MECH 
In academies and private families, it may be erefted in 
the plate allotted for amufement, where it will furnilh en¬ 
tertainment for many a vacant hour. When it has loft 
its novelty, the Ihaft may from time to time be taken down, 
and a living may be fufpended in its place. Edgewortk's 
PraElical Education, vol. ii. chap. 17. 
HYDRO-MECHANICS. 
Hydro-Mechanics, or the fcience which treats of the 
nature and properties of fluids, has been ufually divided 
into the following branches ; Hydrojiatics, which comprife* 
the doftrine of the equilibrium of non-elaftic fluids, as 
water, mercury, See. Hydraulics, which relates to the mo¬ 
tion of thofe fluids; and Pneumatics, which treats on the 
properties of the different kinds of airs. But thefe are 
now all included under the general term Hydrostatics. 
A fluid is a body whole parts are put in motion one 
amongft another by any force imprefled ; and which, when 
the imprefled force is removed, reftores itfelf to its former 
ftate. By its former fate, we do not mean that every ar¬ 
ticle is re-inftated in its former fituation, but that the 
whole body recovers its former dimenlions and figure. 
Fluids may be divided into elallic and non-elaftic. An 
elaflic fluid is one, whofe dimenlions are diminiihed by in- 
creafing the preffure, and increafed by diminilhing the 
preffure, upon it; of which defeription are the different 
kinds of airs. A ncn-elaflic fluid is one, whofe dimen¬ 
lions are not, at lead as to lenfe, affefted by any increafe 
of preffure, as water, mercury, See. As many bodies, by 
cold, from a ftafe of fluidity become folids, fuch bodies 
are fluids fo long as their furfaces, when difturbed, will 
reftore themfelves to their former polition. The definition 
i'uppofes a partial preffure ; for, if the fluid be incompref- 
fible, under an equal and general preffure none of the 
parts will be moved. Different fluids have different de¬ 
grees of fluidity, according to the facility with which the 
particles are moved one amongft another. Water and mer¬ 
cury are the moft perfeft non-elaftic fluids. Many fluids 
have a very lenlible degree of tenacity, and are therefore 
called imperfett fluids. Belides the fluids which come 
under this definition, there are others, as the elettric and 
magnetic fluid, light, and fire, according to the opinion 
of lome, Sec. but thele are not the objeCts of Hydroftatics. 
The fpecific gravity of any folid or fluid body is its weight, 
compared with the weight of another body whofe magni¬ 
tude is the fame. The denfity of a body is as the quan¬ 
tity of matter contained in a given fpace, and therefore in 
proportion to its weight, when the magnitude is the fame. 
Hence, the fpccific gravity of a body is in proportion to its 
denfity. A cubic inch of pure mercury is about fourteen 
times heavier than a cubic inch of water; the fpecific gra¬ 
vity and denfity of the former are therefore about fourteen 
times that' of the latter. As the weight of a body is in 
proportion to its quantity of matter, the fpecific gravity 
and denfity of a body are alfo in proportion to its quan¬ 
tity of matter, when the magnitude is the fame. If the 
magnitude of a body be increafed, the denfity remaining 
the fame, the quantity of matter, and confequently the 
weight, will be increafed in the fame proportion. Hence, 
if the magnitude M, and denfity D, both vary, the quan¬ 
tity of matter, and confequently the weight of the body, 
will vary as M X D, by the compofition of ratios. 
We know fo little of the nature and conftitution of 
fluids, that the application of the general principles of 
motion to the inveftigation of the effefts produced by their 
action, is fuDjeft to great uncertainty. That the differ¬ 
ent kinds of airs are conttituted of particles endued with 
repulfive powers, is manifeft from their expanfion, when 
the force with which they are comprelfed is removed. 
The particles being kept at a diftance by their mutual re- 
pulfion, it is eafy to conceive that they may move very 
freely amongft each other, and that this motion may take 
place in all directions, each particle exerting its repulfive 
qpovver equally on all fides. Thus far we are acquainted 
with the conftitution of thefe fluids; but with what de* 
Vol. XIV. No. 1006. 
