The origins of Clifford Truesdell's Program for Rewriting the History of Early Modern Mechanics

Sandro Caparrini

abstract
The American mathematical physicist Clifford Ambrose Truesdell III (1919-2000) is remembered for having been both one of the founders of modern continuum mechanics and the greatest historian of mechanics in the twentieth century. These two activities cannot be separated. Truesdell began to be interested in the history of mechanics while looking for the origin of the fundamental theorems of continuum mechanics, and his historical work helped him to understand the methodology and aims of mathematical physics. 
A careful examination of Truesdell's historical works shows that they form a vast research program in the history of the exact sciences, and in particular in the history of classical mechanics. This program has influenced generations of historians. To understand its breadth and purpose, it is necessary to reconstruct the early biography of Truesdell, to trace the beginnings of his interest in the history of mechanics and to describe the origins of his great histories of eighteenth-century mechanics, the Rational fluid Mechanics, 1687–1765 (1954) and the Rational mechanics of elastic or flexible bodies, 1638–1788 (1960).

Introduction
With the passage of time, the history of science itself becomes an object of study for historians. Studying the changes in the methodology of the history of science often means looking at the development of science from a different angle.
We have a good example of this in the figure of Clifford Truesdell (1919-2000), one of the most influential mathematical physicists of the twentieth century. Truesdell was one of the creators of modern continuum mechanics. Today the results obtained by him and by the scientists of his school are routinely taught to students of engineering.
Truesdell was also a great historian of classical mechanics. His works have overturned the image of post-Newtonian mechanics that we inherited from Die Mechanik in ihrer Entwickelung: historisch-kritisch dargestellt of Ernst Mach. Whereas Mach saw in the eighteenth century only a purely formal development of Newton’s ideas, Truesdell believed that here was the beginning of modern mathematical physics. While Mach saw in Lagrange the point of arrival of the mathematization of Newton's discoveries, Truesdell identified Euler as the central figure in the history of mechanics and downgraded Lagrange to the role of an arid formalist. These new historical theses, which emerged from his own close reading of the primary sources, painted a new picture of the history of mechanics from Newton onwards.
In several respects, Truesdell seemed to belong to another century. He lived in a house rich with antique furniture, old paintings and eighteenth-century scientific books, was a friend of Federico Zeri and Sir Ernst Gombrich, hosted harpsichord concerts and wrote scientific articles in Latin. Taking into account his wide variety of interests, it is difficult to separate the scientist from the historian and the collector.
All of Truesdell’s historical works are part of a vast program for the foundation of a new historiography of the exact sciences and, in particular, of classical mechanics. This program has influenced, directly or indirectly, all the historians who in the last half century have worked on the history of the mathematical sciences. Truesdell believed that the history of mechanics had not been adequately studied. This gaping hole in our knowledge of the early modern exact sciences had consequences on the historiography of modern physics and mathematics. In order to address this problem it was necessary to form new historians, equally adept in the methodology of historical research and in advanced mathematical physics.
There are several books and articles on Truesdell and his work. They were written mostly by his friends and students, and focus mainly on his later years. Here, with the help of new documents, we will try to provide some previously unknown biographical data, to examine the beginning of his career as a historian and to understand his program for the historiography of the exact sciences.

Early Biography
Clifford Ambrose Truesdell III was born in Los Angeles on January 18, 1919. His father, an architect, left the family when his son was only ten months old and died around 1932. The child grew up in the house of his maternal grandparents. His mother, Helen Truesdell Heath, determined to keep the family going, began teaching literature in high school.
To understand Truesdell's historical work it is necessary to go into some detail on his upbringing, unusual for a twentieth-century American. In the early years the young Truesdell was educated at home by his maternal grandmother. The boy started going to school at eight, but his mother continued to perfect his education. (At that time the family moved to Pasadena.) She taught him to write in clear, direct English, modeled on the classics. Reading aloud was an integral part of this process: she read Gulliver's Travels, Robinson Crusoe and Swiss Family Robinson. She also taught him the different forms of poetry: Horatian dactyl hexameters, Chaucerian couplets and Petrarchian sonnets. Summers were devoted to learning new subjects, like Latin and French, and to read classics like Boswell's Life of Johnson and the diaries of Samuel Pepys.
At school Truesdell was equally interested in science and the humanities. He wrote poetry in Latin. For some time he was interested in chemistry, and of course he had no problems with mathematics. However, he was not the typical good student totally immersed in books: he was a member of the Reserve Officers' Training Corps, actively participated in student debates, took part in amateur theatrical performances and played football. He became passionate about Baroque music. 
