408 
MR. BAILY ON THE CORRECTION OF 
of suspension, or some other influential property : and comprise almost every 
variety of pendulum that is ever likely to be made the subject of experiment. 
In order to prevent confusion in occasionally referring- to them, I shall here 
arrange them in numerical order, and class them according to their form. 
No. 1, 2, 3, 4, are spheres of platina, lead, brass and ivory ; all of the same 
diameter; which is somewhat less than 1^ inch. The platina sphere (No. 1.), 
which has been kindly lent to me, for the occasion, by the Astronomer Royal, 
is of French manufacture, and about T44 inch in diameter; which is the 
same size as that used by M. Biot in his pendulum experiments, and in fact 
appears to have been formed from the same model *. It is furnished with a 
copper calotte, and also with a knife edge, attached to a frame, capable of 
being brought to a state of synchronism with the pendulum with w r hich it is 
used, by means of a screw, in the manner described by M. Borda in the Base 
du Systeme Metrique, vol. iii. page 338. Its specific gravity I found to be 
2T042 ; and it weighed 8963 grains. The copper calotte weighed 8 7 grains, 
and was firmly attached to the platina sphere by means of shell-lac ; as the 
ordinary mode, by greasing the parts, failed in the present experiments. I un- 
fortunately attempted the usual method, in the first instance, not recollecting 
that the adhesion is caused principally by the pressure of the atmosphere ; 
and that when that pressure is removed the sphere would no longer be sup- 
ported. This proved to be the case ; and the platina sphere, in its fall, re- 
ceived a slight cut against the sides of the vacuum apparatus ; but not of suf- 
ficient importance to impair its accuracy in any future experiments. It cer- 
tainly cannot affect the present ones, which are merely comparative. The 
vibrating specific gravity of the mass, including the iron wire to which I shall 
presently allude, was computed to be 20 - 745. The leaden sphere (No. 2), the 
brass sphere (No. 3), and the ivory sphere (No. 4), were ordered to be made of 
the same size as the platina one: but they are somewhat larger, being T46 
inch. They have no calotte, but were tapped for the purpose of inserting a 
brass screw, perforated with a small hole for the insertion of the wire by which 
they were suspended. The screw weighed 19^ grains: and the same screw 
has served for all the experiments, where it was required, except for the platina 
sphere. The wire employed in these and all the subsequent cases, unless 
otherwise expressed, was of iron about 7 Vth of an inch in diameter ; and 
* Base du Systfime Mdtrique, vol. iv. page 449. 
