M E C H A N I C S. 
•f being transferred From one place to another, or of ex¬ 
alting in different parts of 1‘pace. 
All bodies are porous: whence, together with the ex¬ 
treme minutenefs of their particles, it happens, that fluids 
wijl infmuate themfelves into all bodies; that fometimes 
a mixture of two fluids will be lefs in bulk than when 
they are feparate; and that the fame bulk may contain 
different quantities of matter, or maffes. Denfity, flriftly 
fpeaking, denotes vicinity or clofenefs of particles; but 
in mechanical fcience it is ufed as a term of comparifon 
exprefling the proportion of the number of equal mole- 
culte, or the quantity of matter, in one body, to the num¬ 
ber of equal moleculae in the fame bulk of another body. 
Denflty, therefore, is direftly as the quantity of matter, 
and inverfely as the magnitude of the body. 
Motion is a Ample idea, and therefore admits not of de¬ 
finition. When we fay that it is a continual and fucceflive 
change of place, we defcribe it in a periphrafis, by itsfen- 
fible effefts. Or, by another circumlocution, motion 
may be defcribed as that Hate of a body which Is not con¬ 
fident with its continuance in the fame place; or in which 
it is not, in two fucceflive infiants of duration, at the 
fame diftance from divers fixed points in fpace: this ftate 
is oppofed to that of rejl. The motion of bodies is con- 
fidered either as abfolute or relative. A body is faid to be 
in abfolute motion while it is aftually pafling from one point 
in fixed fpace to another ; and to be in relative motion while 
its pofition is varying with refpect to other bodies. It is 
obvious that thefe two kinds of motion can only coincide 
when the bodies, to which the reference is made, are fixed ; 
in other cafes a body in relative motion may or may not 
be in abfolute motion. The determination of the abfo¬ 
lute motions, by means of obfervations on the relative 
motions, is always a matter of great difficulty; nay, is 
generally abfolutely impoflible. Thus, when a ball is 
difcharged from a piece of ordnance, it is poflible, by 
means of the balliftic pendulum, and other contrivances 
of ingenious men, to afcertain its relative motion; that 
is, its motion with refpeft to that place on the earth’s 
furface from which it is projected ; but, in order to deter¬ 
mine its abfolute motion, the diurnal and annual motions 
of the earth about the fun, and probably the motion of 
that luminary about the centre of fome more extenfive 
fyftem, muft; be taken into the account; fo that on the 
whole this apparently Ample enquiry becomes fufficiently 
complex to baffle the proudeft efforts of human intelligence. 
The confideration of motion neceflarily involves that 
of time ; for no motion can be inftantaneous. Abfolute time 
is a portion of duration whofe quantity is only known by 
a comparifon -with another portion ; and confequently 
the relation between any two parts of abfolute time is 
not to be difcovered. Relative time is a part of duration 
■which elapfes during any motion of body, or any fuccef- 
Sion of external appearances. 
Velocity , or celerity , is that affeftion of motion which de¬ 
termines its quantity : it is the name exprefling the rela¬ 
tion between the fpace defcribed by a moving body and 
the time which elapfes during its defcription ; and it is 
meafured by the fpace uniformly defcribed, in a given time. 
A body is faid to move with a uniform , accelerated, or re¬ 
tarded, velocity, according as its rate of motion continues 
the fame, increafes or decreafes; when the increafe orde- 
creafe of velocity is the fame in any equal times, the ac¬ 
celeration or retardation is faid to be uniform ; and, when 
this increafe or decreafe of velocity itfelf increafes or de¬ 
creafes in any equal times, the acceleration or retardation 
increafes or decreafes in the fame ratio. Thefe circum- 
fiances will be brought more fully into confideration as 
we proceed. 
The direBion of a motion is the pofition of the line along 
which it is performed : thus, if a body move from a point 
A to another point B along the ftraight line which joins 
thefe points, AB is called the direction of the body ; if 
the body move from B to A along the fame right line, 
B A, is the direftion. If a body move along a curve line, 
Voi.. XIV. No. iooo. 
cm 
its direElion is continually changing ; it may, however, in 
any given point be regarded as coinciding with the tan¬ 
gent to the curve at that point. 
