Page:EB1911 - Volume 14.djvu/572

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INFINITESIMAL CALCULUS

of gravity and volumes and surfaces of solids, but, as

the project with regard to fluxions. In 1680 Collins sought the assistance of the Royal Society for the publication of the tract, and this was granted in 1682. Yet it remained unpublished. The reason is unknown; but it is known that about 1679, 1680, Newton took up again the studies in natural philosophy which he had intermitted for several years, and that in 1684 he wrote the tract De molu which was in some sense a first draft of the Principia, and it may be conjectured that the fluxions were held over until the Principia should be finished. There is also reason to think that Newton had become dissatisfied with the arguments about infinitesimals on which his calculus was based. In the preface to the De quadralura curvarum (1764), in which he describes this tract as something which he once wrote (“ olim scripsi ”) he says that there is no necessity to introduce into the method of fluxions any argument about infinitely small quantities; and in the Principia (1687) he adopted instead of the method of fluxions a new method, that of “ Prime and Ultimate Ratios.” By the aid of this method it is possible, as Newton. knew, and as was afterwards seen by others, to found the calculus of fluxions on an irreproachable method of limits. For the purpose of explaining his discoveries in dynamics and astronomy Newton used the method of limits only, without the notation of fluxions, and he presented all his results and demonstrations in a geometrical form. There is no doubt that he arrived at most of his theorems in the first instance by using the method of fluxions. Further evidence of Newton's dissatisfaction with arguments about infinitely small quantities is furnished by his tract M ethodns rlijerentialis, published in 1711 by William Jones, in which he laid the foundations of the “ Calculus of Finite Differences.”

24. Leibnitz, unlike Newton, was practically a self-taught mathematician. He seems to have been first attracted to mathematics as a means of symbolical ex ression and P,

Icgxfzts on the occasion of his first visit to London, early in d7s¢, ,, , e, y 1673, he learnt about the doctrine of infinite series which James Gregory, Nicolaus Mercator, Lord Brouncker and others, besides Newton, had used in their investigations. It appears that he did not on this occasion become acquainted with Collins, or see Newton's Analysis per acqualiones, but he purchased Barrow's Lectioncs. On returning to Paris he made the acquaintance of Huygens, who recommended him to read Descartes' Géomélrie. He also read Pascal's Letlres de Detlonville, Gregory of St Vi1icent's Opus geomelricnni, Cavalieri's Indivisibles and the Synopsis geomelrica of Honoré Fabri, a book which is practically a commentary on Cavalieri; it would never have had any importance but for the influence which it had on Leibnitz's thinking at this critical period. In August of this year (1675) he was at work upon the problem of tangents, and he appears to have made out the nature of the solution—the method involved in Barrow's differential triangle for himself by the aid of a diagram drawn by Pascal in a demon station of the formula for the area of a spherical surface. He saw that the problem of the relation between the differences of neighbouring ordinates and the ordinates themselves was the important problem, and then that the solution of this problem was to be effected by quadratures. Unlike Newton, who arrived at differentiation and tangents through integration and areas, Leibnitz proceeded from tangents to quadratures. When he turned his attention to quadratures and indivisible, and realized the nature of the process of finding areas by summing " infinitesimal ” rectangles, he proposed to replace the rectangles by triangles having a common vertex, and obtained by this method the result which we write A

i1f=1'°i+é-'i+ - ~ln

1074 he sent an account of his method, called “ transmutation, ” along with this result to Huygens, and early in 1675 he sent it to Henry Oldenburg, secretary of the Royal Society, with inquiries as to Newton's discoveries in regard to quadratures. In October of 1675 he had begun to devise a symbolical notation for quadratures, starting from Cavalieri's indivisible, At first he proposed to use the word oninirz as an abbreviation for Cavalieri's “sum of all the lines, ” thus writing omnia y for that 54-I

which we write “fy¢l.r, ” but within a day or two he wrote “ fy.” He regarded the symbol “ f” as representing an operation which raises the dimensions of the subject of operation line becoming an area by the operation-and he devised his symbol “ d ” to represent the inverse operation, by which the dimensions are diminished. He observed that, whereas “ f ” represents “ sum, ” “ d ” represents “ difference.” His notation appears to have been practically settled before the end of 1675, for in November he wrote fydy=%y2, just as we do now. 25. In July of 1676 Leibnitz received an answer to his inqui-ry in regard to Newton's methods in a letter written by Newton to Oldenburg. In this letter Newton ave a eneral 3 g Corresstatement

of the binomial theorem and many results pandence relating to series. He stated that by means of such 0fN¢W series he could find areas and lengths of curves, centres mhz; this would take too long to describe, he would illustrate it by examples. He gave no proofs. Leibnitz replied in August, stating some results which he had obtained, and which, as it seemed, could not be obtained easily by the method of series, and he asked for further information. Newton replied in a long letter to Oldenburg of the 24th of October 1676. In this letter he gave a much fuller account of his binomial theorem and indicated a method of proof. Further he gave a number of results relating to quadratures; they were afterwards printed in the tract De quadralura curvarum. He gave many other results relating to the computation of natural logarithms and other calculations in which series could be used. He gave a general statement, similar to that in the letter to Collins, as to the kind of problems relating to tangents, maxima and minima, &c., which he could solve by his method, but he concealed his formulation of the calculus in an anagram of transposed letters. The solution of the anagram was given eleven years later in the Principia in the words we have quoted from Wallis's Algebra. In neither of the letters to Oldenburg does the characteristic notation of the fluxional calculus occur, and the words “ fluxion " and “ fluent ” occur only in anagrams of transposed letters. The letter of October 1676 was not dispatched until May 1677, and Leibnitz answered it in June of that year. In October 1576 Leibnitz was in London, where he made the acquaintance of Collins and read the Analysis per aequaliones, and it seems to have been supposed afterwards that he then read Newton's letter of October 1676, but he left London before Oldenburg received this letter. In his answer of June 1677 Leibnitz gave Newton a candid account of his differential calculus, nearly in the form in which he afterwards published it, and explained how he used it for quadratures and inverse problems of tangents. Newton never replied.

26. In the Acta eriulitornm of 1684 Leibnitz published a short memoir entitled Nova niet/iodus pro niaximis el ileniqne langenlibns, quae nec fraclas nec irralionales minimis,

Leibnltz's

q nan lit ales moratnr, el singulari pro illis calcnli genus. Differ-In this memoir the differential dx of a variable x, "fl" considered as the abscissa of a point of a curve, is said c'“"'"'"" to be an arbitrary quantity, and the differential dy of a related variable y, considered as the ordinate of the point, is defined as a quantity which has to dx the ratio of the ordinate to the sub tangent, and rules are given for operating with differentials. These are the rules for forming the differential of a constant, a sum (or difference), a product, a quotient, a power (or root). They are equivalent to our rules (i.)-(iv.) of § 1 1 and the particular result

d(x"') =mx"""'dx. I

The rule for a function of a function is not stated explicitly but is illustrated by 'examples in which new variables are introduced, in much the same way as in Newton's M elhodus fluxionum. In connexion with the problem of maxima and minima, it is noted that the differential of y is positive or negative according as y increases or decreases when x increases, and the discrimination of maxima from minima depends upon the sign of ddy, the differential of dy. In connexion with the problem of tangents

the differentials are said to be proportional to the momentary