1889.] 



On the Cavendish Experiment. 



265 



result with apparatus of the ordinary construction and usual size is 

 next to impossible, on account chiefly of the great disturbing effect of 

 air currents set up by difference of temperature in the case. The 

 extreme portability of the new instrument is a further advantage, as 

 is evident when the enormous weight and size of the attracting 

 masses in the ordinary apparatus are considered. 



However, this result is only one of the objects of the inquiry which 

 I have now the honour to submit to the Royal Society. The other 

 object which I had in view was to find whether the small apparatus, 

 besides being more sensitive than that hitherto employed, would also 

 be more free from disturbances and so give more consistent results. 

 With this object I have placed the apparatus in a long narrow vault 

 under the private road between the Museum and the Science Schools. 

 This is not a good place for experiments of this kind, for when 

 a cab passes overhead the trembling is so great that loose things 

 visibly move : however, it is the only place at my disposal that is in 

 any degree suitable. A large drain pipe filled with gravel and 

 cement and covered by a slab of stone forms a fairly good table. 

 The scale is made by etching millimetre divisions on a strip of clear 

 plate glass 80 cm. long. This is secured at the other end of the 

 vault at a distance of 1053*8 cm. from the mirror of the instrument. 

 A telescope 132 cm. long and with an object-glass 5*08 cm. in 

 diameter rests on V's clamped to the wall, with its object-glass 

 360 cm. from the mirror. Thus any disturbance that the observer 

 might produce if nearer is avoided and at the same time the field of 

 view comprises 100 divisions. While the observer is sitting at the 

 telescope he can by pulling a string move an albo-carbon light 

 mounted on a carriage so as to illuminate any part of the scale that 

 may happen to be in the field of the telescope. The white and 

 steady flame forms a brilliant background on which the divisions 

 appear in black. The accuracy of the mirror is such that the 

 millimetre divisions are clearly defined, and the position of the cross- 

 wire (a quartz fibre) can be read accurately to one-tenth of a division. 

 This corresponds to a movement of the mirror of almost exactly one 

 second of arc. 



The mode of observation is as follows : When all is quiet with the 

 large masses in one extreme position, the position of rest is observed 

 and a mark placed on the scale. The masses are moved to one side 

 for a time and then replaced which sets up an oscillation. The 

 reading of every elongation and the time of every transit of the mark 

 are observed until the amplitude is reduced to three or four centi- 

 metres. The masses are then moved to the other extreme position and 

 the elongations and transits observed again, and this is repeated as 

 often as necessary. 



On the evening of Saturday, May 18th, six sets of readings were 



