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% David R. Wilkins
% School of Mathematics, Trinity College, Dublin 2, Ireland
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% Trinity College, 1999.

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\centerline{\Largebf ADDITIONAL APPLICATIONS OF THE}

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\centerline{\Largebf THEORY OF ALGEBRAIC QUATERNIONS}

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\centerline{\Largebf By}

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\centerline{\Largebf William Rowan Hamilton}

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\centerline{\largerm (Proceedings of the Royal Irish Academy,
   3 (1847), Appendix, pp.\ li--lx.)}

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\centerline{\largerm Edited by David R. Wilkins}

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\centerline{\largerm 1999}

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\centerline{{\largeit Additional Applications of the Theory of
Algebraic Quaternions.}}

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\centerline{{\largeit By\/}
{\largerm Sir} {\largesc William R. Hamilton.}}

\bigskip

\centerline{Communicated December~8, 1845.}

\bigskip

\centerline{[{\it Proceedings of the Royal Irish Academy},
vol.~3 (1847), Appendix, pp.\ li--lx.]}

\bigskip

The following is the communication made by Sir William R.
Hamilton on some additional applications of his theory of
algebraic quaternions.

It had been shown to the Academy, at one of their
meetings\footnote*{See Appendix No III., page xxxvii.
[Proceedings of the Royal Irish Academy, volume 3.]}
in the last summer, that the differential equations of motion of
a system of bodies attracting each other according to Newton's
law, might be expressed by the formula:
$${{\rm d}^2 \alpha \over {\rm d} t^2}
   =  \Sigma {m + \Delta m
         \over - \Delta \alpha \surd ( - \Delta \alpha^2 )}
   \eqno (1)$$
in which $\alpha$ is the {\it vector\/} of any one such body, or
of any elementary portion of a body, regarded as a material
point, and referred to an arbitrary origin; $m$ the constant
called its mass; $\alpha + \Delta \alpha$, and $m + \Delta m$,
the vector and the mass of another point or body of the system;
$\Sigma$ the sign of summation, relatively to all such other
bodies, or attracting elements of the system; and ${\rm d}$ the
characteristic of differentiation, performed with respect to $t$
the time.

If we confine ourselves for a moment to the consideration of
{\it two\/} bodies, $m$ and $m'$, and suppose $r$ to be the
positive number denoting the variable distance between them, so
that $r$ is the {\it length\/} of the vector $\alpha' - \alpha$,
and, therefore, by the principles of this calculus,
$$r = \surd \{ - ( \alpha' - \alpha )^2 \};
   \eqno (2)$$
we shall have, by the formula~(1), the two equations,
$${{\rm d}^2 \alpha \over {\rm d} t^2}
   = {m' r^{-1} \over \alpha - \alpha'},\quad
  {{\rm d}^2 \alpha' \over {\rm d} t^2}
   = {m' r^{-1} \over \alpha' - \alpha};$$
which may also be thus written,
$$m {{\rm d}^2 \alpha \over {\rm d} t^2}
   =  m m' r^{-3} (\alpha' - \alpha),\quad
  m' {{\rm d}^2 \alpha' \over {\rm d} t^2}
   =  m m' r^{-3} (\alpha - \alpha'),$$
and which give
$$0 = m  \left(
            \delta \alpha \, {{\rm d}^2 \alpha \over {\rm d} t^2}
          + {{\rm d}^2 \alpha \over {\rm d} t^2} \, \delta \alpha
         \right)
    + m' \left(
            \delta \alpha' \, {{\rm d}^2 \alpha' \over {\rm d} t^2}
          + {{\rm d}^2 \alpha' \over {\rm d} t^2} \, \delta \alpha'
         \right)
    = 2 \delta {m m' \over r},$$
$\delta \alpha$, $\delta \alpha'$ being any arbitrary
infinitesimal variations of the vectors $\alpha$,~$\alpha'$, and
$\delta r$ being the corresponding variation of $r$; because
$$\eqalign{
         \delta \alpha \, (\alpha' - \alpha)
       + (\alpha' - \alpha) \, \delta \alpha
       + \delta \alpha' \, (\alpha - \alpha')
       + (\alpha - \alpha') \, \delta \alpha'
      \hskip -12em \cr
   &=  - (\delta \alpha' - \delta \alpha) (\alpha' - \alpha)
       - (\alpha' - \alpha) (\delta \alpha' - \delta \alpha) \cr
   &= - \delta \mathbin{.} (\alpha' - \alpha)^2
    = \delta \mathbin{.} r^2 = 2 r \, \delta r
    = - 2 r^3 \delta \mathbin{.} r^{-1}.\cr}$$