Travel was always a very important aspect of Truesdell's life. At the age of five, his mother took him to Alaska. When he was nine they visited Europe. They went first to England, where they visited London and Canterbury, then to France, where they saw Paris, Mont St. Michel and the the Savoy. In 1933 they visited New England and New York, passing through Chicago on the way up and for the southwest on the way back. But the most important trip was the sojourn of two years in Europe between 1936 and 1938. His mother had to go to Oxford for her thesis on Pepys. They took the opportunity to visit the Orkney, the Shetland and the Hebrides, France, Italy and Greece. While his mother was busy in her studies, Truesdell visited Italy and Germany on his own. It was that time that he began buying old books and antiques. In Oxford he took private lessons of ancient Greek. While visiting Germany and Italy, he learned German and Italian. These cultural experiences had a far-reaching influence on his works.
In 1938 Truesdell enrolled at Caltech, and in three years obtained a B.S. in mathematics and a B.S. in physics, then, in 1941/42, an M.S. in mathematics. His first meeting with the theory of elasticity was a buckling problem assigned to him by the English analyst H. Bateman. In 1942 he obtained an M.S. in applied mechanics at Brown University. In 1943 he was in Princeton, where he took his Ph.D. in mathematics working on questions of analysis and mathematical logic.
After his doctorate, he did various jobs for a few years. He was part of the Radiation Laboratory of MIT (1944-1946) and held positions of responsibility first at the Theoretical Mechanics Subdivision of the Naval Ordnance Laboratory in White Oak, Maryland (1946-1948), then at the Theoretical Mechanics Section of the Naval Research Laboratory in Washington D.C. (1946-1948). By then, he had definitely left pure mathematics behind. 
While at Caltech, Truesdell had met a girl, Beverly. Within a short time, they married and had a son. Years later, at MIT, he met Charlotte Janice Brudno (1922-2008), a student of mathematics employed at the Radiation Laboratory. Truesdell divorced his first wife and in 1951 married Charlotte.
Anyone who deals with mechanics, a science almost as old as mathematics itself, find on his path some references to its history. This also happened to Truesdell. In 1938, in order to improve his German, he had read Mach's Mechanik. Also, the textbook for his course of physics at Caltech had many fine historical notes. Finally, the problems assigned in the courses of Bateman sometimes referred to works dating back to the nineteenth century. The decisive meeting with the original sources occurred in 1946:
When, in 1946, I first began to study the foundations of continuum mechanics, within a few months I had set the whole field in order, to my own satisfaction. I quickly wrote and submitted to an international meeting an expository memoir, which was rejected. In view of the quality of the papers accepted by the same meeting, I was naive enough to be astonished as well as disappointed, and I sent the manuscript for criticism to a number of experts. Most of these did not deign to acknowledge it or reply, but two did. Mr Friedrichs told me I had underestimated the work of earlier authors. Since my information concerning it was drawn from a number of reputable textbooks, I turned, somewhat taken aback, to the sources they cited, and then to the sources cited by these sources, and so on, until within a period of a year I found out how right he was and how little I had seen of the real issues faced by the great natural philosophers of one and two centuries ago.
"The mechanical foundations of elasticity and fluid dynamics" is an encyclopedic work, with a bibliography that goes back in time to Newton. It marked the beginning of Truesdell’s exploration of the history of mechanics. 
After years of temporary contracts across the United States, in 1950, Truesdell was finally hired as a professor of mathematics at Indiana University.

The introductions to euler’s Opera Omnia (1954-1960)
In 1942, at Brown University's summer school of applied mathematics Truesdell had met Paul (Pal) Felix Némenyi, an expert in continuum mechanics of Hungarian origin, which had definitively directed him towards mechanics. After the war, when Truesdell and Némenyi became colleagues at the Naval Research Laboratory, they planned to write a general treatment of fluid dynamics. The work was never finished, but Némenyi managed to complete the historical introduction.
This collaboration with Némenyi was decisive for the evolution of Truesdell’s historical interests. Starting from about 1948, Truesdell began working on an extensive monograph on fluid mechanics, The Kinematics of Vorticity. As in “The mechanical foundations” the bibliography goes back to Newton, but in this case there are also some detailed historical footnotes. One of these notes, for example, is a brief history of the Lagrangian and Eulerian coordinates. The Kinematics can be considered the first of Truesdell's historical work.
