A PENDULUM FOR THE REDUCTION TO A VACUUM. 
463 
space, the results are not sensibly affected : and that they are least so, when the 
surface of the confining body is curved. In the Greenwich vacuum apparatus, 
where the tube is about 13 inches in diameter, Captain Sabine did not find 
any difference in the results of some experiments instituted for the express 
purpose of ascertaining the same ; although the bob of his pendulum was 6 
inches in diameter. In my own apparatus also, I have found the results of 
numerous experiments with the bar pendulums within the tube, agree very 
well with those in free air, before the vacuum apparatus was erected : and cer- 
tainly no discordance has been observable, sufficient to warrant any material 
alteration in the results of the present experiments. In the Greenwich appa- 
ratus, the glass cylinder is formed of three separate pieces, which may be easily 
taken apart ; and the pendulum may thus be, at any time, exposed to the free 
air : whereby the experiments may be alternately made in the confined cylin- 
der, and in the free air. But my apparatus consists of one uniform brass tube, 
and is not adapted to such a change of experiments. 
Anomalies of the knife edge suspension. 
It has been shown by Captain Sabine, in his Account of Experiments, &c. 
page 195, that, in a pendulum with knife edges, a considerable difference may 
arise in the results, if they be used with different planes : but it does not appear 
to have occurred to any one, versed in these experiments, that a much greater 
difference than that which he has recorded may arise from using the same 
knife edge with the same plane. This fact has probably hitherto escaped de- 
tection from the peculiar manner in which pendulum experiments are usually 
conducted : for, on examining the detail of most of those experiments, it will 
be found that after the pendulum at anyone station has been placed in its Y’s, 
it has never been removed therefrom, but merely raised and lowered again as 
occasion may require, till it has been ultimately dismounted, and packed up 
for another station ; whereby any anomaly that might otherwise have occurred, 
is thus avoided, and consequently escapes detection. Experiments, however, 
of this kind, ought to be varied in every possible way, in order to guard against 
any unsuspected source of error. 
When Captain Basil Hall returned from his voyage to the Pacific Ocean, 
where he had undertaken to swing the pendulum at various places, it was 
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