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“CIRCLES OF PROPORTION” AND “TRIGONOMETRIE”
Oughtred wrote and had published three important mathematical books, the Clavis, the Circles of Proportion,37 and a Trigonometrie.38 This last appeared in the year 1657 at London, in both Latin and English.
It is claimed that the trigonometry was “neither finished nor published by himself, but collected out of his scattered papers; and though he connived at the printing it, yet imperfectly done, as appears by his MSS.; and one of the printed Books, corrected by his own Hand.”39 Doubtless more accurate on this point is a letter of Richard Stokes who saw the book through the press:
I have procured your Trigonometry to be written over in a fair hand, which when finished I will send to you, to know if it be according to your mind; for I intend (since you were pleased to give your assent) to endeavour to print it with Mr. Briggs his Tables, and so soon as I can get the Prutenic Tables I will turn those of the sun and moon, and send them to you.40
In the preface to the Latin edition Stokes writes:
Since this trigonometry was written for private use without the intention of having it published, it pleased the Reverend Author, before allowing it to go to press, to expunge some things, to change other things and even to make some additions and insert more lucid methods of exposition.
This much is certain, the Trigonometry bears the impress characteristic of Oughtred. Like all his mathematical writings, the book was very condensed. Aside from the tables, the text covered only 36 pages. Plane and spherical triangles were taken up together. The treatise is known in the history of trigonometry as among the very earliest works to adopt a condensed symbolism so that equations involving trigonometric functions could be easily taken in by the eye. In the work of 1657, contractions are given as follows: s=sine, t=tangent, se=secant, s co=cosine (sine complement), t co=cotangent, se co=cosecant, log=logarithm, Z cru=sum of the sides of a rectangle or right angle, X cru=difference of these sides. It has been generally overlooked by historians that Oughtred used the abbreviations of trigonometric functions, named above, a quarter of a century earlier, in his Circles of Proportion, 1632, 1633. Moreover, he used sometimes also the abbreviations which are current at the present time, namely sin=sine, tan=tangent, sec=secant. We know that the Circles of Proportion existed in manuscript many years before they were published. The symbol sv for sinus versus occurs in the Clavis of 1631. The great importance of well-chosen symbols needs no emphasis to readers of the present day. With reference to Oughtred’s trigonometric symbols. Augustus De Morgan said:
This is so very important a step, simple as it is, that Euler is justly held to have greatly advanced trigonometry by its introduction. Nobody that we know of has noticed that Oughtred was master of the improvement, and willing to have taught it, if people would have learnt.41
We find, however, that even Oughtred cannot be given the whole credit in this matter. By or before 1631 several other writers used abbreviations of the trigonometric functions. As early as 1624 the contractions sin for sine and tan for tangent appear on the drawing representing Gunter’s scale, but Gunter did not use them in his books, except in the drawing of his scale.42 A closer competitor for the honor of first using these trigonometric abbreviations is Richard Norwood in his Trigonometrie, London, 1631, where s stands for sine, t for tangent, sc for sine complement (cosine), tc for tangent complement (cotangent), and sec for secant. Norwood was a teacher of mathematics in London and a well-known writer of books on navigation. Aside from the abbreviations just cited Norwood did not use nearly as much symbolism in his mathematics as did Oughtred.
Mention should be made of trigonometric symbols used even earlier than any of the preceding, in “An Appendix to the Logarithmes, shewing the practise of the Calculation of Triangles, etc.,” printed in Edward Wright’s edition of Napier’s A Description of the Admirable Table of Logarithmes, London, 1618. We referred to this “Appendix” in tracing the origin of the sign ×. It contains, on p. 4, the following passage: “For the Logarithme of an arch or an angle I set before (s), for the antilogarithme or compliment thereof (s*) and for the Differential (t).” In further explanation of this rather unsatisfactory passage, the author (Oughtred?) says, “As for example: sB+BC=CA. that is, the Logarithme of an angle B. at the Base of a plane right-angled triangle, increased by the addition of the Logarithm of BC, the hypothenuse thereof, is equall to the Logarithme of CA the cathetus.”
Here “logarithme of an angle B” evidently means “log sin B,” just as with Napier, “Logarithms of the arcs” signifies really “Logarithms of the sines of the angles.” In Napier’s table, the numbers in the column marked “Differentiae” signify log sine minus log cosine of an angle; that is, the logarithms of the tangents. This explains the contraction (t) in the “Appendix.” The conclusion of all this is that as early as 1618 the signs s, s*, t were used for sine, cosine, and tangent, respectively.