We can follow the subsequent developments thanks to Truesdell’s correspondence. On May 30th, 1951, Truesdell wrote to Andreas Speiser announcing the sending of a typewritten copy of the Kinematics. Speiser was a mathematician interested in art, history and philosophy, who had been a student of Klein, Hilbert and Minkowski in Gottingen. In 1950 he was the director of the Euler-Kommission and the general editor of Euler's Opera omnia. Truesdell expressed his admiration for Speiser’s work on Euler and said he looked forward to the publication of volumes 12 and 13 of the second series of the Opera Omnia, dedicated to fluid mechanics. Speiser replied praising the Kinematics and asking Truesdell to edit the two volumes himself. On August 9th, Truesdell accepted the assignment. He wanted to describe not only Euler's results, but also those of Euler’s predecessors and contemporaries. In other words, he aimed at writing a history of fluids mechanics from Newton to Lagrange.
At that time there were no in-depth texts on fluid mechanics in the eighteenth century. The “Historical sketch” by R. Giacomelli and E. Pistolesi devoted only a handful of pages to the subject and the Histoire de la mécanique by R. Dugas gave little more than a few quotations. Even Némenyi's “History” did not go much beyond the surface. A whole century of fundamental discoveries had not yet been studied.
Editing classics, especially classics of science, is not an easy task. To begin with, it was impossible to find in Indiana most of the original works of the eighteenth century. Some texts ad to be sent by mail from Switzerland; for the others, Truesdell had to go to the Library of Congress in Washington D.C. Euler's calculations had to be carefully examined and sometimes corrected. It was necessary to take into account spelling inconsistencies, oddities of punctuation, unorthodox Latin and old French. The original figures were redrawn, but modern illustrators often added useless details or left out essential aspects, and everything had to be sent back to the other side of the ocean.
On December 3rd, Truesdell asked Speiser if he preferred English or Latin for the introductions and the notes on Euler’s texts, noting without irony that if Latin had been chosen it would have taken longer. Speiser replied that for the notes the language of the original text had to be used, while the introductions could be written in the editor's mother tongue.
The careful examination of the texts, line by line and equation by equation, began to bear fruits. The “Principia motus fluidorum” (1750), for example, contained not only the "Laplace" equation for the velocity potential, as an inframinority of experts already knew, but also the Helmholtz equation for vortex motion and the Killing equation for rigid local motion. It became clear that Euler's works could not be divided easily into watertight compartments. For example, in his commentary to the German translation (1745) of Benjamin Robins' New Principles of Gunnery (1742), an artillery textbook, Euler had enunciated for the first time the famous "d’Alembert paradox", a milestone of hydrodynamics. 
Comparison of the texts revealed the world view of one of the greatest scientists of all time. The Anleitung zur Naturlehre, for example, published as late as 1862, which had always been regarded as a minor curiosity in Euler's work, was in fact the outline of a unitary theory of gravitation, electricity and magnetism by means of the ether, and probably had been conceived when Euler was working on the field equations of fluid mechanics.
According to the original publication plan, the first of the two volumes was dedicated to liquids and the second to gases. Truesdell remarked that it would be more appropriate to dedicate the entire second volume to four long memoirs in Latin, which had first appeared between 1769 and 1771 in the Acta of the Academy of Sciences of St. Petersburg, for in effect they formed a treatise on fluid mechanics. Only the difficulty of finding a printer had led Euler to break the book into four pieces. Thus, a hitherto unrecognized masterpiece of mathematical physics was unveiled to the world.
The introduction to volume 12, dedicated to the dynamics of fluids from 1687 to 1765, was ready in August 1953. Speiser wanted to publish it within the year, but Truesdell continued to revise the work. The latest drafts were sent in early December, and the book appeared in 1954.
Meanwhile Truesdell had sent his introduction in drafts to I. B. Cohen (1914-2003), Harvard professor of history of science and general editor of Isis. Cohen understood the scope of Truesdell’s research and expressed the desire to publish extensive reviews of the recent volumes of the Opera Omnia. This was an important step in spreading the new history of science. According to Truesdell, in the United States even historians were unaware that the Opera Omnia had survived the war.
It remained to write the introduction to volume 13. Once again Truesdell did a thorough literature search. He read the works of Euler, but also those of Newton, the Bernoullis, d'Alembert and Lagrange. What should have been a description of a handful of Euler's memoirs became a history of the theories of wave propagation in the eighteenth century.
