Consummation by Newton

Kepler's laws connected celestial phenomena to a certain degree, before Newton's theory was propounded: but they left this imperfection,—that phenomena which ranked under two of these laws had no necessary connection with each other. Newton brought under one head all the three classes of general facts, uniting them in one more general still; and since that time we have been able to perceive exactly the relation between any two of the phenomena which are all connected with the common theory. As far as we can see, there is nothing more to gain in this direction.

Statical considerations

We have seen what this great conception is in itself. We have now to observe its application to the mathematical explanation of celestial phenomena, and the perfecting of their study. For this purpose, we will recur to our former division of subjects, and contemplate the phenomena of planets as immoveable first, and of planets in motion afterwards; the statical phenomena first, and the dynamical afterwards.

First method of inquiry into masses

To know the mutual gravitation of the heavenly bodies, we must know their masses. Such knowledge once appeared inaccessible from its very nature; but the Newtonian theory has put it within our power, and furnished us with a wholly new set of ideas about these bodies. There are three ways in which the inquiry has been prosecuted, all differing from each other, both in generality and in simplicity. The first method, the most difficult.