i[ 4 
Caufes, that in many Cafes it is extremely difficult to 
affign to each its due Share of Influence; or diftindly 
to point out, what Part of the Motion is the EfFed oi 
one Caufe, and what of another: And whilft the 
joint EfFcds of All are only attended to, great Irre- 
gularities and feeming Inconflflencies frequently oc- 
cur ; whereas, when we are able to allot to each 
particular Caufe its proper EfFed, Harmony and Uni- 
formity ufually enfue. 
Such feeming Irregularities being alfo blended 
with the unavoidable Errors, which Aflronomicai 
Obfervations muft be always liable to, as well from 
the Imperfedion of our Senfes, as of the Inflruments 
that we make ufe of, have often very much per- 
plex'd thofe, who have attempted to folve the Phe- 
nomena : and till Means are difeovered, whereby we 
can feparate and diflinguifh the ‘particular Part of 
the whole Motion, that is owing to each refpedive 
Caufe, it will be impofliblc, to be well aflured of 
the Truth of any Solution. For thefe Reafons, we 
generally find, that the more exad the Inflrumcnrs 
are, that we . make ufe of, and the more regular the 
Series of Obfervations is, that we take ; the fooner 
we are enabled to difeover the Caufe of any new 
'Phenomenon. For when we can be well aflured 
of the Limits, wherein the Errors of the Obferva- 
tions are contain’d ; and have reduced them within 
as narrow Bounds as poflible, by the Perfedion of 
the Inflruments which we employ ; we need not he- 
fitatc to aferibe fuch apparent Changes, as manifeftly 
exceed thofe Limits, to fome other Caufcs. Upon 
thefe Accounts it is incumbent upon the practical 
Aftronomctj 
