August 23, 1894] 



NATURE 



417 



for observation, and there will be very little diminution in its 

 light during the next three or four months. Using recent 

 observations of the position of the comet, M. Schulhof has 

 computed a new orbit. The following positions are extracted 

 from the ephemeris based upon the new elements : — 



Ephemcris for Paris Midnight. 'i 



1894. R.A. Decl. 



Aug. 27 

 29 



3' 

 Sept. 2 



4 



6 



8 

 10 



12 



A New Variable Star. — The Rev. T. E. Espin informs 

 us. through a Wolsingham Observatory Circular, that the star 

 DM + 5o°'225i, the position of which is R.A. i6h. r5m., 

 Decl. + 50' 47', is variable. 



ON THE NEWTONIAN CONSTANT 

 GRAVITATION} 



III. 



OF 



"pIG. 8 is a view of the apparatus with the optical compass in 

 position, and with the microscopes focussed upon the wires. 

 They are then ready to be withdrawn by the focussing slide, 

 so as to transfer the distances directly to the small glass scale, as 

 already described. 



When this is completed the proper windows are put in 

 position, the screen tubes, the octagon house, and the fcli 

 screens are all placed ready for operation lo, in which the 

 dellections are measured, and the period with the balls is 

 determined. As this is the operation in which variations of 

 temperature produce so serious an eftect, I prefer to leave every- 

 thing undisturbed for three days, to quiet down. A few hours 

 are quite useless for the purpose. 



In operation il the period with the counter-weight in th. 

 place of the gold balls is measured ; also the deflection, if any. 

 due to the lid and lead balls upon the mirror alone. This i 

 only i/io division, but its existence is certain. In the latci 

 operations the deflections, if any, due to the lid alone 

 on the mirror alone, and to the lid alone on the ;iiirror 

 and gold balls, are separately determined. Neither of 

 these can be detected. The actual elongation of the fibre may 

 also be observed at this stage, but this is of interest only as 

 bearing on the elastic properties of quartz fibres under longitu- 

 dinal strain. 



Before I come to the treatment of the observations, I should 

 like to refer shortly to the kind of perfection of conditions which 

 by the employment of every practicable refinement that I could 

 devise, I have succeeded in obtaining. Taking experiments as 

 an example, favourable in that the conditions were good, i.e. 

 I was not badly disturbed by trains, wind, or earth tremors, I 

 give the worst and the best sets of four points of rest obtained 

 from six elongations. They were : — 



Best set 

 — position 

 20795-4 

 207957 



20795 '5 iii 

 20795-5 



20795-5 niean. 



Taking all the mean points of rest, as determined above, in 

 groups of three to eliminate slow shifting, if any, of the points 

 "f rest, the series of deflections were : — 



3696-0 

 3696-3 

 3606 o 

 3696-8 



ConciaucJ from p. 363. l\ 

 NO. 1295, VOL. 50] 



- Disturbed by trains. 



(Interval of one hour, in which oscillations of large amplitude 

 were observed for period.) 



(36977) 

 3696-0 



Immediately after the oscillations of large amplitude, which in 

 this case at the end were rather badly disturbed by trains or 

 otherwise, a rather dilTerent deflection was observed, but not 

 seriously different. As examination of the figures shows only 

 one anomalous point of rest immediately after the large ampli- 

 tude disturbance, I feel justified in rejecting the only discordant 

 figure, and in taking the mean of the rest as the true deflection. 

 The unit in this case is i/io division. It corresponds to an 

 angular movement of 1/280000, i.e. about three-quarters of a 

 second of arc. Now a calculation of the angular twist due to a 

 rotation of the air based upon the period, the moment of inertia, 

 and the logarithmic decrement, shows that if the air in the tube 

 were made to whirl round at the rate of one turn in six weeks, 

 so that the air would blow past the gold balls at the rate of 



Fig. 3. _:] 



one inch in a fortnight, the deflection produced would be l/io 

 division, an amount which is greater than the uncertainty of the 

 deflection on this particular night. Again, an examination of 

 the points of rest through the night in the positive and in the 

 negative positions shows a very small steady creep, the same 

 in each case. Creepagc of this sort has been, I believe, men- 

 tioned as a defect of quartz fibres. When it gives trouble it is 

 due to draughts, as already explained, or to imperfect attach- 

 ment of the fibres.' In the present instance the creepage 

 observed corresponds lo a surface rate of movement on the 

 fibre of a millionth of an inch a month. 



An examination of the mobile system of the beam and sus- 

 pended gold balls, of which I e.\hibit a greatly enlarged 

 and working model, at once shows that all the parts are 

 capable of independent movement to an apparently per- 

 plexing degree. This in the theory of the instrument I have 



' See my paper on ' .'Vluchnent of Qu.irtr Fibres," Phil, .^ta;.. May 



i3j.. 



