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HOOKE, ROBERT (b. Freshwater, Isle of Wight,
England, 18 July 1635; d. London, England, 3 March
1702), physics.
bodies,” a dissolution carried out by a salt in
the air and accompanied by intense heat, which we
call fire. He identified the salt with that in saltpeter,
so that combustion, which usually requires air, can
take place in a vacuum when saltpeter is present.
Instead of forestalling Lavoisier, who saw combustion
as a chemical combination, Hooke's theory repeated
the accepted view that fire is an instrument
of analysis that dissolves and separates bodies. There
is no occasion to scorn the insight obtained. Along
with the other three men, Hooke was impressed by
the analogy of combustion and respiration. He carried
out experiments before the Royal Society demonstrating
that a continued supply of fresh air is as
essential to life as it is to fire. By opening the thorax
of a dog, destroying the motion of its lungs, and then
employing a bellows to maintain a stream of air
which passed out of the lungs through holes that he
pricked, he demonstrated conclusively that the function
of respiration is to bring a constant supply of
fresh air into the lungs—not to cool and not to pump,
as prevailing theories held, but solely to supply fresh
air. With Mayow and the others, Hooke identified
the nitrous salt or spirit in the air as the ingredient
essential to life. Although the conceptual expression
of this insight differed radically from Lavoisier's, its
significance cannot be denied; and Hooke's role in
it cannot be ignored.
During the years following Micrographia, Hooke
found time to conduct demonstrations before the
Royal Society and to deliver the Cutlerian lectures
despite his activities as surveyor. Part of this work
extended earlier investigations—for example, both
those on combustion and those on optics—but he also
broke new ground. During the 1670's he published
a series of six brief works which were gathered together
in a single volume, the Lectiones Cutlerianae,
in 1679. The Cutlerian lectures contain at least two
important scientific discoveries. One of these was the
law of elasticity to which Hooke's name is still attached—“ut
tensio sic vis.” That is, the stress is proportional
to the strain. Hooke's law, which was implicit
in much of mechanics before him, was not a
major discovery. Nevertheless, no one before him had
stated it explicitly. Moreover, Hooke perceived intuitively
that a vibrating spring is dynamically equivalent
to a pendulum; and in the lecture that announced
Hooke's law, he undertook one of the early
analyses of simple harmonic motion. He based it on
what he referred to elsewhere as “the General Rule
of Mechanicks”:
Which is, that the proportion of the strength or power
of moving any Body is always in a duplicate proportion
of the Velocity it receives from it ...
That is, the “quantity of strength” employed in moving
a body is proportional to the square of the velocity
it receives. In many ways the passage was typical
of Hooke. The demonstration foundered on its inherent
confusions—although it is necessary to add that
in the seventeenth century only giants such as
Huygens, Leibniz, and Newton succeeded in dispelling
similar confusion in dynamics. In Hooke's case,
the clarity of his mechanical conceptions and the
power of his analysis were not able to match his
intuitive insight.
In another Cutlerian lecture, Hooke announced the
three basic suppositions on which he intended to
construct a system of the world corresponding to the
rules of mechanics:
First, That all Coelestial Bodies whatsoever, have an
attraction or gravitating power towards their own Centers,
whereby they attract not only their own parts, and
keep them from flying from them, as we may observe
the earth to do, but that they do also attract all the
other Coelestial Bodies that are within the sphere of
their activity.... The second supposition is this, That
all bodies whatsoever that are put into a direct and
simple motion, will so continue to move forward in a
streight line, till they are by some other effectual powers
deflected and bent into a Motion, describing a Circle,
Ellipsis, or some other more compounded Curve Line.
The third supposition is, That these attractive powers
are so much the more powerful in operating, by how
much the nearer the body wrought upon is to their own
Centers.
This remarkable statement, together with others that
date back to 1664, has become a major piece of
evidence in the case for Hooke's claim on the law
of universal gravitation. It contains two elements. On
the one hand, it proposes a concept of apparently
universal attraction. It is only apparently universal,
however. An idea of gravitational attractions specific
to each planet, forces by which they maintain the
unity of their systems, was widely held in the seventeenth
century. Although Hooke took a major step
toward generalizing this idea, his understanding of
gravitation never eliminated the notion of a force
specific to certain kinds of matter and hence never
reached the level of universal gravitation. Gravity,
he said elsewhere, is “such a Power, as causes Bodies
of a similar or homogeneous nature to be moved one
towards the other, till they are united....” Planets
are of the same nature as the sun and hence are
attracted to it. Comets are not related, and they are
repelled.
Hooke himself never laid claim to the concept of
universal gravitation. Rather, he asserted his propriety
over the second element in the passage above,