Truesdell's work set precise landmarks in an almost unexplored territory. Until then, the story was told more or less in these terms: Newton had posed the basis of the theory in Book II of the Principia (1687), d'Alembert had discovered the wave equation and its solution in 1746 and Lagrange had provided the definitive treatment in the Méchanique analitique (1788). Truesdell revolutionized this point of view by detailing Newton’s problematic and isolated results, placing a sequence of three long memoirs by Euler published in the 1760s, at the core of the development of the theory, downsizing the value of Lagrange's results and highlighting the role of Daniel Bernoulli. The text of the second introduction was ready in April 1954.
In the meantime, he had agreed to also write the introduction to volumes 10 and 11 of the second series, dedicated to the theory of elasticity. Once again Truesdell decided not to limit himself to the works of Euler alone, but to write a real history of the theory of elasticity, by the Greeks up to Lagrange.
The lack of a truly comprehensive science history library in the United States as regards the seventeenth and eighteenth centuries, it had made work on the first two difficult introductions. Now a sojourn in Basel would have been necessary, but university commitments blocked Truesdell in Indiana. Only the combination of a sabbatical year and a Rockefeller scholarship allowed the realization of the project. The Truesdells stayed in Europe during the academic year 1956/57, spending a lot of time in Basel, Truesdell made good use of European libraries. If the first two introductions were remarkable, The Rational Mechanics of Flexible or Elastic Bodies 1638–1788 was a work monumental in size (430 pages) and content. We quote the opinion of Truesdell, from a letter to Speiser dated July 18, 1958:
I am sorry the work has taken so long and is of so great length, but I am glad to have written it, for it had to be written. You will see what I mean if you turn to Section 63, my evaluation. First, the results obtained in this field and period are of the highest importance for analysis and mechanics as a whole, yet their history and even their scope is unknown to scientists and historians alike. Second, previous historical attempts, particularly the classic by Todhunter and Pearson, are not only fragmentary, but outright mendacious, to the extent that he who reads them is almost more ignorant than he who had read nothing. Third, it is only a complete history that can serve as a proper introduction to Euler's work here. Every result described in Part I, detailing the science prior to Euler's twentieth year, was transferred and extended by him, and every progress made in the eighteenth century was due in large part or sole measure to him. Of the scope and depth of his researches I had no just idea until I completed my study, nor would anyone else conceive them without the detailed analysis presented in this history. The summaries given in Sections 31, 41, and 63 would be dismissed as exaggeration of an enthusiast, were they not preceded by the incontestable facts. I believe this preface is much better than the previous two; I hope you will like it.
The Rational Mechanics of Flexible or Elastic Bodies is a masterpiece of historical research. Truesdell analyzes the works of the scientific giants of their centuries (Galileo, Huygens, Leibniz, the Bernoullis, Lagrange, ...), the relevant contributions of minor scientists (Mersenne, Sauveur, Giordano Riccati, ...), the important results of not well-known authors (Pardies, Parent,…) and, obviously, the fundamental discoveries of Euler. Almost incidentally, he takes up questions that alone should deserve the attention of historians: the discovery of the current form of the Second Law of Motion, the meaning of d’Alembert's Principle and the controversy on the vibrating string. Thanks to the wealth of Swiss libraries, Truesdell enriched his account with data collected from the notebooks and unpublished correspondence of Euler and the Berrnoullis. 
The Rational Mechanics of Flexible or Elastic Bodies is not an easy book to read. It is a book of immense scholarship and refined sensibility, and may be 'above the heads' of many wouldbe readers. But for those who are up to it, this is probably the best single guide to the methods and the achievements of early modern mechanics.

A Historical Evaluation
Apart from their value as historical monographs, Truesdell’s "Introductions" set the stage for a new methodology in the history of the exact sciences.
Up until that time there were very few historical works that detailed the technical history of the mathematical physics of the eighteenth century, as can be seen by leafing through the old issues of Isis and the Notes and Records of the Royal Society. Truesdell took the aristocratic style of the Entwicklungen nach oscillirenden Funktionen und Integration der Differentialgleichungen der mathematischen Physik of H. Burkhardt and made it the model for any serious research in the history of mechanics.
Truesdell was the first historian who accurately described Book II of Newton's Principia, Daniel Bernoulli's Hydrodynamica and Johann Bernoulli's Hydraulica. Back then, they were still unread. It would be as if historians of science had to wait until 1955 to know something about the content of Galileo’s Il Saggiatore and Huygens' Horologium Oscillatorium.