And by extending this reasoning to any system of bodies, we
deduce from the equation~(1) this other formula, by which it may
be replaced:
$${\textstyle {1 \over 2}} \Sigma \mathbin{.} m
         \left(
            \delta \alpha \, {{\rm d}^2 \alpha \over {\rm d} t^2}
          + {{\rm d}^2 \alpha \over {\rm d} t^2} \, \delta \alpha
         \right)
       + \delta \Sigma {m m' \over r}
   =  0.
   \eqno (3)$$

Although it is believed that this result~(3), if regarded merely
{\it as a symbolic form}, is new, as well as the method by which
it has been here obtained; yet if we transform it by the
introduction of rectangular coordinates, $x$,~$y$,~$z$, making
for this purpose
$$\alpha = ix + jy + kz,\quad
  \alpha' = ix' + jy' + kz',
   \eqno (4)$$
and eliminating the squares and products of the three imaginary
units, $i$,~$j$,~$k$, by the nine fundamental relations which
were communicated to the Academy in 1843, namely,
$$\left. \eqalign{
  i^2 &= j^2 = k^2 = -1;\cr
  ij &= k,\quad
  jk = i,\quad
  ki = j;\cr
  ji &= -k;\quad
  kj = -i,\quad
  ik = -j;\cr}
   \right\}
   \eqno (5)$$
we are conducted, from the equation~(3), to a well-known
equation, of Lagrange, which may be written thus:
$$\Sigma \mathbin{.} m
      \left(
         {{\rm d}^2 x \over {\rm d} t^2} \, \delta x
       + {{\rm d}^2 y \over {\rm d} t^2} \, \delta y
       + {{\rm d}^2 z \over {\rm d} t^2} \, \delta z
      \right)
   =  \delta \Sigma \mathbin{.} {m m' \over r};$$
where $r$, by (2), (4), (5), is equal to the known expression,
$$r = \surd \{ (x' - x)^2 + (y' - y)^2 + (z' - z)^2 \}.$$

If the law of attraction were supposed different from that of the
inverse square, a different function of $r$, instead of $r^{-1}$,
should be multiplied by the product of the two masses.

