MAC 
are either simple or compound. Tlie sim- 
ple machines are the mechanical powers, 
viz. the lever, the wheel and axis, the pul- 
ley, the inclined plane, the wedge and the 
screw. See Mechanics. 
These simple machines serve for different 
purposes, and it is the business of the skil- 
ful mechanician to select and combine tlieni 
in such a manner, as may be best adapfed 
to produce the effect of which he stands in 
need. Compound machines are formed 
from these simple ones. Tliese may be in- 
definitely varied, and they belong to all the 
branches of science. Descriptions of many 
of the most useful, and which serve to exhi- 
bit the principles of machinery, will be 
found in various parts of our work. See 
Engine, Hydraulics, Pneumatics, &c. 
&c. 
The modes of applying mechanical forces 
are almost as various as the machines that 
are constructed, and the purposes for which 
they are employed. In general the human 
strength is applied by means of levers, or 
winches, or by walking wheels, which slide 
beneath them as they attempt to ascend. 
The force of other animals i.s applied by a 
horizontal arm projecting from a vertical 
axis, to which they are harnessed. When 
motion is simply communicated to a sub- 
stance placed before the moving body, 
such materials are used as are capable of 
exerting a repulsive force ; but when the 
body to be moved is behind the moving 
power, and is pulled along with it, chains or 
ropes are sometimes more convenient. 
When the direction of motion communi- 
cated is also to be changed, levers or cranks 
may be employed, united by joints or 
hinges of various kinds. Sometimes a long 
series of connected rods is suspended by 
other rods or chains, so as to convey the ef- 
'fect of the force to a considerable distance ; 
in this case the motion is generally alter- 
nate, as when pumps are worked by means 
of a water-wheel at a distance from the 
shafts in w'hich the pumps are placed. For 
the communication of a rotatory motion. 
Dr. Hooke’s universal joint, formed by a 
cross, making the diameters of two semicir- 
cles, one of which is fixed at the end of each 
axis, is frequently used. The best mode of 
connecting a rotatory motion with an alter- 
nate one is, in all common cases, to employ 
a crank, acting on one end of a long rod 
which has a joint at the other. If the rota- 
tory motion of the crank be equable, the 
progressive motion of the rod wull be gra- 
dually accelerated and retarded, and for a 
VOL. IV. 
MAC 
considerable part of the revolution the 
force exerted will be nearly uniform. The 
force applied to a machine may, in general, 
be divided into two portions, the one em- 
ployed in opposing another force, so as to 
produce equilibrium only, the other in ge- 
nerating momentum. With respect to the 
first portion, a single crank has the incon- 
venience of changing continually the me- 
chanical advantage of the macliinery ; w'ith 
regard to the second, its motion in the se- 
cond quarter of its revolution is accele- 
rated, instead of being retarded, by the 
inertias which this portion of the force is in- 
tended to overcome, hence the motion is ir- 
regular. This difficulty may be remedied 
by employing cranks in pairs, one of which 
being fixed so as to make a right angle with 
the other, which is moreover the best posi- 
tion for two winches to be turned by two 
labourers ; since the point of the circle, in 
which a man can exert his greatest strength, 
is nearly at the distance of a right angle, or 
a little more, from the point at which his 
force is smallest. But of all the modes of 
communicating motion, the most exten- 
sively useful is employment of wheel-work, 
which is capable of varying its direction and 
its velocity without any limit. See Wheel- 
worh. 
Machine, electric. The electric ma- 
chine consists of three parts, the electric 
body, which is rubbed ; the rubber, which 
is a compounded conductor ; and the prime 
conductor, which is destined to receive 
and convey the electricity, in making expe- 
riments. The first electiical apparatus con- 
sisted of a tube of glass, or a stick of seal- 
ing-wax, rubbed by the hand. Glass globes 
whirled quickly on an axis, were substi- 
tuted as an improvement, and the rub- 
ber was still the hand ; but subsequently a 
round concave cushion. These w'ere suc- 
ceeded by glass cylinders, which are cheap, 
safe, and considerably powerful; but the 
present fashion determines in favour of flat 
glass plates, on account of the advantage 
of a large surface, nibbed by two or more 
pairs of cushions, and the equality of pres- 
sure which causes the supply of electricity 
to be steady and without undulation, as to 
its quantity. Machines of very great power 
(see “ Nicholson’s Journal,” quarto), have 
been made by M. Walckiers, consisting of 
an endless web or jack-tow el of silk, pass- 
ing between two pair of cylindrical rub- 
bers, faced with cat-skin; the electricity 
being communicated to a prime conductor, 
lying between the parallel pieces of the silk. 
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