PHYSICAL Anp LITERARY. 97 
common projectiles, it is not capable of 
retarding them fenfibly in their motion 
from the moft diftant fixed ftars to our 
eye? Do not thefe and many other diffi- 
culties; in the phyfical part of optics, 
whofe folution is fought for in vain from 
any principles hitherto difcovered, fhew 
the neceffity of extending our views and 
inlarging our ftock of principles by fur- 
ther experimental inquiry? Such objec- 
tions are not to be confidered as demon- , 
{trations of the -falfhood of our prefent 
theory, but as proofs of its narrownefs, 
partiality, and imperfection. 
Des CarTEs, contenting himfelf with 
a fuperficial and inaccurate knowledge of 
the laws of impulfe, vainly dreamed, that 
he had got poffeffion of the univerfal caufe 
from whence all effects in nature are de- 
rived; when, in truth, he was unable to 
_deduce from them the fimpleft cafes of 
collifion. » Many in this age, who write 
and fpeculate on phyfical fubjects, feem 
to fall into a like error; while they em- 
ploy their whole fiudy in endeavouring 
Vor. IE. N to 