But further, it is not difficult so to operate on the
formula~(3), as to deduce from it another equation which shall be
equivalent to the forms that were proposed by the present author,
in his papers ``On a General Method in Dynamics'' (published in
the Philosophical Transactions),\footnote*{1834, Part~II.  1835,
Part~I.}
as being, at least theoretically, forms for the {\it integrals\/}
of the differential equations of motion of any system of
attracting bodies.  For if we observe, that by the principles of
the calculus of variations, combined with those of the method of
vectors, we have the identity,
$$\delta \alpha \, {\rm d}^2 \alpha
       + {\rm d}^2 \alpha \, \delta \alpha
   =  {\rm d} ( \delta \alpha \, {\rm d} \alpha
       + {\rm d} \alpha \, \delta \alpha )
       - \delta ( {\rm d} \alpha^2 );$$
and if we write
$$v = \sqrt{\vphantom{\biggl\{}}
         \left\{
          - \left( {{\rm d} \alpha \over {\rm d} t} \right)^2
         \right\},
   \eqno (6)$$
denoting by $v$ the magnitude or degree (but not the direction)
of the velocity of the body of which the vector is $\alpha$; we
may transform the equation~(3) into the following:
$${{\rm d} \over {\rm d} t} \Sigma \mathbin{.} {m \over 2}
         \left(
            \delta \alpha \, {{\rm d} \alpha \over {\rm d} t}
          + {{\rm d} \alpha \over {\rm d} t} \, \delta \alpha
         \right)
       + \delta
         \left(
            \Sigma {mv^2 \over 2}
          + \Sigma {m m' \over r}
         \right)
   =  0;
   \eqno (7)$$
which, when operated upon by the characteristic
$\displaystyle \int_0^t {\rm d} t$,
that is, when integrated once with respect to the time from $0$
to $t$, becomes
$$\Sigma \mathbin{.} {m \over 2}
         \left(
            \delta \alpha \, {{\rm d} \alpha \over {\rm d} t}
          - \delta \alpha_0 \, {{\rm d} \alpha_0 \over {\rm d} t}
          + {{\rm d} \alpha \over {\rm d} t} \, \delta \alpha
          - {{\rm d} \alpha_0 \over {\rm d} t} \, \delta \alpha_0
         \right)
       + \delta {\sc f}
   =  0,
   \eqno (8)$$
if we make for abridgment
$${\sc f} = \int_0^t {\rm d} t
         \left(
            \Sigma {mv^2 \over 2}
          + \Sigma {m m' \over r}
         \right),
   \eqno (9)$$
and denote by $\delta \alpha_0$ and
$\displaystyle {{\rm d} \alpha_0 \over {\rm d} t}$
the values which the variation of $\alpha$, and the differential
coefficient of that vector taken with respect to $t$, are
supposed to have at the origin of time.  The definite integral
denoted here by the letter~${\sc f}$ is the same with that which
was denoted by the letter~${\sc s}$ in the Essays already
referred to, and which was called, in one of those Essays, the
{\it Principal Function\/} of the motion of a system of bodies,
and if we now regard it as a function of the time~$t$, and of all
the final and initial vectors
$\alpha, \alpha' \, \ldots, \alpha_0, \alpha_0' \, \ldots$
of the various bodies of the system, and suppose (as we say) that
its variation, taken with respect to all those vectors, is
determined by an equation of the form,
$$0 = 2 \, \delta {\sc f}
       + \Sigma (
            \sigma \, \delta \alpha
          - \sigma_0 \, \delta \alpha_0
          + \delta \alpha \mathbin{.} \sigma
          - \delta \alpha_0 \mathbin{.} \sigma_0 ),
   \eqno (10)$$
in which $\sigma$,~$\sigma_0$ are vectors, we are conducted, by
comparison of the coefficients of the arbitrary variations of
vectors, in the equations (8) and (10), to the two following
systems of formul{\ae}:
$$m {{\rm d} \alpha \over {\rm d} t} = \sigma,\quad
  m' {{\rm d} \alpha' \over {\rm d} t} = \sigma',\quad\ldots;
   \eqno (11)$$
$$m {{\rm d} \alpha_0 \over {\rm d} t} = \sigma_0,\quad
  m' {{\rm d} \alpha_0' \over {\rm d} t} = \sigma_0',\quad\ldots;
   \eqno (12)$$
of which the former may be regarded as intermediate, and the
latter as final integrals of the differential equations of
motion.  The determination of the (vector) coefficient~$\sigma$,
from the variation of the (scalar) function~${\sc f}$, is an
operation of the same kind as the known operation of taking a
partial differential coefficient, and may, in these new
calculations, be called by the same name; but in order to be
fully understood, it requires some new considerations, of which
the account must be postponed to another occasion.

