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P>. PV, PV. which forces being as thcfe three lines, 
fliall move along the diagonal of the parallelopipid 
made by thele three lines ; fo that, in the fecond mo- 
ment of time, the body, inftead of moving from R'' 
to fhall move from P'' to P'\ 
Draw the lines SP, SP^^ and TP\TP''\ as alfo 
S/, rp. 
Now, the folid STPP'— folid STP'y>^ for they 
Hand upon equal bafes T P'P, T P^, and have one 
common vertex 8, or their common altitude is the 
perpendicular drawn from S to the plane VTp. 
And the folid STFP' = folid STP>; for they 
iland upon the fame bafe S T P\ and lie between the 
fame parallel planes P", jof. 
Therefore the folid ST PP'— folid STFPA 
In like manner, in the third moment of time, the 
body at P" being adted upon i by three forces, in the 
diredtions P'P", P"S, P"T, fliall move along the line 
P"P"', fo as to make the folid STP"P''"— folid 
ST P'P"; and fo in all fucceeding equal moments of 
time,, the triangle formed by right lines drawn from 
the body to the two fixed points S,T, fhall conftantly 
defcribe little folids, each equal to the folid ST P P\ 
Therefore, the moments of the folids being pro- 
portional to the moments of the time in which they 
are defcribed ; the folid itfelf is proportional to the 
time in which it is defcribed. Q^E. D. 
Some difficulties may, perhaps, feem to arife upon 
a flight view of only particular cafes of this propo- 
rtion ; but, it is conceived, all fuch muit 'vuuffi, 
when the fame is thoroughly confidered. 
For, as in two bodies T and S ; if T is adted 
upon by S, fo as to defcribe a right line, diat is, 
if 
