Environmental Engineering Reference
In-Depth Information
sailships) or mostly from animate power (medieval Eu-
rope, China until two generations ago) had little physical
security and only a highly privileged affluence to offer.
They could increase available power only through the
mass concentration of labor and by deploying ingenious
devices designed to overcome some limitations of human
and animal bodies.
Despite the abundance of free slave labor, no ancient
civilization took effective steps toward true mass manu-
facturing. What Christ (1984), writing about the Ro-
mans, calls atomization of production remained the
norm. Ancient massed labor thus left its most impressive
legacy in buildings of stunning dimensions and un-
surpassed esthetic appeal. But their construction could
not rely solely on massed animate power because its di-
rect applications have obvious logistical limitations. Only
a limited number of people can fit around the perimeter
of a heavy object in order to grasp and lift it or simply
push it. Similarly, only a limited number of animals can
be harnessed together as a coherent team in order to per-
form a demanding task.
Without mechanical aids that help to overcome the
effects of gravity and friction, individual human capacities
to lift and carry loads are limited to modest burdens. Ne-
pali Sherpas, the world's ablest load carriers, shoulder
between 30 kg and 35 kg while walking to a base camp
and less than 20 kg above it. Roman saccarii lifted and
carried 28-kg sacks over short distances (Utley 1925).
In the light version of the traditional Chinese sedan
chair, two porters conveyed a customer, a load of 25-35
kg per porter (luxurious litters had as many as eight
carriers). More efficient use of human power required
devices that confer a significant mechanical advantage,
usually by deploying a lesser force over a longer distance
(Bloomfield 1997).
The Greeks and Romans were masters of five simple
machines capable of such action (first enumerated by
Philo in the third century B . C . E .): wheel and axle, lever,
system of pulleys, wedge, and endless screw. The three
simplest designs, levers, inclined planes, and pulleys,
were used by all the Old World's high cultures (Lacey
1935; Needham 1965; Burstall 1968). Levers are rigid,
usually slender objects whose pivoting around a fulcrum
conveys mechanical advantage equal to the quotient
of the length of their effort and resistance arms (mea-
sured from the pivot point). Archimedes' famous boast
(quoted by Pappus of Alexandria more than 500 years
after the physicist's death)—DOS MOI POY STA KAI
KINW THN GHN (Give me a place to stand, and I will
move the Earth)—testifies to the ancient understanding
of the lever's efficacy.
Three classes of levers are distinguished by the point at
which the force is applied in relation to the object and
the fulcrum (fig. 7.2). In levers of the first class, the ap-
plied force and load are at opposite ends with the ful-
crum between them, and the force acts in a direction
opposite to that of the displaced load. Familiar applica-
tions are seesaws, crowbars, scissors, and pliers (the latter
are double levers). In levers of the second class, the ful-
crum is at one end, and hence the force moves in the
same direction as the load, the design shared by wheel-
barrows and nutcrackers (a double lever). Levers of the
third class do not offer any mechanical advantage because
the applied force is greater than the force of the load,
but the load moves faster because the force acts over a
shorter distance than the displaced object. This is the
case when one eats: the elbow is the fulcrum, force is
applied to lift the forearm, and the food carried to mouth
moves further (hence faster) than the actuated muscles.
Catapults, hoes, and scythes are in this category, as
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