Kraft, Jens

(b. Fredrikstad, Norway, 2 October 1720; d. Søro, Denmark, 18 March 1765)

mathematics, physics, anthropology, philosophy.

Kraft’s mother, Severine Ehrensfryd Scolt, died when he was only two, and his father, Anders Kraft, a senior lieutenant in the Norwegian army, died when he was live years old. He was privately educated in Denmark at the manor of his uncle, Major Jens Kraft, and took the master’s degree in Copenhagen in 1742. Kraft was married twice, to Bodil Cathrine Evertsen, who died in 1758, and to Sophie Magdalene Langhorn, who survived him.

A traveling grant enabled him to study philosophy with Christian Wolff in Germany, and mathematics and physics in France. Later he often expressed his admiration for Wolff, Daniel Bernoulli, Clairaut, and d’Alembert, whose works changed his general scientific outlook. On his return in 1746, he was admitted as a fellow of the Royal Danish Academy of Science and Letters. The following year he became the first professor of mathematics and philosophy in the reestablished academy for the nobility at Sorø, where he remained until his death. An eminent teacher, Kraft’s lectures and private colloquia helped to diminish the prevailing influence of Cartesianism, and to bring Danish science back into the mainstream of the eighteenth century.

Kraft’s best-known work is a textbook on theoretical and technical mechanics (1763-1764). The book, written in an easy and fluent style, contains a series of lectures baied on Newtonian principle?. Each lecture is provided with a supplement giving a more advanced mathematical exposition of the subject matter. In Denmark this work gave theoretical physics a firm basis as an academic subject, while its large section on machines stimulated the expansion of industry. The book was favorably received abroad and was trans. lated into Latin and German.

Kraft’s broad cultural interests were also reflected in a book on the life and manners of primitive peoples which is regarded as a pioneer work in social anthropology. It was written in the belief that a study of savage cultures would reveal the general origin of human institutions and beliefs.

His first paper, presented to the Royal Danish Academy of Science and Letters in 1746, was a clear exposition of the systems of Descartes and Newton. In opposition to his admired teacher, Christian Wolff, Kraft sided with Newton by showing thai the Carte. sian vortex theory was incompatible with accepted mechanical principles. Kraft did write several text. books, nevertheless, on logic, ontology, cosmology, and psychology, inspired primarily by Wolffian philosophy.

Mathematics was one of Kraft’s major areas of interest. Two early theses (written in 1741 and 1742) present no really new contributions to mathematics, but they show Kraft to have been a skilled and well. read mathematician. For example, the theses contain discussions of equations which are solved by means of Descartes’s method of cuts between parabolas and circles. In 1748-1750 Kraft published two mathe. matical treatises. In the first he proved that if

has two equal roots α, then α is also a root in dy/dx. In the second paper Kraft discusses the following problem: Given an equation

A=αx^ry^f + βx^my^t + δx^py^h + …

with rational exponents, y can be found as a series

y= Bxⁿ + Cx^n+k + Dx^n+2k + …

with rational exponents. In his introduction, Kraft mentioned Newton, Leibniz, Maclaurin, Sterling, and’s Gravesande as examples of mathematicians who had treated this problem before, and Kraft’s own method is a refinement of that of ’s Gravesande.

Furthermore, in two small treatises from 1751-1754 Kraft argued that the concepts of infinitely large and infinitely small do not exist in an absolute sense in mathematics and physics, and that they must be conceived as relative quantities.

BIBLIOGRAPHY

I. Original Works. Kraft’s textbook on mechanics was published in two volumes. The first, Forelæsninger overmekanik med hosfoiede tillæg (Sorø, 1763) was translated into Latin, Mechanica Latine (Wismar, 1773), and into German, Mechanik, aus Lateinischen mit Zusätzen ver. mehrten Uebersetzung Tetens ins Deutsche übersetzt und hin und wieder verbessert von Joh. Chr. Aug. Steingrüber (Dresden, 1787); the other appeared as Forelæsninger over statik og hydrodynamik med Maskin-Væsenets theorier (Sorø, 1764).

His book on ethnology, Kort fortælning af de vilde folks fornemmeste indretninger, skikke on meninger, til oplysning af det menneskeliges oprindelse og fremgang i almindelighed (Sorø, 1760), was translated into German, Die Sitten der Wilden zur Aufklärung des Ursprungs und Aufnahme der Menschheit (Copenhagen, 1766), and Dutch, Verhandeling over de zeden en gewoontens der oude en hedendaagsche wilde volker (Utrecht, 1779).

Kraft’s paper on the systems of Descartes and Newton,“Betænkning over Neutons og Cartesii systemer med nye Anmærkninger over Lyset,” was published in Det Kioben. havnske Selskabs Skrifter, 3 (1747), 213-296. Kraft’s mathematical papers include Explicationum in Is. Neutoni Arithmeticam universalem particulam primum (Copenhagen, 1741); Theoria generalis succincta construendi aeqvationes analyticas (Copenhagen, 1742); “Anmcrkning over de Liigheder, i hvilke af flere Værdier af den ubekiendte Størrelse er lige store” in Det Kiøbenhavnske Selskabs Skrifter, 5 (17501, 303-309; and “Metode at bevise, hvorledes man i alle Tilfælde kand bestemme den eue Ubekiendle ved en u-endelig Følge af Tcrminis. som gives ved den anden, i de algebraiske Liigheder, som indeholde to Ubekiendte,” ibid., 324-354.

His most important philosophical papers are Systema mundi deductum ex principiis monadis, Dissertation, qui a remporté le prix proposé par l’Académie des sciences et belles lettres sur le système des monades avec les pièces, qui ont concouru (Berlin, 1748); and “Afhandling om en Deel Contradictioner, som findes i det sædvanlige Systema over Materien og de sammensatte Ting,” in Det Kiøben. havnske Selskabs Skrifter, 6 (1754), 189-216.

II. Secondary Literature. See Dansk Biografisk Leksikon. An account of Kraft’s contribution to ethnology is given by Kaj Birket-Smith, “Jens Kraft, A Pioneer of Ethnology in Denmark” in Folk, Dansk etnografisk tidsskrift, 2 (Copenhagen. 1960), 5-12.

Kurt MØller Pedersen