system in relation to each other and call the mass of the one m, and of the other m2, and the distance separating them r, then we have found that the force acting between these bodies must be proportional to these masses and inversely proportional to r2.

This is Newton's law of gravitation. Although deduced from slightly faulty premises, it has been strengthened almost to full substantiation by the very remarkable results achieved with it in accounting for most complicated phenomena of motion in the solar system.

It should be observed, however, that the definition of mass introduced in the statement of the law is entirely arbitrary. But to this point we shall return later.

Having studied the law of force in the solar system, we shall take up the question of the law of force prevailing outside of our solar system in the case of double stars.

Observation has shown that when stars revolve about one another they describe ellipses. But as their motion, as seen from the earth, is projected on a plane perpendicular to the line of vision, it is not possible to say whether or not the observed ellipse is the real orbit. Since the law of areas is found to be satisfied, by observation, by one star with reference to the other as primary, double stars move under the action of central forces. The position of the primary star, however, is in general not found to be at the focus or at the center of the ellipse, but lies anywhere within it.

Applying under these conditions the usual mathematical test for the nature of the law, mathematical difficulties are encountered in establishing a law of force which is independent of the angle 6, the orientation. When the law is arbitrarily assumed to be independent of the orientation, as was found to be the case in the solar system, then two possibilities arise, namely, either that the force is in direct proportion to the distance r or that the Newtonian law applies. It can be shown, however, that when, in the case of an elliptic orbit, the force is proportional to r, then the

primary star must be in the center of the ellipse. As this has never been found to be the case, the only alternative is the Newtonian law.

It should be observed that all demonstrations regarding the law of gravitation depend upon observational material which is imperfect in accuracy and completeness. But with every advance in observation which has indicated a possible departure from the rigor of the law of gravitation, there has come a more precise mathematical application of the law which removed apparent discrepancies. The law has been applied to account for past and to predict future motions of hundreds of comets and asteroids, and has not been found wanting. Experience has shown that the solution for such differences of observed motion as still remain to be accounted for, as in the case of the moon and the planet Mercury, will be found in some form under the law of gravitation. The exceptions existing are of two classes. One class concerns bodies of finite mass. The other class of exceptions includes the motions of small particles, as in the case of the motion of particles in comets' tails. Possibly the irregularities in the motion of Encke's comet may fall in this class.

On the other hand, remarkable verification has come from the study of spectroscopic binary stars. In a visual binary system, by the application of the law of gravitation, it is possible to determine the mass of the system and the actual linear dimensions of the orbit, including the component of the velocity in the line of vision, provided the distance of the system from the sun has been independently derived by observation. By the application of the spectroscope, the component of velocity in the line of vision may be directly measured on the basis of the wave theory of light. When this has been done for a visual binary, then the distance of the binary system from the sun may be calculated and in the few cases where this has been attempted, a complete agreement has been found between the values of the distance of the system derived directly

from instrumental measures of position and those derived from the spectroscopic observations. This indicates that the mathematical formulation of the theory of gravitation and of the wave theory of light is sufficient to account for the phenomena to which reference has been made.

If we disregard the defects in our demonstration that arise from the imperfection and the incompleteness of our observational material, then we may accept Newton's law as universal for finite masses, however small. The principal difficulty lies in its application to very small particles, as the particles in comets' tails. In spite of many theories, the correctness of which is established, such as involve light pressure, electrical phenomena, et cetera, which have been rigorously applied in connection with Newton's law, no wholly satisfactory representation of the observed motions of particles in comets' tails has been accomplished.

The difficulties frequently referred to, which concern the representation of the motion of a number of bodies of finite mass acting on each other, are purely mathematical and arise from the imperfection of our present mathematical methods.

With reference to the law as applying to small particles, the suggestion presents itself that the identification by Newton of what we have chosen to call the force-index of a central body, solely with the quantity of matter or mass, may be too specific, particularly in the light of recent physical investigations of electricity as affecting the mass of particles. The suggestion has been advanced by Kapteyn that the force acting between the particles of nebulae in their earliest stages is solely electrical and that the Newtonian law of force develops with the mass as the particles combine in later stages of development.

JOHN MORTON ESHLEMAN-A TRIBUTE*

GUY C. EARL

Mr. President, Members of the Faculty, and Students of the University:

Upon this occasion I desire to avoid undue eulogy, I wish to preserve the true perspective and to indulge in no extravagance of commendation in speaking of Jack Eshleman. He was my friend. I knew him intimately and I knew that extravagant indulgence in the language of adulation and unrestrained eulogy would be most offensive to him. It seemed, therefore, the most fitting thing for me at this time to tell you the story in brief of Jack Eshleman's heroic life, and let the facts themselves speak his eulogy.

A year or two ago, in a conversation that took a most intimate turn, Jack, as we loved to call him, briefly told me. of some of the difficulties with which he had had to contend. While telling me, his face was at times very sad and his voice husky. I am going to tell you the story, even if in doing so I am shaken by the pathos of it all. He told me that his father was a soldier in the Northern army in the Civil War, and, shot to pieces, was taken in an almost dying condition to a Northern hospital. There, through the tender nursing of the daughter of the clergyman who was in charge of the hospital, the father of John Morton Eshleman was made to live. He fell in love with this gracious lady who had kept off the king of terrors and the two were married. They lived in Southern Illinois. Several children were born, the youngest of whom we are thinking of today. Jack told me of the limitations and poverty of his home. His

* Address delivered by Guy C. Earl, March 17, 1916, at the Memorial Meeting at the University of California.

father never fully recovered of his wounds and was incapacitated thereby from labor. The mother, on the small compensation of twenty or thirty dollars a month, taught school for a few months each year in her various struggles to provide the food supply, all insufficient for the children. During the last five years of his life, the father was utterly bedridden. Jack told me that frequently the cupboard was absolutely bare, and that there was not a mouthful of food in the house, and that often as a child he had hunger days. Many a time he had seen the neighbors bring in the food necessary to keep the family from starving.

When the father died, Jack was given a home for one year with an uncle in New York. He spent the year as a freshman in the high school. Returning to Illinois, he went to work. But that frail body, born into a house of privation, insufficiently cared for and nourished from the very beginning, became a prey to dread disease, and this youth found himself, at an age when other youths are in the full blossom of buoyant and vigorous bodily health, suddenly companioned by the White Plague that began to gnaw at his vitals. The County Clerk of the county where Jack lived had been similarly afflicted and had gone to Colton, California. Jack thought of but one word-Colton -and getting enough money to pay the poorest passage by train to that place, he fled to it. He sat up on the car all the way because he did not have money to buy a bed. Arriving at Colton, he went to work in a fruit orchard during the harvest, and then, with pick and shovel, he went to work for the Southern Pacific Company as a railroad track hand. After a while he became a dishwasher and scullion to a gang of railroad men who lived in one of the maintenance-of-the-way cars. He then became steward on this car and for two years he lived in the car in that capacity, at a salary of thirty or forty dollars a month, in the roughest environment, in railroad building. Did he yield to these obstacles and settle down to the life of those about him?