have done.  Let him also write a “Mecanique Celeste,” let him employ those consummate talents which he possesses in developing his noble subject to the utmost.  Let him utilise every result that I or any other mathematician have arrived at, but not trouble himself unduly with unimportant historical details as to who discovered this, and who discovered that; let him produce such a work as he could write, and I shall heartily welcome it as a splendid contribution to our science.”  Certain it is that Laplace and Lagrange continued the best of friends, and on the death of the latter it was Laplace who was summoned to deliver the funeral oration at the grave of his great rival.

The investigations of Laplace are, generally speaking, of too technical a character to make it possible to set forth any account of them in such a work as the present.  He did publish, however, one treatise, called the “Systeme du Monde,” in which, without introducing mathematical symbols, he was able to give a general account of the theories of the celestial movements, and of the discoveries to which he and others had been led.  In this work the great French astronomer sketched for the first time that remarkable doctrine by which his name is probably most generally known to those readers of astronomical books who are not specially mathematicians.  It is in the “Systeme du Monde” that Laplace laid down the principles of the Nebular Theory which, in modern days, has been generally accepted by those philosophers who are competent to judge, as substantially a correct expression of a great historical fact.

[Plate:  Laplace.]

The Nebular Theory gives a physical account of the origin of the solar system, consisting of the sun in the centre, with the planets and their attendant satellites.  Laplace perceived the significance of the fact that all the planets revolved in the same direction around the sun; he noticed also that the movements of rotation of the planets on their axes were performed in the same direction as that in which a planet revolves around the sun; he saw that the orbits of the satellites, so far at least as he knew them, revolved around their primaries also in the same direction.  Nor did it escape his attention that the sun itself rotated on its axis in the same sense.  His philosophical mind was led to reflect that such a remarkable unanimity in the direction of the movements in the solar system demanded some special explanation.  It would have been in the highest degree improbable that there should have been this unanimity unless there had been some physical reason to account for it.  To appreciate the argument let us first concentrate our attention on three particular bodies, namely the earth, the sun, and the moon.  First the earth revolves around the sun in a certain direction, and the earth also rotates on its axis.  The direction in which the earth turns in accordance with this latter movement might have been that in which it revolves around the