14. One and the same body is always of one and the same magnitude. For seeing a body and the magnitude and place thereof cannot be comprehended in the mind otherwise than as they are coincident, if any body be understood to be at rest, that is, to remain in the same place during some time, and the magnitude thereof be in one part of that time greater, and in another part less, that body's place, which is one and the same, will be coincident sometimes with greater, sometimes with less magnitude, that is, the same place will be greater and less than itself, which is impossible. But there would be no need at all of demonstrating a thing that is in itself so manifest, if there were not some, whose opinion concerning bodies and their magnitudes is, that a body may exist separated from its magnitude, and have greater or less magnitude bestowed upon it, making use of this principle for the explication of the nature of _rarum_ and _densum_. [Sidenote: Velocity, what it is.] 15. Motion, in as much as a certain length may in a certain time be transmitted by it, is called VELOCITY or _swiftness_: &c. For though _swift_ be very often understood with relation to _slower_ or _less swift_, as great is in respect of less, yet nevertheless, as magnitude is by philosophers taken absolutely for extension, so also _velocity_ or _swiftness_ may be put absolutely for motion according to length. [Sidenote: Equal, greater, and less, in times, what they are.] 16. Many motions are said to be made in equal times, when every one of them begins and ends together with some other motion, or if it had begun together, would also have ended together with the same. For time, which is a phantasm of motion, cannot be reckoned but by some exposed motion; as in dials by the motion of the sun or of the hand; and if two or more motions begin and end with this motion, they are said to be made in equal times; from whence also it is easy to understand what it is to be moved in greater or longer time, and in less time or not so long; namely, that that is longer moved, which beginning with another, ends later; or ending together, began sooner. [Sidenote: Equal, greater, and less, in velocity, what.] 17. Motions are said to be equally swift, when equal lengths are transmitted in equal times; and greater swiftness is that, wherein greater length is passed in equal time, or equal length in less time. Also that swiftness by which equal lengths are passed in equal parts of time, is called _uniform_ swiftness or motion; and of motions _not uniform_, such as become swifter or slower by equal increasings or decreasings in equal parts of time, are said to be _accelerated_ or _retarded uniformly_. [Sidenote: Equal, greater, and less, in motion, what.]