_A._ It is evident; as also that every point of the sun’s body shall do the like. And not only so, but also if one end describe any other figure, all the other points of the body shall describe like and equal figures to it. _B._ You see by this, that this parallel motion is compounded of two motions, one circular upon the superficies of a sphere, the other a straight motion from the centre to every point of the same superficies, and beyond it. _A._ I see it. _B._ It follows hence, that the sun by this motion must every way repel the air; and since there is no empty place for retiring, the air must turn about in a circular stream; but slower or swifter according as it is more or less remote from the sun; and that according to the nature of fluids, the particles of the air must continually change place with one another; and also that the stream of the air shall be the contrary way to that of the motion, for else the air cannot be repelled. _A._ All this is certain. _B._ Well; then if you suppose the globe of the earth to be in this stream which is made by the motion of the sun’s body from east to west, the stream of air wherein is the earth’s annual motion will be from west to east. _A._ It is certain. _B._ Well. Then if you suppose the globe of the earth, whose circle is moved annually, to be _l m_, the stream of the air without the ecliptic falling upon the superficies of the earth _l m_ without the ecliptic, being slower, and the stream that falleth within swifter, the earth shall be turned upon its own centre proportionally to the greatness of the circles; and consequently their diameters shall be parallel; as also are other straight lines correspondent. _A._ I deny not but the streams are as you say; and confess that the proportion of the swiftness without, is to the swiftness within, as the sun’s ecliptic to the ecliptic of the earth; that is to say, as the angle H A B to the angle _m_ B _b_. And I like your argument the better, because it is drawn from Copernicus his foundation. I mean the compounded motion of straight and circular. _B._ I think I shall not offer you many demonstrations of physical conclusions that are not derived from the motions supposed or proved by Copernicus. For those conclusions in natural philosophy I most suspect of falshood, which require most variety of suppositions for their demonstrations. _A._ The next thing I would know, is how great or little you suppose that circle _a_ M? _B._ I suppose it less than you can make it: for there appears in the sun no such motion sensible. It is the first endeavour of the sun’s motion. But for all that, as small as the circle is, the motion may be as swift, and of as great strength as it is possible to be named. It is but a kind of trembling that necessarily happeneth in those bodies, which with great resistance press upon one another. _A._ I understand now from what cause proceedeth the annual motion. Is the sun the cause also of the diurnal motion?