NEWTONIAN CONSTANT OF GRAVITATION. 37 
is this that it is quite easy by pinching the tube properly with the fingers to bring 
the movements of the mirror, when the counterweight but not the balls are suspended, 
down to one unit, corresponding to 7539900 Inch movement of the gold balls if they 

were in their place. 
Conclusion of Part I. 
The apparatus and optical compass were made by the Cambridge Scientific 
Instrument Company. I cannot lose this opportunity of expressing my great 
indebtedness to Mr. Pyx, who, in the absence of Mr. Darwin through illness, 
entered into every detail with the greatest care and faithfully carried out all the 
directions as to modes of construction upon which, after consultation with him, I 
finally decided. 
As already mentioned, Mr. CoteBrook constructed the special tools and the very 
numerous extraneous apparatus. He also made all the special windows. Mr. Munro 
made the tools for compressing the lead balls, and being his work they are of course 
accurate in the highest degree. Mr. Stantey undertook the large scale, but though 
the execution of the etching is excellent, the accuracy is not good. This, however, 
matters little, for the errors are eliminated by the calibration. 
The actual work of making the gold balls, the lead balls, the finish of the beam 
mitror, the quartz fibre work, the gilding and polishing of the inside of the central 
tube, and a great deal of the general fittings I did myself, either alone or with the 
help of Mr. CoapmAN and Mr. Cotesrook of the Physical Laboratory. 
The apparatus, which belongs to the Science Collection of the South Kensington 
Museum, will, I hope, on the completion of the experiments, be set up in a special 
place in the Museum so that it may be seen in action by anyone interested. I intend 
to leave also permanently in the Museum a series of photographs of the apparatus as 
it appears 77 situ when each one of the operations is being carried out. Besides this, 
the note books and all the calculations will be left there permanently so that 
reference may be made to them in case any point is insutliciently explained. 
Parr II. 
The Mode of Procedure. 
The actual method of carrying out the experiment, though in the main obvious to any 
one who has read the first part of this paper, is nevertheless in some particulars by no 
means evident. As, moreover, a careful explanation of the several operations and 
the purposes which they serve will make the mere numerical details of the actual 
experiments, which form the third part of the paper, more intelligible, I shall describe 
them in order, under distinguishing numbers. 
The operations, as described, are fourteen in number, but they are of very different 
