( 30 ? ) 
by the Obfervations that have hitherto been commu- 
nicated to the Publick ; as may be gathered from the 
20th Propofition of the third Book of Sir Ifaac New- 
ton's Principia, where they are compared as well 
with each other, as with the Theory of that illuftri- 
ous Author. It were therefore to be wilhed, that 
more of this kind of Experiments could be made with 
greater Accuracy in proper Places, by fuch Perfons 
as have fufficient Skill and Opportunities to do it ; 
that we might thereby be enabled to judge with more 
Certainty, concerning the true Figure of the Earth, 
and the Nature of its conflituent Parts. 
As an Inducement to fuch as may have it in their 
Power to put the like again into P raftice ; I fhall 
lay before the Society, an Account of a very curious 
Experiment of this fort lately made in Jamaica , by 
our worthy Member Colin Campbell , Efq* whole 
Knowledge and Abilities in every relpeft qualifies 
him for the Improvement of Arts and Sciences ; and 
whofe Genius prompts him to cultivate them lb affi- 
duoufly, that I doubt not but we (hall foon have the 
Satisfaction of receiving many other valuable Obfer- 
vations from him, particularly fuch as relate to Aftro- 
nomy j he having furnilhed himfelf with an Appara- 
tus of Inflruments not unworthy the Obfervatory of 
a Prince. Among thefe is a Clock whofe Pendulum 
vibrates Seconds, made by our ingenious Member 
Mr. George Graham , juflly efteem’d for his great 
Skill in Mechanicks •, who judging that an Opportu- 
nity was now offered of trying with the utmoft Ex- 
acinefs, what is the true Difference between the 
Lengths of Ifochronal Pendulums at London and Ja- 
maica^ 