Force, or power, in a mechanical fenfe, is that which 
caufes a change in the ftate of a body, whether that ftate 
be reft or motion. We fpeak here of proximate caufes, 
for it is not the bufinefs of mechanics to fearch into the 
eflential and hidden caufes of motion. The enquiry whe¬ 
ther they are material or fpiritual may exercife the talents 
of ingenious fpeculatills, and may, perhaps, be of fome 
importance in a moral point of view ; but certainly forms 
no part of the principles of mechanical fcience. The 
mufcular power of animals, as likewife preftiire, impaft, 
gravity, eleftricity, &c. are'by us looked upon as forces, 
or fources of motion; for it is an incontrovertable faff, 
that bodies expofed to the free aflion of either of thefe 
are put into motion, or have the ftate of their motion 
changed. All forces, however various, are meafured by 
the eft’efls they produce in like circumftances; whether 
the effefts be creating, accelerating, retarding, or defleft. 
ing, motions ; the effeft of fome general and commonly- 
obferved force is taken for unity; and with this any 
others may be compared, and their proportions reprefented 
by numbers or by lines ; in this point of view they are 
confidered by the mathematician ; all elfe falls within the 
province of the univerfal philosopher or the metaphy- 
fician. When we fay that a force is reprefented by a 
right line A B, it is to be underftood that it would caufe 
a material point fituated at reft in A, to run over the line 
A B (which we name the direBion. of the power) fo as to 
arrive at B at the end of a given time ; while another 
power fhould caufe the fame point to have moved a greater 
or lefs diftance from A in the fame time. 
Among other forces, it has been cuftomary to fpeak of 
the vis inertia, or inert force, of matter ; applying the 
term to that property of bodies by which they tend to 
retain their prefent ftate, or are indifferent to motion or 
reft. But, while we admit that much of the language re¬ 
lating to powers, forces, actions, Sec. is metaphorical, we 
muft objeft to fuch life of it in the prefent cafe; this pro¬ 
perty being improperly called a force : ift. Becaufe, were 
it a (finally fuch, it muft be of fome definite quantity in a 
given body, and therefore an impreffed force lefs than 
that would not move the body ; whereas any impreffect 
force, however fmall, will move any body however great, 
adly. Becaufe it feems to indicate an aftive power refi- 
dent in matter; or, rather, it implies an abfolute contra, 
diction, namely, that a body fhould be both aftive and 
inaftive at the fame time. It is defirable, therefore, that 
only the term inertia, or inertnefs, fhould be retained ; for 
this term will imply, as it ought to do, that matter is a 
merely paflive thing. A fail which needs no laboured 
proof; for this inertia prefents itfelf immediately in all 
our obfervations and experiments upon matter, and is in. 
feparable from it, even in idea. When we confider any 
of the aftive powers of nature, as they are called, fuch as 
gravitation, magnetifm, eleflricity, or the attraftions and 
repulfions which take place in the cohefions and fepara- 
tions of the fmall particles of natural bodies, and endea. 
vour to refolve them into fome higher and Ampler prin¬ 
ciples, the inertia is always the common bafis upon which 
we endeavour to ereft our folutions. For the aftive party 
which is fuppofed to generate the gravitation, magnetifm, 
&c. in the paflive one, muft have a motion, and inertia, 
whereby it continues in that motion, elfe it could have no 
power ; and, by parity of reafon, the paflive party muft be 
inert alfo, elfe it could not re-aft againft the aftive party, 
nor imprefs motion on a foreign body. And this, by tire 
way, if the reader will pardon a flight digreflion, fuggefts 
a brief but cogent argument for the immateriality of the 
Supreme Being. For, as the acute Hartley obferves, “ Let 
us proceed as far as we pleafe in a feries of fucceflive fo¬ 
lutions, we ftiall always find an inertia inherent in mat¬ 
ter, and a motion derived to it from fome foreign caufe. 
If this caufe be fuppofed matter always, we fhall be car. 
7 If rie4 