A-NIC8. 701 
gree of facility the particles move, and how this may be 
affected under different degrees of compreflion, are cir- 
cumftances of which we are totally ignorant. With re- 
lpeft to the nature of thofe fluids which are denominated 
liquids, we are ftill lefs acquainted. If we fuppofe their 
particles to be in contaft, it is very difficult to conceive 
how they can move amongft each other with fuch extreme 
facility, and produce effefts in directions oppofite to the 
imprefled force, without any fenfible iofs of motion. To 
account for this, the particles have, by fome, been fup- 
pofed to be perfectly fmooth and fpherical. If we were 
to admit this fuppofition, it would yet remain to be Ihown 
how it would lblve all the phaenomena, for it is by no 
means felf-evident that it would. If the particles be not 
in contaCt, they muft be kept at a diftance by fome repul¬ 
five power. But it is manifeft that thefe particles attract 
each other, from the drops of all perfeCt fluids endeavour¬ 
ing to form themfelves into fpheres. We muft therefore 
admit in this cafe both powers, and that where one power 
ends the other begins, agreeably to fir I. Newton’s idea of 
what takes place, not only in refpeCt to the conftituent par¬ 
ticles of bodies, but to the bodies themfelves. The in- 
comprefllbility of liquids (for we know no decifive expe¬ 
riments which have proved them to be compreflible) feems 
moft to favour the former fuppofition, uniefs we admit, in 
the latter hypothefis, that the repulfive force is greater 
than any human power which can be applied. The ex¬ 
panfion of water by heat, and the poffibility of aClfiaily 
converting it into two permanent elaftic fluids, according 
to fome late experiments, feem to prove that a repulfive 
power exifls between the particles, for it is hard to con¬ 
ceive that heat can actually create any fuch new powers, 
or that it can of itfelf produce any fuch effeCts. 
A fluid being compofed of an indefinite number of cor- 
pufcles, we muftconfider its action, either as the joint ac¬ 
tion of all the corpufcles, elfimated as fo many diitinft 
bodies, or we muft confider the aCtion of the whole as a 
mafs, or as one body. In the former cafe, the motion of 
the particles being Subject to no regularity, or at leaft to 
none that can be difeovered by any experiments, it is irn- 
poffible, from this confidcration, to compute the effeft; 
for no calculation of effefts can be applied, when pro¬ 
duced by caufes which are fubjeft to no law. And, in the 
latter cafe, the effeCts of the aftion of one fluid upon another 
differ fo much, in many refpefts, from what would be its 
aClion as a folid body, that a computation of its effefts can 
by no means be deduced from the fame principles. In 
Mechanics, no equilibrium can take place between two 
bodies of different weights, uniefs the lighter afts at l'ome 
mechanical advantage; but, in Hydroftatics, a very fmall 
weight of fluid may, without its afting at any mechanical 
advantage whatever, be made to balance a weight of any 
magnitude. In Mechanics, bodies aft only in the direc¬ 
tion of gravity ; but the property which fluids have of ail¬ 
ing equally in all directions, produces effefts of fuch an 
extraordinary nature as to furpafs the power of inveftiga¬ 
tion. The indefinitely-fmall corpufcles of which a fluid 
is compofed, probably poflefs the fame powers, and would 
be fubjeft to the fame laws of motion, as bodies of finite 
magnitudes, could any two of them aft upon each other by 
contaft; but this is a circumttance which certainly never 
tikes place in any of the aerial fluids, and probably not 
in any liquids. Under the circumftances, therefore, of 
an indefinite number of bodies afting upon each other by 
repulfive powers, orbyabfolute contaft, under the uncer¬ 
tainty of the friftion which may take place, and of what 
variation of effefts may be produced by different degree* 
of compreflion, the concluiions deduced from any theory 
mult be fubjeft to confiderable errors, except from that 
which is founded upon fuch experiments, as include in 
them the confequences of thofe principles which are lia¬ 
ble to any degree of uncertainty. 
Sir I. Newton feems to have been w/ell aware of all thefe 
difficulties, and therefore, in his Principia, he has de¬ 
duced his laws of refiftance, and the principles upon which 
8 Q the 