Truesdell's “Introductions” changed the way of publishing the complete works of the great scientists of the past. Until the end of the nineteenth century these were simply reprints of the original texts, sometimes tacitly corrected; see, for example, the Oeuvres of Lagrange, Laplace, Fourier and Cauchy. Things began to change at the beginning of the twentieth century, when a few competent historians added historical introductions to each volume of the editions entrusted to them: Huygens' Oeuvres and Euler's Opera are the best examples. With his work on eighteenth-century mechanics, Truesdell demonstrated that it was necessary to broaden the scope of the research to the predecessors and the contemporaries alike. Today this idea has been generally accepted, and the introductions to the modern collected works of Euler, the Bernoullis, d'Alembert and Newton are ground-breaking researches that illuminates a period in history. 
While Truesdell had reached a new level of sophistication for the 1950s, today it is clear that some of his ideas needs to be clarified, extended and complemented with supplementary research. Truesdell is not entirely to be blamed for these problems, for he lacked a solid ground on which to stand. At that time there were no scientific biographies of mathematicians and physicist of the eighteenth century, and very little of their correspondence had been published. We recall, for example, that there were no reliable editions of Newton's works and correspondence, and most of his manuscripts still lay unopened. Truesdell wrote of Euler’s mathematical theory of fluids without being able to rely on any serious history of hydraulics, and meticulously described the mathematical details of Euler’s works on elasticity without the help of a history of statics in the eighteenth century. Despite these limitations, Truesdell's "Introductions" are still accepted today as the basis of all subsequent research.

The Classical Field Theories (1960) and The Non-Linear Field Theories of Mechanics (1965)
While Truesdell was busy writing on the three introductions to Euler’s Opera Omnia, he was also working, in collaboration with R. Toupin, on an article for the new Handbuch der Physik, the "Classical Field Theories”. This "article" is a work of about 500 pages which describes pretty much everything that had been produced up to the 1940s in continuum mechanics. Even today it remains unequalled as a reference to classic results.
In the tradition of the Encyklopädie der mathematischen Wissenschaften, Truesdell provides correct attributions for each result. The bibliography is impressive: the first author cited is Robert Hooke (1679), followed by Newton and Euler, arriving, after fifty pages, to 1951. Clearly the “Classical Field Theories" was a direct descendant of the encyclopedic articles that Truesdell had written in the late 1940s. With the evolution of his interests, he had further developed the aspects relating to the history of mechanics:
We have tried to supply full and correct attributions, not only for historical perspective but also in plain justice. […] Our subject is largely the creation of Euler and Cauchy. If we present their results in forms often very different from the original, in return we have included many of their discoveries that have not previously found a place in expositions. Not only will these names be the most frequently encountered, but also their appearances are at the crucial theorems and definitions. Next such as Stokes, Helmholtz, Kirchhoff, Kelvin, Maxwell and Huguniot. In the twentieth century, Hadamard and Hilbert continued and deepened the tradition. That no one later name is frequently cited does not indicate that the subject is dead. Rather, after a generation of quiescence, in very recent years it has experienced a revival in a form more compact and general, and, we believe, closer to nature.
The notes, comments and bibliographic references of the "Classical Field Theories" outline the development of continuum mechanics over the course of about two and a half centuries. 
In 1965 Truesdell, in collaboration with W. Noll, published a second article for the Handbuch der Physik, the “Non-Linear Field Theories of Mechanics”, dedicated to modern theories of continuum mechanics. This was the main reference for two or three generations of scientists working in a rapidly developing field. Like his predecessor, the “Non-Linear Field Theories” too is enriched by an impressive bibliography, and is destined to become a valuable aid for future historians.

The essays in the History of Mechanics (1968)
The "Introductions" to Euler had revealed the structure of the development of mechanics in century after Newton. However, only the history of fluid dynamics and the theory of elasticity they had been thoroughly examined; everything else was essentially an uncharted territory. Obviously, it would take the work of several historians over the course of many years to complete the task. 
During the 1960s, Truesdell outlined what remained to be done. He published several essays on the history of mechanics, elegantly written and accessible to a wide audience of readers. In 1968 they were collected under the title Essays in the History of Mechanics. This is his best-known work, certainly the most cited. The Essays is not a research book, but rather a kind of programmatic manifesto of a new historiography of the exact sciences.
