134 ESSAYS ann OBSERVATIONS ' 
of bodies are performed immediately by thefe 
bodies themfelves, without the influence of 
other matter, or any other power; there is 
an end of all enquiries into caufes and effects ; 
philofophy muft be degraded to a bare know- 
ledge of facts, a hiftory of nature. 
. Tuere is an argument, in the form of a 
reduétio ad abfurdum, brought againft the re- 
ceived opinion of the equality of action and 
reaction, which had almoft been overlooked. 
The fubftance of it is this, that as the preflure 
undequaque, is a confequence of the law of 
equal action and reaction in fluids; fo the + 
fame fort of preflure ought to be a property 
of folid bodies likewife, if the fame law be 
univerfally extended to all bodies, folids as 
well as fluids. For a ready anfwer to this ob- 
jection, we need only recollect the definition 
of a fluid, as diftinguifhed from a folid body. 
An undequaque preffure cannot obtain in a 
folid body, unlefs the particles of it did yield 
to any force exercifed upon them; and, in 
yielding, were eafily put in motion amongft 
themfelves; or in other words, unlefs the 
folid body was converted intoa fluid. A 
parcel of dry fand may bear fome fmall re- 
femblance to a fluid ; its particles may be mo- 
ved 