Consider a system of {\it three\/} attracting masses
$m$,~$m'$,~$m''$, with their corresponding vectors,
$\alpha$,~$\alpha'$,~$\alpha''$;
and make for abridgment $\alpha' - \alpha = \beta$, and
$\alpha'' - \alpha = \gamma$; we shall have, by (1), for the
differential equations of motion of these three masses, referred
to an arbitrary origin of vectors, the following:
$$\left. \eqalign{
{{\rm d}^2 \alpha \over {\rm d} t^2}
   &=    {m'  \over - \beta  \surd ( - \beta^2  )}
       + {m'' \over - \gamma \surd ( - \gamma^2 )}; \cr
{{\rm d}^2 (\alpha + \beta)  \over {\rm d} t^2}
   &=    {m   \over   \beta  \surd ( - \beta^2  )}
       + {m'' \over (\beta - \gamma) \surd \{ - (\beta - \gamma)^2 \}}; \cr
{{\rm d}^2 (\alpha + \gamma) \over {\rm d} t^2}
   &=    {m   \over   \gamma \surd ( - \gamma^2 )}
       + {m'  \over (\gamma - \beta) \surd \{ - (\gamma - \beta)^2 \}}; \cr}
   \right\}
   \eqno (13)$$
which give, for the internal or relative motions of $m'$ and
$m''$ about $m$, the equations:
$$\left. \eqalign{
{{\rm d}^2 \beta  \over {\rm d} t^2}
   &= {m + m'  \over \beta  \surd ( - \beta^2  )}
       + m''
         \left\{
            {(\beta - \gamma)^{-1}
               \over \surd \{ - (\beta - \gamma)^2 \}}
          + {\gamma^{-1} \over \surd ( - \gamma^2 )}
         \right\};\cr
{{\rm d}^2 \gamma \over {\rm d} t^2}
   &= {m + m'' \over \gamma \surd ( - \gamma^2 )}
       + m'
         \left\{
            {(\gamma - \beta)^{-1}
               \over \surd \{ - (\gamma - \beta)^2 \}}
          + {\beta^{-1}  \over \surd ( - \beta^2 )}
         \right\}.\cr}
   \right\}
   \eqno (14)$$

If we suppress the terms multiplied by $m''$ in the first of
these equations~(14), or the terms multiplied by $m'$ in the
second of those equations, we get the differential equation of
motion of a binary system, under a form, from which it was shewn
to the Academy last summer, that the laws of Kepler can be
deduced.  If we take account of the terms thus suppressed, we
have, at least in theory, the means of obtaining the
perturbations.

Let $m$ be the earth, $m'$ the moon, $m''$ the sun; then $\beta$
and $\gamma$ will be the geocentric vectors of the moon and sun;
and the laws of the disturbed motion of our satellite will be
contained in the two equations (14), but especially in the first
of these equations.  By the principles of the present calculus we
have the developments,
$$(\gamma - \beta)^{-1}
   =  \gamma^{-1}
       + \gamma^{-1} \beta \gamma^{-1}
       + \gamma^{-1} \beta \gamma^{-1} \beta \gamma^{-1}
       + \ldots,
   \eqno (15)$$
and
$${\surd ( - \gamma^2 ) \over \surd \{ - (\gamma - \beta)^2 \}}
   =  \left\{
         1
       - {\beta \gamma + \gamma \beta \over \gamma^2}
       + {\beta^2 \over \gamma^2}
      \right\}^{-{1 \over 2}}
   =  1 + {\beta \gamma + \gamma \beta \over 2 \gamma^2}
       + \ldots;
   \eqno (16)$$
if then we reject the terms of the same order as
$m'' \beta^2 \gamma^{-4}$, that is, terms depending on the
inverse fourth power of the distance of the sun from the earth,
the disturbing part of the expression for the second differential
coefficient, taken with respect to the time, of the moon's
geocentric vector, will reduce itself in this notation to the
following:
$${- m'' \over \surd ( - \gamma^2 )}
      \left(
         {(\gamma - \beta)^{-1} \surd ( - \gamma^2 )
            \over \surd \{ - (\gamma - \beta)^2 \}}
       - \gamma^{-1}
      \right)
   =  {\textstyle {1 \over 2}} m'' (-\gamma^2)^{-{3 \over 2}}
         (\beta + 3 \gamma^{-1} \beta \gamma).
   \eqno (17)$$