The book's central essay is “A Program Toward Rediscovering the Rational Mechanics of the Age of Reason”, originally published in 1960. Essentially, it was an appeal to historians of science:
To answer these questions, I call for a program of rediscovery of the Rational Mechanics of the Age of Reason. This note is an outline of such a program. Parts have been completed and published as my three essays [on Euler]; parts remain scarcely charted. Rather than let the skeleton of the whole wait to be discerned under the flesh built upon it when, years hence, the work is finished, I have thought others might be led to share the pleasure of discovery for themselves if now I give out this short survey of the riches waiting for anyone willing to take down the dusty volumes and read them, as they have not been read for two centuries, with consequences of the mathematics.
In the eighteenth century described by Truesdell, Newton's Principia opens up problems to be solved by the next three generations of mathematicians, Varignon is hardly mentioned and Jacob Bernoulli is considered almost as influential as Newton. In opposition to Mach and Dugas, here Lagrange's Méchanique analitique plays a secondary role:
Like the first half of Book I of Newton's Principia, it is largely a retrospective work, ordering within a single formal scheme a selection from the discoveries of the immediately preceding generations. In the main, the results presented are slight generalizations of some of Euler's, set within Lagrange's severely formal style. Unlike Book I of the Principia, it contains no major discovery, although some of its original contents are taken from earlier original works by Lagrange himself. Neither is there anything corresponding to Book II and III Book I of the Principia, nothing looking forward rather than backward, and no comparison of theoretical and observed values. Mechanics appear as a physically closed subject, a branch of the theory of differential equations.
These claims are substantially correct, but they do not do justice to Lagrange. It is true, for example, that Lagrange reduces theoretical mechanics to the study of certain classes of equations differentials, but that was precisely his goal; from there, an entire branch of mathematics was born.
The following essay, entitled “Reactions of Late Baroque Mechanics to Success, Error, and Failure in Newton's Principia", can be considered a follow up to the previous one. What was actually the influence of the Principia on the mechanics of the eighteenth century? Truesdell has in mind Book II, dedicated to fluid mechanics:
With its bewildering alternation of mathematical proof, brilliant hypothesis, as well guessing, bluff and plain error, Book II has long been praised, in whole or in part, and praised justly, as affording the greatest sign of Newton's genius. To the geometers of the day, it offered an immediate challenge: to correct the errors, to replace the guesswork by clear hypotheses, to embed the hypotheses at their just station in a rational mechanics, to brush away the bluff by mathematical proof, to create new concepts so as to succeed where Newton had failed. It is not an exaggeration to say that rational mechanics, and hence mathematical physics as a whole and the general picture of nature accepted today, grew from the challenge as it was accepted by the Basel school of mathematicians: the three Bernoullis and Euler, on the basis of whose work Lagrange, Fourier, Poisson, Navier, Cauchy, Green, Stokes, Kelvin, Helmholtz, Kirchhoff, Maxwell and Gibbs constructed what is now called classical physics.
In the two subsequent essays Truesdell focuses the attention on two fundamental theorems of rational mechanics. The very idea of ​​studying the history of a theorem of mathematical physics was relatively new at the time. In "The Creation and Unfolding of the Concept of Stress" he examines the evolution of the concept of contact force, from the first hints in the works of Stevin and Galilei the seventeenth century to the definitive formulation given by Cauchy in the 1820s. Truesdell is interested in the conceptual development of a mathematical idea:
My purpose here is to mark the conceptual stages we now recognize as predecessors of Cauchy's great and unifying idea. Few of these were cited by Cauchy himself, and perhaps he did not know of them. […] In the history of ideas, the progress of the uncited central or common domain is just as important as the cited points of departure from it, or perhaps more so. It is mainly this progress I wish to trace.
The next essay, "Whence the Law of moment of Momentum?" traces the origin of the theorem of elementary mechanics that derives the law of moments of forces from the second law of Newton:
Here I wish to open a historical analysis. […] I have not yet uncovered all I seek on either question, but it seems to me that research on the history of rational mechanics is still enough of a novelty that an example of method during the working, before it is yet known which conjectures are right and which leads point to ends, might not be unwelcome in a circle accustomed to studies of an older, less formally mathematical philosophy.
The other essays in the volume deal with the history of the kinetic theory of gases and with some aspects of the history of modern mechanics. Even from this short summary, the programmatic character of the Essays is quite clear. 

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