Dec. 19, 1.S89] 



NA TURE 



159 



It will be noticed that the true position of rest is slightly 

 rising in value, and this rise was found to continue at the 

 rate of o'36 centimetre an hour during the whole course 

 of the experiment, and to be the same when the large 

 masses were in the positive or negative position. The 

 motion was perfectly uniform, and in no way interfered 

 with the accuracy of the experiments. It was due, I 

 believe, to the shellac fastening of the fibre, for I find 

 that immediately after a fibre has been attached, this 

 movement is very noticeable, but after a few days it 

 almost entirely ceases ; it is, moreover, chiefly evident 

 when the fibre is loaded very heavily. At the time that 

 the experiment was made the instrument had only been 

 set up a few hours. 



The mean decrement of three positive sets was o'Soii, 

 and of three negative sets, o'8o35. The observed mean 

 period of three positive sets was 79'g8, and of three 

 negative sets, 8003 seconds, from both of which 0*20 

 must be deducted as the time correction for damping. 



The deflections, in centimetres, obtained from the six 

 sets of observations taken in groups of three, so as to take 

 into account the effect of the slow change of the position 

 of rest, were as follows : — ■ 



From sets i, 2, and 3 



,, 2, 3, and 4 



,, 3, 4, and ^ 



,, 4, 5, and 6 



17-66 ± Q-oi 



17-65 ± 0-02 



17-65 ± 0-02 



17-65 ± 002 



An examination of these figures shows that the deflec- 

 tion is known with an accuracy of about one part in two 

 thousand, while the period is known to the 4000th part 

 of the whole. As a matter of fact, the discrepancies are 

 not more than may be due to an uncertainty in some of 

 the observations of A millimetre or less, a quantity which, 

 under the circumstances, is hardly to be avoided. 



The result of these experiments is complete and satis- 

 factory. As a lecture experiment, the attraction between 

 small masses can be easily and certainly shown, even 

 though the resolved force causing motion is, as in the 

 present instance, no more than the 1/200,000 of a dyne 

 (less than 1/10,000,000 of the weight of a grain), and this 

 is possible with the comparatively short half period of 

 80 seconds. Had it been necessary to make use of such 

 half periods as three to fifteen minutes, which have been 

 employed hitherto, then, even though a considerable deflec- 

 tion were produced, this could hardly be considered a 

 lecture experiment. So perfectly does the instrument 

 behave, that there can be no difficulty in making a fairly 

 accurate measure of the attraction between a pair of 

 No. 5, or, I believe, even of dust shot. 



The very remarkable agreement between successive 

 deflections and periods shows that an absolute measure 

 made with apparatus designed for the purpose, but on 

 the lines laid down above, is likely to lead to results of 

 far greater accuracy than any that have been obtained. 

 For instance, in the original experiment of Cavendish 

 there seems to have been an irregularity in the position 

 of rest of one-tenth of the deflection obtained, while the 

 period showed discrepancies of five to fifteen seconds in 

 seven minutes. The experiments of Baily,made in the most 

 elaborate manner, were more consistent, but Cornu was 

 the first to obtain from the Cavendish apparatus results 

 having a precision in any way comparable to that of 

 other physical measurements. The three papers, pub- 

 lished by him in the Coinptes rendiis of 1878, referred to 

 above, contain a very complete solution of some of the 

 problems to which the investigation has given rise. The 

 agreement between the successive values, decrement, and 

 period is much the same as I have obtained, nevertheless 

 the means of the summer and of the winter observations 

 differ by about I per cent. 



I have not referred to the various methods of determin- 

 ing the constant of gravitation in which a balance, 

 whether with the usual horizontal beam, or with a vertical 



beam on the metronome principle, is employed. They 

 are essentially the same as the Cavendish method, except 

 that there is introduced the friction of the knife-edges 

 and the unknown disturbances due to particles of dust at 

 these points, and to buoyancy, without, in my opinion, 

 any compensating advantage. However, it would appear 

 that if the experiment is to be made with a balance, the 

 considerations which I have advanced in this paper 

 would point to the advantage of making the apparatus 

 small, so that attracting masses of greater proportionate 

 size may be employed, and the disturbance due to 

 convection reduced. 



It is my intention, if I can obtain a proper place in 

 which to make the observations, to prepare an apparatus 

 specially suitable for absolute determinations. The scale 

 will have to be increased, so that the dimensions may be 

 determined to a ten-thousandth part at least. Both pairs 

 of masses should, I think, be suspended by fibres or by 

 wires, so that the distance of their centres from the axis 

 may be accurately measured, and so that, in the case of 

 the little masses, the moment of inertia of the beam, 

 mirror, &c., may be. found by alternately measuring the 

 period with and without the masses attached. The un- 

 balanced attractions between the beam, &c., and the 

 large masses, and between the little masses and anything 

 unsymmetrical about the support of the large masses, will 

 probably be more accurately determined experimentally 

 by observing the deflections when the large and the small 

 masses are in turn removed, than by calculation. 



If anything is to be gained by swinging the small 

 masses in a good Sprengel vacuum, the difficulty will not 

 be so great with apparatus made on the scale I have in 

 view, i e. with a beam about 5 centimetres long, as it 

 would with large apparatus. With a view to reduce the 

 considerable decrement, I did try to maintain such a 

 vacuum in the first iustrument, in which a beam V2 

 centimetre long was suspended by a fibre so fine as to 

 give a complete period of five minutes, but though the 

 pump would click violently for a day perhaps, leakage 

 always took place before long, and so no satisfactory 

 results were obtained. 



With an apparatus such as I have described, but 

 arranged to have a complete period of six minutes, it will 

 be possible to read the scale with an accuracy of 1/10,000 

 of the deflection, and to determine the time of vibration 

 with an accuracy about twice as great. 



I hope early next year, in spite of the difficulty of 

 finding a suitable place to observe in, to prepare appa- 

 ratus for absolute determinations, and I shall be glad to 

 receive any suggestions which those interested may be 

 good enough to offer. C. V. BOYS. 



WILLIAM RAMSAY McNAB. 



XiriLLIAM RAMSAY McNAB, M.D., whose sudden 

 * * death from heart-disease we have already re- 

 corded, was born in Edinburgh in November 1844. He 

 was educated at the Edinburgh Academy, and after- 

 wards in the Univei'sity of that city, obtaining the degree 

 of Doctor of Medicine when twenty-two years of age. 



His grandfather and father, in succession, held office 

 as Curators of the Edinburgh Botanic Garden ; and the 

 late Dr. McNab early manifested an inherited capacity 

 for botanical work ; for, while still an undergraduate, he 

 was appointed assistant to Prof. Balfour, who then held 

 the Edinburgh botanical chair. He also entered the 

 University of Berlin as a student — in botany under Profs. 

 Braun and Koch, and in pathological anatomy and 

 histology under Prof. Virchow. Three years of his 

 later life were spent in medical practice ; but his love 

 of botany was his dominant feeling, and in 1870 he 

 embarked upon a purely biological career, having been 

 then appointed to the Professorship of Natural History 



