RESISTANCE. 
By distilling to dryness common rosin is 
obtained. When water is added, while it is 
yet fluid, and incorporated by agitation, 
wliat is called yellow rosin is formed. 
Pitch is a resinous juice obtained from 
the pinus picea, pitch pine. It is purified 
by melting and squeezing it through linen 
bags, and it is then known by the name of 
white, or Burgundy pitch. White pitch 
mixed with lamp-black forms black pitch. 
Sandarac. This resinous substance is 
extracted from the juniper. It is a sponta- 
neous exudation from this plant in the form 
of brown tears, which are semitransparent 
and brittle. See Balsam, Copal, Guia- 
CUM, &C. 
RESISTANCE, or Resisting /orce, in 
philosophy, denotes, in general, any power 
which acts in an opposite direction to ano- 
ther, so as to destroy or diminish its effect. 
Hence the force wherewith bodies, moving 
in fluid raedioms, are impeded or retarded, 
is the resistance of those fluids. Authors 
have established it as a certain rule, that, 
whilst the same body moves in the same 
medium, it is always resisted in the dupli- 
cate proportion of its velocity; that is, if 
the resisted body move in one part of its 
track with three times the velocity with 
wliicli it moved in some other part, then its 
resistance to the greater velocity will be 
nine times the resistance to the lesser : if 
the velocity in one place be four times the 
velocity in another, the resistance to the 
greater velocity will be sixteen times the 
resistance to the lesser, and so on. This 
rule, though very erroneous, when taken in 
a general sense, is yet undoubtedly very 
near the truth, when confined within cer- 
tain limits. 
In order to conceive the resistance of 
fluids to a body moving in them, Mr. Ro- 
bins distinguishes between those fluids, 
which being compressed by some incum- 
bent weight, perpetually close up the space 
deserted by the body in motion, without 
permitting, for an instant, any vacuity to 
remain behind it ; and those fluids in which, 
they being not sufficiently compressed, the 
space left behind the moving body remains 
for some time empty. These differences 
in the resisting fluids will occasion very re- 
markable varieties in the law’s of their resis- 
tance, and are absolutely necessary to be 
considered in the determination of the 
action of the air in shot and shells ; for the 
air partakes of both these affections, accord- 
ing to the different velocities of the pro- 
jected body. If a fluid were so constituted. 
that all the particles composing it were at 
some distance from each other, and there 
was no action between tiiem, then the re- 
sistance of a body moving therein would be 
easily computed from the quantity of mo- 
tion communicated to these particles : for 
instance, if a cylinder moved in such a fluid 
in the direction of its axis, it would commu- 
nicate to the particles it met with a velocity 
equal to its own, and in its own direction, 
supposing that neitlier the cylinder nor the 
parts of the fluid were elastic ; whence, if 
the velocity and diameter of the cylinder 
be known, and also tlie density of the fluid, 
there would thence be determined the 
quantity of motion communicated to the 
fluid, whidi (action and re action being 
equal) is the same with the quantity lost by 
the cylinder, consequently the resistance 
would be hereby ascertained. 
In this kind of discontinued fluid, the 
particles being detached from each other, 
every one of them can pursue its own mo- 
tion in any direction, at least for some time, 
independently of the neighbouring ones j 
wherefore, if instead of a cylinder moving in 
the direction of its axis, a body, with a sui’- 
face oblique toils direction, be supposed to 
move in such a fluid, the motion the parts 
of the fluid will hereby acquire, will not be 
in the direction of the resisted body, but 
perpendicular to its oblique surface ; whence 
the resistance to such a body will not be 
estimated from the whole motion communi- 
cated to the particles of the fluid', but from 
that part of it only which is in the direction 
of the resisted body. In fluids then, where 
the parts are thus discontinued in each 
other, the different obliquities of that sur- 
face, wdiich goes foremost, will occasion 
considerable changes in the resistance ; al- 
though the section of the solid, by a plain 
perpendicular to its direction, should in all 
cases be the same. And Sir Isaac Newton 
has particularly determined, that in a fluid 
thus constituted the resistance of a globe is 
but half the resistance of a cylinder of the 
same diameter, moving in the direction of 
its axis with the same velocity. 
But though the hypothesis of a fluid, thus 
constituted, be of great use in explaining 
the nature of resistances, yet in reality no 
such fluid does exist within our knowledge : 
all the fluids with which we are conversant 
are so formed, that their particles either lie 
contiguous to each other, or at least act on 
each other in the same manner as if they 
did ; consequently, in tliese fluids, no one 
particle, contiguous to the resisted body, 
