26 PROFESSOR C. V. BOYS ON THE 
centre by an amount corresponding very nearly with that which he had deduced 
from his gravitational observations. I do not feel satisfied myself that this affords any 
proof of the existence ofan actual cavity, asa gradual variation of density, such as might 
easily occur in an alloy, might have produced the same effect in each case. For this 
reason, when a much higher degree of accuracy is being sought for, I consider that alloys 
are fatal to success. Professor Cornu has without any doubt avoided any uncertainty 
as to cavities or uniformity of density, or probably truth of form, by employing 
mercury aspirated from one pair of spherical hollow cast-iron moulds to another pair 
so placed as to reverse the attraction. By this means it seems to me everything may be 
known with more than abundant accuracy, except the actual position of the centres of 
the spheres, or what comes to the same thing, their actual distances from the centres 
of the attracted masses. As I explained in my first paper, in my arrangement, the 
difficult geometrical measurements are almost all made of secondary importance. A 
small uncertainty in the levels is, as in previous arrangements, of secondary importance, 
as in this sense they are at a position of maximum effect. A small uncertainty of the 
angle of azimuth does not matter, for this also is at a position of maximum effect. 
If there is a small eccentricity of position of the gold with respect to the lead balls, 
either in the plane of the lead balls, or across that plane, again the effect is infini- 
tesimal, for the departure is from a minimum of effect in the first case, and a maximum 
in the second. The only measures of serious importance, on the accuracy of which 
the result directly depends, is the distance 7m plan from the centre of one lead ball to 
the centre of the other, and the corresponding distance in the case of the gold balls. 
In the first there must not be an uncertainty of 3959 of an inch, or in the second 
of zotoo Of an inch. I do not think it would be possible on Professor CorNnv’s plan 
to obtain a knowledge of the positions of the centres of the mercury spheres, especially 
when one is six inches above the other, with anything like this degree of accuracy, 
and therefore, though with the large apparatus he used, and the proportionally 
lower degree of accuracy that was sufficient, the plan is most excellent, and answers 
perfectly, it would not be suitable in the present case. There is a second objection 
depending upon the magnetic quality of the iron moulds. For though to ordinary 
tests the beam and gold balls are not affected by magnetism, I have felt that in 
measurements of forces of such supreme delicacy it is safest to avoid introducing 
magnetic materials, lest any systematic disturbance should be introduced. I have 
however, satisfied myself by experiment since putting up the apparatus, that a 
magnetic force much greater than that due to the earth produces no effect. 
The plan that I have adopted seems to me to be free from the objections that I 
have urged, to be easily carried out, and to be specially adapted to the purpose of 
exact measurement of the distance in plan from the centre of one ball to the centre of 
the other. 
Mr. Munro, who has special experience in accurate spherical work, made the cast- 
iron mould shown in figs. 16 and 17. The internal hemispheres are turned out so 
