C2i :M ECH 
cimen Theories Turbinum, 1765. lie demonftrated, that, if a 
body of any form or magnitude, after it has received rota¬ 
tory motions in all directions, be left entirely to itfelf, it 
will always have three principal axes of rotation ; or, in 
other words, all the rotatory motions with which it is 
affetied, may be reduced to three, which are performed 
round three axes, perpendicular to each, pafling through 
the centre of gravity of the revolving body, and preferv- 
ing the fame pofition in abfolute fpace, while the centre 
of gravity is either at reft or moving uniformly in a 
ftraight line. 
The force of torfwn began at this time to be inveftigated 
by Coulomb, who publifhed two ingenious papers on the 
fubjeCt, in the Memoirs of the French Academy. He 
fuccefsfully employed this principle in feveral phyfical 
refearches, but particularly in determining the law of 
magnetic aCtion, and in finding the laws of the refiftance 
of fluids when the motions are extremely flow. It was 
by means of an elegant experiment on the principle of 
torfion that Mr. Cavendifh determined the mutual attrac¬ 
tion of two mafles of lead, and thence deduced the mean 
denfity of the earth. We are alfo indebted to Coulomb 
for a complete fet of experiments on the nature and effects 
of friftion. By employing large bodies and ponderous 
weights, and conducting his experiments on a large fcale, 
he has corrected errors which neceflarily arofe from the 
limited experiments of preceding writers; he has brought 
to light many new and interefting faCts, and confirmed 
others which had hitherto been partially eftablifhed. The 
molt curious refult of thefe experiments is the efftCl of 
time in increafing the friction between two furfaces. In 
fome cafes the friction reaches its maximum after the 
rubbing furfaces have remained in contaCt for one minute; 
and in other cafes five or fix days were neceflary before 
this effeCt was produced. The increafe of friCtion, which 
js generated by prolonging the time of contaCt, is fo great, 
that a body, weighing 1650 pounds was moved with a 
force of 64. pounds when firft laid upon the correfponding 
furface; but, after remaining in contaCt for the fpace of 
three feconds, iop pounds were neceflary to put it in mo¬ 
tion; and, when the time was prolonged to fix days, it 
could fcarcely be moved with a power of 622 pounds. 
One of the molt important treatifes on the fcience of 
motion is the Mechanics of the celebrated Euler, publiflied 
in 1736. It contains the whole theory of rectilineal and 
curvilineal motion in an infulated body, affeCted by any 
accelerating forces, either in vacuo or in a refitting me¬ 
dium. In the Mecaniquc Analytique of Lagrange, publiflied 
in 1788, all the mechanical problems are reduced to ge¬ 
neral formulae, which, being developed, furnifti us with 
the equations that are neceflary for the folution of each 
problem ; and the different principles which have been 
dilcovered for facilitating the folutions of mechanical 
queftions, are brought under one point of view', and their 
connection and dependence clearly pointed out. The 
Architedure Hydraulique, by M. Prony, publifhed in 1790, 
and the Mecanique Philofophique of the fame author, pub- 
lilhed in 1799, contains all the late improvements in me¬ 
chanics, and a complete view both of the theory and ap¬ 
plication of that fcience. The firft of thefe works is in¬ 
tended chiefly for the ufe of the engineer, though an 
extenfive acquaintance with the higher geometry is ne- 
ceffary for perilling it with advantage. His Mecanique 
Philofophique is a profound work, in which, without the 
aid of a Angle diagram, he gives all the formulae, and the 
various theorems and problems which belong to the fei- 
ences of mechanics and hydiodynamics. Every alternate 
page contains a methodical table of the refults obtained 
in the preceding page, the defeription of the fymbols, and 
the theorems, problems, and formulae, which may have 
been obtained. The Trails de Mecanique Elementaire, by 
M. Franceur, publiflied in 1802, in one volume oCtavo, is 
an excellent abridgment of the works of Prony, and is in¬ 
tended as an introduction to the Mecanique Philofophique 
of that author, to the Mecanique Analytique of Lagrange, 
\ N I C S. 
and to the Mecanique Celefte of Laplace. None of thefe 
works have been tranflated into Englifh ; but their place 
is w'ell fupplied by “ Principles of Mechanics,” by J. 