This symbolic result admits of a simple geometrical
interpretation.  The symbol $\gamma^{-1} \beta \gamma$ denotes a
vector in the plane of the two vectors~$\beta$ and $\gamma$,
which has the same length as $\beta$, and is inclined at the same
angle to $\gamma$, but at the other side of that line; so that
$\gamma$ bisects the angle between $\beta$ and
$\gamma^{-1} \beta \gamma$.  If then we conceive a fictitious
moon among the stars, so situated that either the sun, or a point
opposite to the sun, as seen from the earth, bisects the arc of a
great circle on the celestial sphere, between the fictitious and
the actual moon (the bodies being here treated as points); and if
we decompose the sun's disturbing force on the moon into two
others, directed respectively towards the extremities of that
celestial arc which is in this manner bisected: one component
force, resulting from this decomposition, will be constantly
ablatitious, tending directly to increase the distance of the
moon from the earth, and bearing to the attractive force in the
moon's undisturbed relative orbit, a ratio compounded of the
direct ratio of half the mass of the sun to the sum of the masses
of the earth and moon, and of the inverse ratio of the cubes of
the distances of the sun and moon from the earth; and {\it the
other component force, directed towards the fictitious moon},
will be {\it exactly the triple of the ablatitious force\/} thus
determined; provided that we still neglect all terms depending on
the inverse fourth power of the sun's distance, as we have done
in deducing the equation~(17), of which the theorem here
enunciated is an interpretation.  A similar result, of course,
hold good, for every satellite disturbed by the central body of a
system.  The theorem admits of being proved by considerations
more elementary, but was suggested to the author by the analysis
above described; which may be extended, by continuing the
developments (15), (16), to the case of one planet disturbed by
another, and to a more accurate theory of a satellite.

Without entering into any farther account at present of the
attempts which he has made to apply the processes and notation of
his calculus of quaternions, or method of vectors, to questions
of physical astronomy, the author wished to state that he had
found those processes, and that notation, adapt themselves with
remarkable facility to questions and results respecting Poinsot's
Theory of Mechanical Couples.  A single {\it force}, of the
ordinary kind, is naturally represented by a {\it vector},
because it is constructed or represented, in mathematical
reasoning, by a straight line having direction; but also a
{\it couple}, of the kind considered by Poinsot, is found, in Sir
William Hamilton's analysis, to admit of being regarded as
{\it the vector part of the product of two vectors}, namely, of
those which represent respectively one of the two forces of the
couple, and the straight line drawn to any point of its line of
direction from any point on the line of direction of the other
force.  Composition of couples corresponds to addition of such
vector parts; and the laws of equilibrium of several forces,
applied to various points of a solid body, are thus included in
the two equations,
$$\Sigma \beta = 0;\quad
  \Sigma (\alpha \beta - \beta \alpha) = 0;
   \eqno (18)$$
the vector of the point of application being $\alpha$, and the
vector representing the force applied at that point being
$\beta$.  The condition of the existence of a single resultant is
expressed by the formula,
$$       \Sigma \beta
         \mathbin{.} \Sigma (\alpha \beta - \beta \alpha)
       + \Sigma (\alpha \beta - \beta \alpha)
         \mathbin{.} \Sigma \beta
   =  0.
   \eqno (19)$$

Instead of the two equations of equilibrium (18), we may employ
the single formula
$$\Sigma \mathbin{.} \alpha \beta = - c
   \eqno (20)$$
$c$ here denoting a scalar (or real) quantity, which is
independent of the origin of vectors, and seems to have some
title to be called the total {\it tension\/} of the system.