John Speidell, in his Breefe Treatise of Sphaericall Triangles, London, 1627, uses Si. for sine, T. and Tan for tangent, Se. for secant, Si. Co. for cosine, Se. Co. for cosecant, T. Co. for cotangent.
The innovation of designating the sides and angles of a triangle by A, B, C, and a, b, c, so that A was opposite a, B opposite b, and C opposite c, is attributed to Leonard Euler (1753), but was first used by Richard Rawlinson of Queen’s College, Oxford, sometimes after 1655 and before 1668. Oughtred did not use Rawlinson’s notation.43
In trigonometry English writers of the first half of the seventeenth century used contractions more freely than their continental contemporaries; even more freely, indeed, than English writers of a later period. Von Braunmühl, the great historian of trigonometry, gives Oughtred much praise for his trigonometry, and points out that half a century later the army of writers on trigonometry had hardly yet reached the standard set by Oughtred’s analysis.44 Oughtred must be credited also with the first complete proof that was given to the first two of “Napier’s analogies.” His trigonometry contains seven-place tables of sines, tangents, and secants, and six-place tables of logarithmic sines and tangents; also seven-place logarithmic tables of numbers. At the time of Oughtred there was some agitation in favor of a wider introduction of decimal systems. This movement is reflected in those tables which contain the centesimal division of the degree, a practice which is urged for general adoption in our own day, particularly by the French.
SOLUTION OF NUMERICAL EQUATIONS
In the solution of numerical equations Oughtred does not mention the sources from which he drew, but the method is substantially that of the great French algebraist Vieta, as explained in a publication which appeared in 1600 in Paris under the title, De numerosa potestatum purarum atque adfectarum ad exegesin resolutione tractatus. In view of the fact that Vieta’s process has been described inaccurately by leading modern historians including H. Hankel45 and M. Cantor,46 it may be worth while to go into some detail.47 By them it is made to appear as identical with the procedure given later by Newton. The two are not the same. The difference lies in the divisor used. What is now called “Newton’s method” is Newton’s method as modified by Joseph Raphson.48 The Newton-Raphson method of approximation to the roots of an equation f(x)=0 is usually given the form a-[f(a)/f´(a)], where a is an approximate value of the required root. It will be seen that the divisor is f´(a). Vieta’s divisor is different; it is
|f(a+s₁) -f(a)|-s₁ⁿ,
where f(x) is the left of the equation f(x)=k, n is the degree of equation, and s₁ is a unit of the denomination of the digit next to be found. Thus in x³+420000x=247651713, it can be shown that 417 is approximately a root; suppose that a has been taken to be 400, then s₁=10; but if, at the next step of approximation, a is taken to be 410, then s₁=1. In this example, taking a=400, Vieta’s divisor would have been 9120000; Newton’s divisor would have been 900000.
A comparison of Vieta’s method with the Newton-Raphson method reveals the fact that Vieta’s divisor is more reliable, but labors under the very great disadvantage of requiring a much larger amount of computation. The latter divisor is accurate enough and easier to compute. Altogether the Newton-Raphson process marks a decided advance over that of Vieta.
As already stated, it is the method of Vieta that Oughtred explains. The Englishman’s exposition is an improvement on that of Vieta, printed forty years earlier. Nevertheless, Oughtred’s explanation is far from easy to follow. The theory of equations was at that time still in its primitive stage of development. Algebraic notation was not sufficiently developed to enable the argument to be condensed into a form easily surveyed. So complicated does Vieta’s process of approximation appear that M. Cantor failed to recognize that Vieta possessed a uniform mode of procedure. But when one has in mind the general expression for Vieta’s divisor which we gave above, one will recognize that there was marked uniformity in Vieta’s approximations.
Oughtred allows himself twenty-eight sections in which to explain the process and at the close cannot forbear remarking that 28 is a “perfect” number (being equal to the sum of its divisors, 1, 2, 4, 7, 14).
The early part of his exposition shows how an equation may be transformed so as to make its roots 10, 100, 1000, or 10m times smaller. This simplifies the task of “locating a root”; that is, of finding between what integers the root lies.
Taking one of Oughtred’s equations, x⁴-72x³+238600x=8725815, upon dividing 72x³ by 10, 238600x by 1000, and 8725815 by 10,000, we obtain x⁴-7·2x³+238·6x=872·5. Dividing both sides by x, we obtain x³+238·6-7·2x²=x)872·5. Letting x=4, we have 64+238·6-115·2=187·4.
But 4)872·5(218·1; 4 is too small. Next let x=5, we have 125+238·6-180=183·6.
