Motion  by  ImpdB  and  Gravity.  371 
loft  is  always  equal  to  the  motion  communicated  in  an 
oppofite  direction : in  the  fecond  the  motion  loft  is  to  be 
increafed  or  dimmifhed  in  the  ratio  of  the  levers  before 
it  will  be  equal  to  the  motion  communicated.  The  pro- 
perties of  the  lever  are  well  underftood  and  eafily  ap- 
plied, and  becaufe  their  evidence  depends  upon  expe- 
rience, and  is  as  firmly  eftablifhed  as  the  third  law  of 
motion  itfelf,  it  is  always  beft  to  make  ufe  of  thofe  two 
univerfal  principles,  inftead  of  others  which  are  more 
liable  to  deceive  us^. 
In  all  cafes  concerning  the  motion  of  a fingle  body,  or 
fyftem  of  bodies,  where  there  is  any  rotatory  motion,  the 
confideration  of  the  lever  becomes  requifite,  and  that, 
with  a juft  application  of  the  laws  of  motion,  is  fufficient 
for  the  refolution  of  the  moft  arduous  problems.  It  is 
(h)  Tt  is  acknowledged,  that  the  experiments  which  have  been  made  to  deter- 
mine the  effe&s  of  wind  and  water-mills  do  not  agree  with  the  computations  of 
mathematicians;  but  this  is  no  objection  to  the  principles  here  maintained. 
Writers  generally  propofe  fuch  examples  with  a view  rather  of  illuflrating  the 
methods  of  calculation  by  algebra  and  fluxions,  than  of  making  any  ufeful  im- 
provements in  pra£tice.  They  fuppofe  the  particles  of  the  fluid  to  move  in 
flraight  lines,  and  to  ftrike  the  machine  with  a certain  velocity  ; and  after  that, 
to  have  no  more  effeft.  As  fuch  fuppofitions  are  evidently  inconfiftent  with  the 
known  properties  of  a fluid,  we  are  not  at  a lofs  to  account  for  a difference 
between  experiment  and  theory  ; and  therefore  it  fhould  feem  unreasonable  to 
affert,  that  certain  authors  of  reputation  have  negle£led  the  collateral  circum- 
flances  of  time,  fpacje,  or  velocity,  in  the  refolution  of  thefe  problems,  unlefs  wc 
were  able  to  point  out  fuch  omiflions. 
Vol,  LXVIIL 
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