In mentioning finally some applications of his algebraic method
to central surfaces of the second order, the author could not but
feel that he spoke in the presence of persons, of whom several
were much better acquainted with the general geometrical
properties of those surfaces than he could pretend to be.  But,
while deeply conscious that he had much to learn in this
department from his brethren of the Dublin School, as well as
from mathematicians elsewhere, he ventured to hope that the
novelty and simplicity of the symbolic forms which he was about
to submit to their notice might induce some of them to regard the
future development of the principles of his method as a task not
unworthy of their co-operation.  He finds, then, that if $\alpha$
and $\beta$ denote two arbitary but constant vectors, and if
$\rho$ be a variable vector, the equation of an ellipsoid with
the three arbitrary, and, in general, unequal axes, referred to
the centre as the origin of vectors, may be put under the
following form
$$(\alpha \rho + \rho \alpha)^2
       - (\beta \rho - \rho \beta)^2 = 1.
   \eqno (21)$$

One of its circumscribing cylinders of revolution is denoted by
the equation
$$- (\beta \rho - \rho \beta)^2 = 1;
   \eqno (22)$$
the plane of the ellipse of contact by
$$\alpha \rho + \rho \alpha = 0;
   \eqno (23)$$
and the systems of the two tangent planes parallel hereto, by
$$(\alpha \rho + \rho \alpha)^2 = 1.
   \eqno (24)$$

A hyperboloid of one sheet, touching the same cylinder in the
same ellipse, is denoted by the equation
$$(\alpha \rho + \rho \alpha)^2
       + (\beta \rho - \rho \beta)^2 = - 1;
   \eqno (25)$$
its asymptotic cone by
$$(\alpha \rho + \rho \alpha)^2 + (\beta \rho - \rho \beta)^2
   =  0;
   \eqno (26)$$
and a hyperboloid of two sheets, with the same asymptotic
cone~(26), and with the tangent planes~(24), is represented by a
the formula
$$(\alpha \rho + \rho \alpha)^2
       + (\beta \rho - \rho \beta)^2 = 1.
   \eqno (27)$$

By changing $\rho$ to $\rho - \gamma$, in which $\gamma$ is a
third arbitrary but constant vector, we introduce an arbitrary
origin of vectors, or an arbitary position of the centre of the
surface as referred to such an origin; and the general problem of
determining that individual surface of the second order (supposed
to have a centre, until the calculation shall show in any
particular question that it has none), which shall pass through
{\it nine given points}, may thus be regarded as equivalent to
the problem of finding {\it three constant vectors}, $\alpha$,
$\beta$, $\gamma$, which shall for nine given values of the
variable vector~$\rho$, satisfy one equation of the form
$$\{ \alpha (\rho - \gamma) + (\rho - \gamma) \alpha \}^2
      \pm \{ \beta (\rho - \gamma) - (\rho - \gamma) \beta^2 \}^2
   =  \pm 1;
   \eqno (28)$$
with suitable selections of the two ambiguous signs, depending
on, and in their turn determining, the particular nature of the
surface.  It is not difficult to transform the equation~(28), or
those which it includes, so as to put in evidence some of the
chief properties of surfaces of the second order, with respect to
their circular sections.