But 5)872·5(174·5; 5 is too large. We take the lesser value, x=4, or in the original equation, x=40. This method may be used to find the second digit in the root. Oughtred divides both sides of the equation by x², and obtains x²+x)238600-72x=x²)8725815. He tries x=47 and x=48, and finds that x=47.
He explains also how the last computation may be done by logarithms. Thereby he established for himself the record of being the first to use logarithms in the solution of affected equations.
As an illustration of Oughtred’s method of approximation after the root sought has been located, we have chosen for brevity a cubic in preference to a quartic. We selected the equation x³+420000x=247651713. By the process explained above a root is found to lie between x=400 and x=500. From this point on, the approximation as given by Oughtred is as shown on p. 43.
In further explanation of this process, observe that the given equation is of the form Lc+CqL=Dc, where Lc is our x, Cq=420000, Dc=247651713. In the first step of approximation, let L=A+E, where A=400 and E is, as yet, undetermined. We have
Lc=(A+E)³=A³+3A²E+3AE²+E³
and
CqL=420000(A+E).
Subtract from 247651713 the sum of the known terms A³ (his Ac) and 420000 A (his CqA). This sum is 232000000 the remainder is 15651713.
Next, he evaluates the coefficients of E in 3A²E and 420000E, also 3A, the coefficient of E². He obtains 3A²=480000, 3A=1200, Cq=420000. He interprets 3A² and Cq as tens, 3A as hundreds. Accordingly, he obtains as their sum 9120000, which is the divisor for finding the second digit in the approximation. Observe that this divisor is the value of |f(a+s₁) -f(a)|-s₁ⁿ in our general expression, where a=400, s₁=10, n=3, f(x)=x³+420000x.
Dividing the remainder 15651713 by 9120000, he obtains the integer 1 in ten’s place; thus E=10, approximately. He now computes the terms 3A²E, 3AE² and E³ to be, respectively, 4800000, 120000, 1000. Their sum is 9121000. Subtracting it from the previous remainder, 15651713, leaves the new remainder, 6530713.
From here on each step is a repetition of the preceding step. The new A is 410, the new E is to be determined. We have now in closer approximation, L=A+E. This time we do not subtract A³ and CqA, because this subtraction is already affected by the preceding work.
We find the second trial divisor by computing the sum of 3A², 3A and Cq; that is, the sum of 504300, 1230, 420000, which is 925530. Again, this divisor can be computed by our general expression for divisors, by taking a=410, s₁=1, n=3.
Dividing 6530713 by 925530 yields the integer 7. Thus E=7. Computing 3A²E, 3AE², E³ and subtracting their sum, the remainder is 0. Hence 417 is an exact root of the given equation.
Since the extraction of a cube root is merely the solution of a pure cubic equation, x³=n, the process given above may be utilized in finding cube roots. This is precisely what Oughtred does in chap. xiv of his Clavis. If the foregoing computation is modified by putting Cq=0, the process will yield the approximate cube root of 247651713.
Oughtred solves 16 examples by the process of approximation here explained. Of these, 9 are cubics, 5 are quartics, and 2 are quintics. In all cases he finds only one or two real roots. Of the roots sought, five are irrational, the remaining are rational and are computed to their exact values. Three of the computed roots have 2 figures each, 9 roots have 3 figures each, 4 roots have 4 figures each. While no attempt is made to secure all the roots – methods of computing complex roots were invented much later – he computes roots of equations which involve large coefficients and some of them are of a degree as high as the fifth. In view of the fact that many editions of the Clavis were issued, one impression as late as 1702, it contributed probably more than any other book to the popularization of Vieta’s method in England.
Before Oughtred, Thomas Harriot and William Milbourn are the only Englishmen known to have solved numerical equations of higher degrees. Milbourn published nothing. Harriot slightly modified Vieta’s process by simplifying somewhat the formation of the trial divisor. This method of approximation was the best in existence in Europe until the publication by Wallis in 1685 of Newton’s method of approximation.
It should be stated that, before the time of Newton, the best method of approximation to the roots of numerical equations existed, not in Europe, but in China. As early as the thirteenth century the Chinese possessed a method which is almost identical with what is known today as “Horner’s method.”
LOGARITHMS
Oughtred’s treatment of logarithms is quite in accordance with the more recent practice.49 He explains the finding of the “index” (our “characteristic”); he states that “the sum of two Logarithms is the Logarithm of the Product of their Valors; and their difference is the Logarithm of the Quotient,” that “the Logarithm of the side [436] drawn upon the Index number [2] of dimensions of any Potestas is the logarithm of the same Potestas” [436²], that “the logarithm of any Potestas [436²] divided by the number of its dimensions [2] affordeth the Logarithm of its Root [436].” These statements of Oughtred occur for the first time in the Key of the Mathematicks of 1647; the Clavis of 1631 contains no treatment of logarithms.
