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John Wallis (November 22, 1616 - October 28, 1703) was
an English mathematician who
is given partial credit for the development of modern calculus. He is also credited
with introducing the symbol ∞ for infinity.
John Wallis was born at Ashford, Kent. He was educated at Felstead school, and at the age of fifteen mastered arithmetic after becoming fascinated by
his brother's book on the topic. As it was intended that he should be a doctor, he was sent to Emmanuel College, Cambridge, while there he kept an
act on the doctrine of the circulation of the blood; that was said to have been the first occasion in Europe on which
this theory was publicly maintained in a disputation. His interests, however, centred on mathematics.
He was elected to a fellowship at Queens'
College, Cambridge, and subsequently took orders, but on the whole adhered to the Puritan party, to whom he rendered great assistance in deciphering the royalist despatches. He, however, joined the
moderate Presbyterians in signing the remonstrance against the execution of Charles I, by which he incurred the lasting hostility of the Independents. In spite of their
opposition he was appointed in 1649 to the Savilian chair of geometry at Oxford University, where he lived until his death on October 28, 1703. Besides his mathematical works he
wrote on theology, logic, and philosophy, and he was the first to devise a system for teaching deaf-mutes.
The analytic geometry expounded in La Géométrie by Descartes was
both difficult and obscure to many of his contemporaries. In 1655, Wallis published a treatise on conic sections in which they
were defined analytically. This work did something to make the method intelligible to all mathematicians: it is the earliest book
in which these curves are considered and defined as curves of the second degree.
The most important of Wallis's works was his Arithmetica Infinitorum, which was published in 1656. In this treatise
the methods of analysis of Descartes and Cavalieri were systematised and greatly extended, but their logical exposition is open to criticism. It is
prefaced by a short tract on conic sections. He begins by developing the standard notation for powers:
-
-
- x1 / n = n-th root of x,xp / q = q-th
root of xp,
Leaving the numerous algebraic applications of this discovery, he next proceeds to find, by the method of indivisibles, the
area enclosed between the curve y = xm, the axis of x, and any ordinate x =
h, and he proves that the ratio of this area to that of the parallelogram on the same base and of the same altitude is
1/(m + 1). He apparently assumed that the same result would be true also for the curve y =
axm, where a is any constant, and m any number positive or negative; but he only discusses
the case of the parabola in which m = 2, and that of the hyperbola in which m = −1. In the latter case,
his interpretation of the result is incorrect. He then shows that similar results might be written down for any curve of the
form
-
and hence that, if the ordinate y of a curve can be expanded in powers of x, its area can be determined:
thus he says that if the equation of the curve is y = x0 + x1 +
x2 + ..., its area would be x + x2/2 + x3/3 + ... He then applies
this to the quadrature of the curves y = (x − x2)0, y =
(x − x2)1, y = (x − x2)2,
etc., taken between the limits x = 0 and x = 1. He shows that the areas are respectively 1, 1/6, 1/30, 1/140,
etc. He next considers curves of the form y = x1/m and establishes the theorem that the area bounded
by this curve and the lines x = 0 and x = 1 is equal to the area of the rectangle on the same base and of the
same altitude as m : m + 1. This is equivalent to computing
-
He illustrates this by the parabola, in which case m = 2. He states, but does not prove, the corresponding result for
a curve of the form y = xp/q.
Wallis showed considerable ingenuity in reducing the equations of curves to the forms given above, but, as he was unacquainted
with the binomial theorem, he could not effect the quadrature of
the circle, whose equation is y = , since he was unable to expand this in powers of x. He laid down, however,
the principle of interpolation. Thus, as the ordinate of the circle y = is the geometrical mean between the ordinates of
the curves y = and y = , it might be suppose that, as an approximation, the area of the semicircle dx which is
might be taken as the geometrical mean between the values of
dx and dx
that is, 1 and ; this is equivalent to taking or 3.26... as the value of . But, Wallis argued, we have in fact a series 1, , ,
,... and therefore the term interpolated between 1 and ought to be chosen so as to obey the law of this series. This, by an
elaborate method, which I need not describe in detail, leads to a value for the interpolated term which is equivalent to taking [
<-- This needs a lot of work! ]
In this work also the formation and properties of continued
fractions are discussed, the subject having been brought into prominence by Brouncker's use of these
fractions.
A few years later, in 1659, Wallis published a tract containing the solution of the
problems on the cycloid which had been proposed by Blaise Pascal. In this
he incidentally explained how the principles laid down in his Arithmetica Infinitorum could be used for the
rectification of algebraic curves; and gave a solution of the problem to rectify (i.e. find the length of) the semi-cubical
parabola x3 = ay2, which had been discovered in 1657 by his pupil William
Neil. Since all attempts to rectify the ellipse and hyperbola had been (necessarily) ineffectual, it had been supposed that
no curves could be rectified, as indeed Descartes had definitely asserted to be the case. The logarithmic spiral had been rectified by Torricelli, and was the first curved line (other than the circle) whose length was determined, but the extension
by Neil and Wallis to an algebraic curve was novel. The cycloid was the next curve rectified; this was done by Wren in 1658.
Early in 1658 a similar discovery, independent of that of Neil, was made by van Heuraët, and this was published by van
Schooten in his edition of Descartes's Geometria in 1659. Van Heuraët's method is as follows. He supposes the curve to be
referred to rectangular axes; if this be so, and if (x, y) be the coordinates of any point on it, and
n be the length of the normal, and if another point whose coordinates are (x, ) be taken such that : h = n :
y, where h is a constant; then, if ds be the element of the length of the required curve, we have by similar triangles ds : dx =
n : y. Therefore h ds = dx. Hence, if the area of the locus of the point (x,) can be found, the first curve can be rectified. In
this way van Heuraët effected the rectification of the curve y³ = ax² but added that the rectification of the parabola y² = ax is
impossible since it requires the quadrature of the hyperbola. The solutions given by Neil and Wallis are somewhat similar to that
given by van Heuraët, though no general rule is enunciated, and the analysis is clumsy. A third method was suggested by Fermat in
1660, but it is inelegant and laborious.
The theory of the collision of bodies was propounded by the Royal Society in 1668 for the consideration of mathematicians.
Wallis, Wren, and Huygens sent correct and similar solutions, all depending on what is now called the conservation of momentum;
but, while Wren and Huygens confined their theory to perfectly elastic bodies, Wallis considered also imperfectly elastic bodies.
This was followed in 1669 by a work on statics (centres of gravity), and in 1670 by one on dynamics: these provide a convenient
synopsis of what was then known on the subject.
In 1685 Wallis published Algebra, preceded by a historical account of the development of the subject, which contains
a great deal of valuable information. The second edition, issued in 1693 and forming the second volume of his Opera, was
considerably enlarged. This algebra is noteworthy as containing the first systematic use of formulae. A given magnitude is here
represented by the numerical ratio which it bears to the unit of the same kind of magnitude: thus, when Wallis wants to compare
two lengths he regards each as containing so many units of length. This perhaps will be made clearer by noting that the relation
between the space described in any time by a particle moving with a uniform velocity is denoted by Wallis by the formula
s = vt, where s is the number representing the ratio of the space described to the unit of length;
while the previous writers would have denoted the same relation by stating what is equivalent to the proposition . It is curious
to note that Wallis rejected as absurd the now usual idea of a negative number as being less than nothing, but accepted the view
that it is something greater than infinity.
Reference
The initial text of this article was taken from the public domain
resource A Short Account of the History of Mathematics by W. W. Rouse Ball (4th Edition, 1908).
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