Repulsion resulting from Radiation. 151 



movements on a graduated circle. The beam is cemented to the 

 torsion-fibre, and the whole is enclosed in glass and connected with 

 the mercury pump and exhausted as perfectly as possible. A 

 weight of 0*01 grain is so arranged that it can be placed on the 

 pith or removed from it at pleasure. A ray of light from a lamp 

 reflected from a mirror in the centre of the beam to a millimetre- 

 scale 4 feet off shows the slightest movement. When the reflected 

 ray points to zero, a turn of the torsion-handle in one or the other 

 direction will raise or depress the pith end of the beam, and thus 

 cause the index ray to travel along the scale to the right or to the 

 left. If a small weight is placed on one end so as to depress it, 

 and the torsion -handle is then turned, the tendency of the glass 

 fibre to untwist itself will ultimately balance the downward pres- 

 sure of the weight, and will again bring the index ray to zero. 

 It was found that when the weight of the y^- of a grain was placed 

 on the pith surface, the torsion-handle had to be turned 27 revo- 

 lutions and 353°, or 10073° before the beam became horizontal. 

 The downward pressure of the T ^-§ of a grain was therefore 

 equivalent to the force of torsion of the glass thread when twisted 

 through 10073°. 



The author next ascertained what was the smallest amount of 

 weight which the balance would indicate. He found that 1° of 

 torsion gave a very decided movement of the index ray of light, a 

 torsion of 10073° balancing the -^ of a grain, while 10074° 

 overbalanced it. The balance will therefore turn to 100 Q 9 9 Q 000 of 

 a grain. 



Divide a grain weight into a million parts, place one of them on 

 the pan of the balance, and the beam will be instantly depressed. 



"Weighed in this balance the mechanica] force of a candle 12 

 inches off was found to be 0*000444 grain ; of a candle 6 inches off 

 0*001772 grain. At half the distance the weight of radiation 

 should be four times, or 0*001776 grain ; the difference between 

 theory and experiment being only four millionths of a grain is a 

 sufficient proof that the indications of this instrument, like those 

 of the apparatus previously described by the author, follow rigidly 

 the law of inverse squares. An examination of the differences 

 between the separate observations and the mean shows that the 

 author's estimate of the sensitiveness of his balance is not excessive, 

 and that in practice it will safely indicate the millionth of a grain. 



One observation of the weight of sunlight is given ; it was 

 taken on December 13 ; but the sun was so obscured by thin clouds 

 and haze that it was only equal to 10*2 candles 6 inches off. 

 Calculating from this datum, it is seen that the pressure of sun- 

 shine is 2*3 tons per square mile. 



The author promises further observations with this instrument, 

 not only in photometry and in the repulsion caused by radiation, 

 but in other branches of science in which the possession of a balance 

 of such incredible delicacy is likely to furnish valuable results. 