If the characteristic of a logarithm is negative, Oughtred indicates this fact by placing the – above the characteristic. He separates the characteristic and mantissa by a comma, but still uses the sign |_ to indicate decimal fractions. He uses the contraction “log.”
INVENTION OF THE SLIDE RULE; CONTROVERSY ON PRIORITY OF INVENTION
Oughtred’s most original line of scientific activity is the one least known to the present generation. Augustus De Morgan, in speaking of Oughtred, who was sometimes called “Oughtred Aetonensis,” remarks: “He is an animal of extinct race, an Eton mathematician. Few Eton men, even of the minority which knows what a sliding rule is, are aware that the inventor was of their own school and college.”50 The invention of the slide rule has, until recently,51 been a matter of dispute; it has been erroneously ascribed to Edmund Gunter, Edmund Wingate, Seth Partridge, and others. We have been able to establish that William Oughtred was the first inventor of slide rules, though not the first to publish thereon. We shall see that Oughtred invented slide rules about 1622, but the descriptions of his instruments were not put into print before 1632 and 1633. Meanwhile one of his own pupils, Richard Delamain, who probably invented the circular slide rule independently, published a description in 1630, at London, in a pamphlet of 32 pages entitled Grammelogia; or the Mathematicall Ring. In editions of this pamphlet which appeared during the following three or four years, various parts were added on, and some parts of the first and second editions eliminated. Thus Delamain antedates Oughtred two years in the publication of a description of a circular slide rule. But Oughtred had invented also a rectilinear slide rule, a description of which appeared in 1633. To the invention of this Oughtred has a clear title. A bitter controversy sprang up between Delamain on one hand, and Oughtred and some of his pupils on the other, on the priority and independence of invention of the circular slide rule. Few inventors and scientific men are so fortunate as to escape contests. The reader needs only to recall the disputes which have arisen, involving the researches of Sir Isaac Newton and Leibniz on the differential and integral calculus, of Thomas Harriot and René Descartes relating to the theory of equations, of Robert Mayer, Hermann von Helmholtz, and Joule on the principle of the conservation of energy, or of Robert Morse, Joseph Henry, Gauss and Weber, and others on the telegraph, to see that questions of priority and independence are not uncommon. The controversy between Oughtred and Delamain embittered Oughtred’s life for many years. He refers to it in print on more than one occasion. We shall confine ourselves at present to the statement that it is by no means clear that Delamain stole the invention from Oughtred; Delamain was probably an independent inventor. Moreover, it is highly probable that the controversy would never have arisen, had not some of Oughtred’s pupils urged and forced him into it. William Forster stated in the preface to the Circles of Proportion of 1632 that while he had been carefully preparing the manuscript for the press, “another to whom the Author [Oughtred] in a louing confidence discouered this intent, using more hast then good speed, went about to preocupate.” It was this passage which started the conflagration. Another pupil, W. Robinson, wrote to Oughtred, when the latter was preparing his Apologeticall Epistle as a reply to Delamain’s countercharges: “Good sir, let me be beholden to you for your Apology whensoever it comes forth, and (if I speak not too late) let me entreat you, whip ignorance well on the blind side, and we may turn him round, and see what part of him is free.”52 As stated previously, Oughtred’s circular slide rule was described by him in his Circles of Proportion, London, 1632, which was translated from Oughtred’s Latin manuscript and then seen through the press by his pupil, William Forster. In 1633 appeared An Addition vnto the Vse of the Instrvment called the Circles of Proportion which contained at the end “The Declaration of the two Rulers for Calculation,” giving a description of Oughtred’s rectilinear slide rule. This Addition was bound with the Circles of Proportion as one volume. About the same time Oughtred described a modified form of the rectilinear slide rule, to be used in London for gauging.53
In 1633 there was added the following, with a separate title-page: An addition vnto the Vse of the Instrvment called the Circles of Proportion… London, 1633, this being followed by Oughtred’s To the English Gentrie etc. In the British Museum there is a copy of another impression of the Circles of Proportion, dated 1639, with the Addition vnto the Vse of the Instrument etc., bearing the original date, 1633, and with the epistle, To the English Gentrie, etc., inserted immediately after Forster’s dedication, instead of at the end of the volume.
