PHYSICAL any LITERARY. 99 
different refrangibility can be thence me- 
chanically explained. 
_ Tue whole fyftem of nature is one im- 
menfe feries of caufes and effects, whofe 
beginning and end are equally hid in the 
depths of infinity. Only a fmall, a very 
{mall portion of it, comes under our im- 
_ mediate obfervation ; being expofed alike 
to the fight and other fenfes of all man- 
kind. Almoft every phaczomenon is, at once, 
the caufe of manifold effets; and one ef- 
fet, among many, of a fuperior caufe. 
‘The butinefs of {cience is to extend our 
views, by unfolding the latent caufes 
which exift in nature ; and thence explain- 
ing their manifeft effe@s. The difcovery 
of one fuch real caufe, unknown before, 
if it be of general or very extenfive influ- 
ence, as that of univerfal gravity, is to be 
e{teemed a great advancement of natural 
philofophy. To undervalue fuch a difco- 
very, as fome have done, becaufe the taufe 
of that caufe cannot yet be afligned, ‘is 
highly abfurd: Since the fame objection 
muit for ever lie againft all caufes, except 
primary 
