls are not precisely the !^ame as those 

 of the lead balls, again the departure is from a position of maxi- 

 mum cfTecl. All these three eccentricities can be determined 

 wilh an accuracy of l/iooo inch. Errors of l/too inch would 

 make ■ barely perceptible effect upon the result. The design, 



NO. 1292, VOL. 50] 



Ihe screen a drawing of the vault in which the experiments have 

 been made. Prof. Clifton has kindly allowed me the free use 

 of the vault under Ihc Clarendon Lab iratory at Oxford. 

 This is shown in Fig. 2, of which the upper portion represents 

 an elevation, and the lower part a plan. The instrument it>el( 

 stands upon the table A, in the corner, where it is screened from 

 temperature disturbances, which my presence in the distant 

 corner and a very small flame produce, by an oclagon house of 

 double wood lined with cottonwool and by double (elt screens, 

 /, /"j. On Ihc second t.ible, A„ aie pLiced a large astronomical 

 telescope, T, through which the large .'■calc, s, is seen by ic 

 flection from the mirror in the apparatus, a small reading 

 telescope, /, lo rc.id the angle of the lid and vernier, a pulley- 

 wheel, /,, and a diivingwhecl, (i. The pulley-wheel /, keeps 

 the cord ii which passes round/, and/j. and is allaclicd to the 

 cart, X, always tightly stretched, so lh.it the observer at Ihc tele- 

 scope can always keep a little flame carried by the cart 

 immediately bchnid the parlicular division under obscrvalion. 