Wood, B. D. of Cambridge, 1799,” and “ A Treatifeon 
Mechanics, Theoretical, Practical, and Defcriptive, by 
Olinthus Gregory, A. M.” publiflied in 1806, and con¬ 
taining a complete view of the lateft improvements, both 
in theory and practice. 
THEORY OF MECHANICS. 
The theory of mechanics, next to that of geometry, i& 
juftly reckoned the molt certain ; for, next to that, it pro- 
pofes the feweft properties of matter as objects of con¬ 
templation. Generally fpeaking, this fcience propofes 
for consideration, time, force, or power, and the following 
properties of bodies : viz. extenfion, figure, impenetrability, 
majs, mobility, and inertia. The application of this fcience 
to the various practical purpofes in human life, leads to 
many other branches of enquiry; fuch as, the nature of 
machinery, the ad vantages and difadvantages of different 
materials, the effefts of friction, See. Each of which will 
be brought under difeuflion, after the feveral parts of the 
theory are laid down. 
Matter is a term by which we denote that fubftance of 
which every thing our fenfes perceive is imagined to be 
compofed. So far as relates to mechanics, its effence may 
be regarded as conlifting in extenfion, impenetrability, and 
inertnefs. 
Body is fuch a collected quantity of matter as is palpa¬ 
ble, or obvious to fome of the fenfes. We fay that a body 
is folid, when it is compofed of particles, or moleculas, fo 
adhering the one to the other, that they cannot be fepa- 
• rated without effort: fuch are metals, ftone, wood, See. 
The term fiuid we apply to fuch ftibftances as are com¬ 
pofed of particles adhering very flightly, and which, yield¬ 
ing to any fmall effort, are ealily moved among each 
other : fuch are water, air, Sc c. 
All bodies, it is manifeft, are extended ; and therefore 
are found exilting under figure, or ftiape, which is the 
boundary of extenfion. It appears alfo effential to mat¬ 
ter, that it prevents all other fubftances of the fame kind 
from occupying its place; and that it requires the exer¬ 
tion of fomething ab extra to remove it from the iituation, 
or to change its Itate. The former of thefe properties is 
called folidity or impenetrability ; the latter, inertnefs or iner¬ 
tia-, ot which we (hall have occafion to fay more. 
All bodies are divifible either in reality or in imagina¬ 
tion. By art they may be divided into parts of fur- 
priling minutenefs; as by the operations of grinding, 
hammering, wiredrawing, Sec. In fome chemical folu¬ 
tions this may be carried farther (till. And the confti- 
tuent particles of the folids and fluids of the myriads of 
animalcules which are dilcovered by the microfcope, mull 
be fmall beyond conception ; fo that we can fcarcely help 
imagining the capacity of divifibility to be without limit. 
What, however, are called mathematical demonltrations 
of the infinite divifibility of matter, fail in their objeft; 
they merely prove the infinite divifibility of extenfion, in 
a geometrical fenfe. It is probable that, with refpect to 
matter, the aflual divifion may admit of being carried to 
a different extent in different bodies; but the inquiry is 
of little, if any, confequence to the mechanician 
Space is commonly defined as, the order of things which 
co-exift: in this fenfe it is a mere abltrafit idea, arifing 
from our notion of the actual or pofiible fituation of things 
amongft themfelves. We rather choofe to call fpace an 
extenfion confidered as without bounds, immoveable but 
penetrable by matter: this may be regarded as abfolute 
Jpace. Relative fpace is that variable dimenfion or mea- 
lure of abfolute ipace which our fenfes define by its rela¬ 
tion to bodies within it. 
Place , or abfolute place, is that part of the infinite fpace 
which a body poflelfes. Relative place is the fpace a body 
occupies confidered with relation to other objects. 
Mobility is a property of body, by which it is capable 
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