Salviatus who speaks for Galileo,
Sagredus who represents an intelligent layman, and
Simplicius who represents an unthinking believer in Aristotle's system:
Salviatus: ... Aristotle says that is a most convincing argument of the Earth's immobility to see that projectiles thrown or shot upright return perpendicularly by the same line unto the same place from whence they were shot or thrown. And this holds true, although the motion be of a very great height. So that hither may be referred the argument taken from a shot fired directly upwards from a cannon, as also that other used by Aristotle and Ptolemy, of the heavy bodies that, falling from on high, are observed to descend by a direct and perpendicular line to the surface of the Earth. Now, that I may begin to untie these knots, I demand this of Simplicius: in case one should deny to Ptolemy and Aristotle that weights in falling freely from on high descend by a right and perpendicular line, that is, directly to the centre, what means would he use to prove it?
Simplicius: The means of the senses, which assure us that that tower or other altitude is upright and perpendicular, and show us that that stone slides along the wall, without inclining a hair's breadth to one side or another, and lights on the ground just under the place from where it was let fall.
Salviatus: But if it should happen that the terrestrial globe did move round, and consequently carry the tower also along with it, and that the stone did then also graze and slide along the side of the tower, what must its motion be then?
Simplicius: In this case we may rather say its motions, for it would have one wherewith to descend from the top to the bottom and should then have another to follow the course of the said tower.
Salviatus: So that its motion should be compounded of two; from this it would follow that the stone would no longer describe that simple straight and perpendicular line but one transverse and perhaps not straight.
Simplicius: I can say nothing of its nonrectitude, but this I know very well: that it would of necessity be transverse.
Salviatus: You see then that, merely observing the falling stone to glide along the tower, you cannot certainly affirm that it describes a line which is straight and perpendicular unless you first suppose that the Earth stands still.
Simplicius: True; for, if the Earth should move, the stone's motion would be transverse and not perpendicular.
Salviatus: Aristotle's defence then consists in the impossibility, or at least in his esteeming it an impossibility, that the stone should move with a motion mixed of right and circular. For, if he did not hold it impossible that the stone could move at once to the centre and about the centre, he would have understood that it might come to pass that the falling stone might in its descent graze the tower as well when it moved as when it stood still. Consequently, he ought to have perceived that from this grazing nothing could be inferred touching the mobility or immobility of the Earth. But this does not any way excuse Aristotle; because he ought to have expressed it, if he had had such a notion, it being so material a part of his argument. Also because it cannot be said that such an effect is impossible or that Aristotle did esteem it so. The first cannot be affirmed, for by and by I shall show that it is not only possible but necessary; nor can the second be averred, for Aristotle himself grants that fire moves naturally in a right line, and moves about with the diurnal motion, imparted by the heavens to the whole element of fire and the greater part of the upper air. If therefore he held it possible to mix the straight motion upwards with the circular communicated to the fire and air from the concave of the sphere of the Moon, much less ought he to account impossible the mixture of the straight motion of the stone downwards with the circular which we presuppose natural to the whole terrestrial globe, of which the stone is a part.
Here Salviatus has shown that Aristotle's 'proof' that the earth is stationary is invalid. However, later in the Dialogue, he proposes an experiment to verify his claims:
Salviatus: Your making a greater scruple of this than of the other instances depends, if I mistake not, upon the birds being animated, and thereby enabled to use their strength at pleasure against the primary motion inbred in terrestrial bodies. For example, we see them fly upwards, a thing which ought to be altogether impossible for heavy bodies; whereas, when dead, they can only fall downwards. And therefore you hold that the reasons that hold for all kinds of missiles above named cannot hold for birds. Now this is very true, and, because it is true, therefore we see live birds behaving different from falling bodies. If from the top of a tower you let fall a dead bird and a live one, the dead bird shall do the same that a stone does, that is, it shall first follow the general diurnal motion, and then the motion of descent, just like a stone. But if the bird let fall be alive, what shall hinder it (there ever remaining in it the diurnal motion) from soaring by help of its wings to what point of the horizon it shall please? And this new motion, as being peculiar to the bird, and not participated in by us, must of necessity be visible to us. In short, the effect of the flight of birds differs from the missiles shot or thrown to any part of the world in nothing, except that the missiles are moved by an external projector, and the birds by an internal principle.
For a final proof of the nullity of all the experiments before alleged, I conceive it now a convenient time and place to demonstrate a way how to make an exact trial of them all. Shut yourself up with some friend in the largest room below decks of some large ship and there procure gnats, flies, and such other small winged creatures. Also get a great tub full of water and within it put certain fishes; let also a certain bottle be hung up, which drop by drop lets forth its water into another narrow-necked bottle placed underneath. Then, the ship lying still, observe how those small winged animals fly with like velocity towards all parts of the room; how the fishes swim indifferently towards all sides; and how the distilling drops all fall into the bottle placed underneath. And casting anything towards your friend, you need not throw it with more force one way than another, provided the distances be equal; and jumping broad, you will reach as far one way as another. Having observed all these particulars, though no man doubts that, so long as the vessel stands still, they ought to take place in this manner, make the ship move with what velocity you please, so long as the motion is uniform and not fluctuating this way and that. You shall not be able to discern the least alteration in all the forenamed effects, nor can you gather by any of them whether the ship moves or stands still. Of this correspondence of effects the cause is that the ship's motion is common to all the things contained in it and to the air also; I mean if those things be shut up in the room; but in case those things were above deck in the open air, and not obliged to follow the course of the ship, differences more or less notable would be observed in some of the forenamed effects, and there is no doubt but that smoke would stay behind as much as the air itself; the flies also and the gnats, being hindered by the air, would not be able to follow the motion of the ship, if they were separated at any distance from it; but keeping near thereto, because the ship itself, as being an anfractuous structure, carries along with it part of its nearest air, they would follow the ship without any pains or difficulty. For the like reason we see sometimes, in riding post, that the troublesome horseflies do follow the horses flying sometimes to one, sometimes to another, part of the body. But in the falling drops the difference would be very small and in the jumps and projections of grave bodies altogether imperceptible.
Sagredus: Though it came not into my thoughts to make trial of these observations when I was at sea, yet I am confident that they will succeed in the manner that you have related. In confirmation of this, I remember that being in my cabin I have wondered a hundred times whether the ship moved or stood still; and sometimes I have imagined that it moved one way, when it moved the other way. I am therefore satisfied and convinced of the nullity of all those experiments that have been produced in proof of the negative part.
There now remains the objection founded upon that which experience shows us, namely, that a swift whirling about has a faculty to extrude and disperse the matters adherent to the machine that turns round. On this fact many based the opinion, and Ptolemy amongst the rest, that, if the Earth should turn round with such great velocity, the stones and creatures upon it should be tossed into the sky and that there could not be a mortar strong enough to fasten buildings to their foundations so that they should not suffer a like extrusion. ...