f6] 
2. In like manner, when any phyfical theory is 
deduced from obfervations, its accuracy will dill be 
in proportion to that of the obfervations, on which it 
is founded. Sir Ifaac Newton, we find, in com- 
puting the ratio of the earth’s axis to its equatorial 
diameter, confines himfelf to a reafonable approxi- 
mation, and to three places of figures (229 to 230) ; 
becaufe, whether that ratio is deduced from the dif- 
ferent lengths of ifochronous pendulums in different 
latitudes, from the meafurement of diflant degrees 
of a meridian, or from both j the elements of the 
calculus can fcarcely furnifh a greater degree of ex- 
aCtnefs. And of the fame judicious caution we have 
many other examples in the works of that incompa- 
rable author. 
3. On the other hand, when obfervations and 
theories are brought together, and compared, nothing 
can be juftly inferred againfl: a theory from its dif- 
agreement with the obfervations, unlefs that difagree- 
ment is greater, than can be fairly imputed to the im- 
perfection of inftruments, and to the unavoidable 
miftakes of an obferver 5 efpecially, if the difference 
fhould be fometimes in excefs, and at other times in 
defeCl ; or, according to fome of the obfervations, 
fhould intirely vanifh. 
4. Altho’ thefe rules, manifestly well-founded, 
have been followed by all the beft writers, our au- 
thor obferves, that feveral ingenious men, both in 
France, and in Italy, have deviated from them ; par- 
ticularly in treating of the famous queftion concern- 
ing the figure of the earth. Some, with Meflieurs 
Clairaut and Bougucr, attributing too much to the 
obfervations, that have been made, and taking them 
