since bodies fall more readily in vacuo than in air. There is an important distinction between kinematic and dynamic reactions. The first, or primary reactions, not only can but always do take place at a distance, and in view of the undoubted fact any discussion of the possibility is idle in the extreme. But it is very different in the case of secondary or dynamic effects. There is, strictly speaking, no reaction between the bodies concerned. They have no attraction for each other. They merely happen to get in each other's way when one or both are undergoing some kinematic reaction. A falling body may meet a second body thrown upward, and the path of each will be notably altered, but the reaction will be purely dynamic, and as such absolutely dependent on contact. These secondary reactions happen to be the ones with which we are the more familiar and upon which we have allowed our minds to play the more freely. The old question, "Can a body act where it is not?" is only applicable to dynamic reactions, but is not at all pertinent in the discussion of kinematic reactions. In the first case, no one doubts the inability of a body to push or pull another without actual contact. In the second case, the facts leave no room for discussion.
The concluding book, on gravitation, is naturally the pièce de resistance of the whole, and seems to us of high interest and importance. The other books lead up to it very cleverly. Newton himself, as we have seen, had no theory of gravitation, or at least was extremely careful not to publish it if he ever had one. Later disciples, however, have been less cautious, and the Newtonian view has been erected into something of a theory. This assumes that all matter attracts all matter, and this quite independently of its state of excitation. The intensity of the attraction has been formulated in the well-known law that gravity is proportional to the mass and inversely proportional to the square of the distance. But mass, as Newton used the term, is synonymous with weight, and weight is simply the measure of gravity, so that this fine-sounding phrase amounts only to this, that gravity is proportional to gravity, a statement which can not be said to materially help on the cause of truth. The Newtonian view assumes the constancy of mass or weight, but does so without the least experimental verification, and indeed in the teeth of much contrary evidence. It is true that modern physicists distinguish between mass and weight, making the former term stand for amount of matter (whatever that may mean) and the latter for the measure of gravity. But in this sense mass is a purely metaphysical quantity.
Now, there are two classic experiments for determining the density of the earth that seem at first sight to establish the current view that all matter attracts all matter. These are the