The recent expressions may be abridged, if we agree to employ the
letters ${\sc s}$ and ${\sc v}$ as characteristics of the
operations of taking separately the scalar and vector parts of
any quaternion to which they are prefixed; for then we shall have
$$\alpha \rho + \rho \alpha
   =  2 {\sc s} \mathbin{.} \alpha \rho,\quad
  \beta \rho - \rho \beta
   =  2 {\sc v} \mathbin{.} \beta \rho;
   \eqno (29)$$
so that, by making for abridgment $2 \alpha = \alpha'$,
$2 \beta = \beta'$, the equation~(21) of the ellipsoid (for
example) will take the shorter form,
$$({\sc s} \mathbin{.} \alpha' \rho)^2
       - ({\sc v} \mathbin{.} \beta' \rho)^2
   =  1.
   \eqno (30)$$

Another modification of the notation, which, from its general
character, will often be found useful, or at least illustrative,
may be obtained by agreeing to denote by the geometrical symbol
${\sc b} {\sc a}$ the vector $\beta - \alpha$, which is the
difference of two other vectors $\alpha$ and $\beta$ drawn to the
two points ${\sc a}$ and ${\sc b}$, from any common origin; so
that ${\sc b} {\sc a}$ is the vector {\it to\/}~${\sc b}$
{\it from\/}~${\sc a}$.  Denoting also by the symbol
${\sc c} {\sc b} {\sc a}$ the quaternion
${\sc c} {\sc b} \times {\sc b} {\sc a}$,
which is the product of the two vectors ${\sc c} {\sc b}$ and
${\sc b} {\sc a}$; by ${\sc d} {\sc c} {\sc b} {\sc a}$ the
continued product
${\sc d} {\sc c} \times {\sc c} {\sc b} \times {\sc b} {\sc a}$,
and so on: the foregoing equations of central surfaces may be
transformed, and a great number of geometrical processes and
results expressed under concise and not inelegant forms.  For
example, the symbols
$${{\sc v} \mathbin{.} {\sc a} {\sc b} {\sc c}
   \over {\sc a} {\sc c}},
   \eqno (31)$$
and
$${{\sc s} \mathbin{.} {\sc a} {\sc b} {\sc c} {\sc d}
   \over {\sc v} \mathbin{.} {\sc a} {\sc b} {\sc c}},
   \eqno (32)$$
will denote, in length and direction, the perpendiculars let
fall, respectively, from the summit~${\sc b}$ on the base
${\sc a} {\sc c}$ of a triangle, and from the summit~${\sc d}$ on
the base ${\sc a} {\sc b} {\sc c}$ of a tetrahedron: the sextuple
area of this tetrahedron ${\sc a} {\sc b} {\sc c} {\sc d}$ being
expressed in the same notation by the symbol
${\sc s} \mathbin{.} {\sc a} {\sc b} {\sc c} {\sc d}$.

The developments (15) and (16), with a great number of others,
may be included in a formula which corresponds to Taylor's
theorem, namely, the following:
$$f(\alpha + {\rm d} \alpha)
   =  \left(
         1
       + {{\rm d} \over 1}
       + {{\rm d}^2 \over 1 \mathbin{.} 2}
       + \ldots
      \right)
      f \alpha;
   \eqno (33)$$
the only new circumstance being, that in interpreting or
transforming the separate terms, for example, the term
${1 \over 2} {\rm d}^2 f \alpha$,
of the resulting development of the function
$f(\alpha + {\rm d} \alpha)$, if $\alpha$ and its differential
${\rm d} \alpha$ denote vectors, we must in general employ
{\it new rules of differentiation}, having indeed a close
affinity to the known rules, but modified by the non-commutative
character of the operation of multiplication in this calculus of
vectors or of quaternions.  It is thus that, instead of writing
${\rm d} \mathbin{.} \alpha^2 = 2 \alpha \, {\rm d} \alpha$,
$\delta  \mathbin{.} \alpha^2 = 2 \alpha \, \delta  \alpha$,
we have been obliged to write
$${\rm d} \mathbin{.} \alpha^2
   =     \alpha \mathbin{.} {\rm d} \alpha
       + {\rm d} \alpha \mathbin{.} \alpha;
   \eqno (34)$$
$$\delta  \mathbin{.} \alpha^2
   =     \alpha \mathbin{.} \delta  \alpha
       + \delta  \alpha \mathbin{.} \alpha.
   \eqno (35)$$

\bye



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