( 1785 ) 
IV. Experiments about the Motion of fendulums in Va- 
cuo. % the Reverend Ur^W. Derham^ F. (^. S. 
I -Have long defired to know, whether there be any, or 
what Difference, between the Vibration of ^mdnhtms 
in VacHo^ and the Air. And therefore have divers times 
recommended the Experiment to many ingenious perfons, 
who had better opportunities af trying it than L And 
at laft, having feen' Mr Hawkeshees Experiments in his 
Pneumatick Engines (hewn before the R. Society, I ac- 
compHflfd my defire, by recomriiendiog the Experiment, 
to him 5 who readily undertook it. And having pro- 
vided himfelt with a proper Receiver, and all other 
things neceffary, he, with a Friend of mine m London^ 
made the Experiment. The Movements he tryed with 
were aiT Eight day Clock vibrating feconds, and an Half-, 
feconds Movement of mine, which I fhall fj^eak more o£. 
prefently. The iffoe of their Experiment was, My Pen- 
dulum vibrated two tenths of an Inch on each fide farther. 
in Vacuo, than it did in the Free Air, and went 7 feconds 
flower in 20 minutes, than the orher Movement. Bat in 
the open Air, my Pendulum in 20 minutes, went only 3 i 
feconds (lower than the otlier Pendulum. 
This Experiment (although I diftrufted neither the . 
Skill, or Fidelity of the Experimenters ) I was dcfirous to . 
try over my felf. And by Mr Havpk,esbe€S generous kind- 
nefs, in lending me one of his Air-Pump ( which I Qiall . 
always gratefully acknowledge ) I was fcon furnifhed . 
with proper I nftruments. Which ! ihair give fome ac*. 
count of, that others may judge how far the Experiment 
Tttctt tttt 2 may 
