This theory of course seems liable to several objections. As noticed by our very ingenious friend Porzio, the compressive force of air has one and the same vigour and energy. Consequently, it will always produce the same effect. Therefore, whenever its action is exerted against two unequal resistances, a greater and more considerable operation will occur in the less resistant subject than in the other. Let us consider two fluids of unequal specific weights, quicksilver and fresh water. A drop of mercury is fourteen times heavier than a drop of water of the same volume. Since the external force of the air compressing these two fluids has always the same vigour, it will not be able to make spherical a drop of mercury as big as another drop of water. Since the mercury is heavier and thus more resistant, it requires a greater compressive force than water which is lighter. Therefore, a drop of mercury which must be rounded by the same energy of air has to be a fourteenth of the magnitude of a drop of water also rounded spherically. Consequently, air cannot possibly make a sphere of mercury larger than one of water. But actually this is what occurs, as taught by experience: the drops of mercury which are rounded spherically and displaced over a flat table do not appear to be smaller than the drops of water which usually become round on cabbage leaves. Consequently, the compressive force of air is not the actual cause of the sphericity of water or mercury.

Moreover, if the energy of gravity or the elastic force of air is what swells the drops of fluid spherically by compressing them from everywhere, the drops which are surrounded by very rare air or air expanded ad infinitum should be compressed less than those in abundant and maximally condensed air. Consequently, after vacuum has been achieved in a tube of Torricelli where no, or at most very few, particles of air are found, the droplets of water sustained by cabbage leaves should be lower and more squeezed or at least they should form reduced and very small globules as compared with those in air considerably condensed in a vessel by way of a pair of bellows or of a pump. But this does not occur. Drops of water bulging equally spherically are suspended and raised to the same height and magnitude in the vacuum of Torricelli by the rarest air as by air considerably dense and concentrated. We experimented this at the Accademia del Cimento.

If I am right, it will be possible to prove by an obvious demonstration that the drops of water are not made spherical by the external compressive force of air. If indeed we ponder why all the water element is wrapped spherically round the centre of an elementary system, we shall understand that this occurs because all the parts of water have the force to move straight towards the centre of the earth and that such motive force in the same homogeneous body of water has not always the same magnitude unless the external parts are equally distant from the centre of the earth.

Let point E be the centre of the terrestrial globe and let us suppose that the water ABCD is unequally distant from the centre E (Table 5.​2, Fig. 7). For example, the mountain of water MAK is elevated above the remainder of the spherical surface BCD by an external force. If the motive force impelling downwards towards the centre E had the same energy in the water A and in B, the water A above could not be lowered by expelling and overcoming the resistance of the water B or D since an equal power cannot prevail over its equal. Therefore, the elevated water MAK must necessarily have a greater compressive force than the water B. This is very obvious since the mass of water EA which is higher, more abundant and thus heavier will overcome the resistance of the smaller mass of the lower water EB. Consequently, the actual cause why the water element is wrapped spherically round the centre of the earth is that the parts of water more elevated from the centre of the earth and in continuity with the others have a greater compressive force than the other parts less remote from this centre.

Let us suppose that the mercury ABCD has no force to unite, i.e. it has no gravity (Table 5.​2, Fig. 7). Obviously, if this mercury was set around the centre E of all the elementary region, it would not become spherical spontaneously but would retain the hillock MAK. Then let us suppose that the mercury is surrounded by a sphere of air FGHI and that this ambient fluid has some gravity and a motive principle towards the centre E. Thus, the mass of mercury ABCD is compressed from everywhere by the ambient fluid FGHI. This fluid is homogeneous, i.e. it is uniformly heavy. I claim that there is no reason for the ambient fluid to round up the mercury ABCD spherically. The ambient fluid indeed compresses the surrounded fluid by exactly as much as is required by the magnitude of its gravity. The ratio of the gravity of the fluid FA to the gravity of the other part BG is equal to the ratio of the height or mass of the fluid FA to those of the fluid BG (since the fluid is supposed to be homogeneous). The mass of fluid FA is smaller than GB. Consequently, FA weighs less and thus compresses the subjacent fluid AE less than the fluid GB compresses the underlying fluid EB. But the smaller compressive force of the ambient fluid FA cannot possibly impel downwards and squeeze the fluid hillock MAK without expelling upwards the weaker part EB of this fluid. This cannot be expelled upwards unless the force of gravity of all the fluid GB falls. Consequently, the greater force of gravity of all the water BG would have to be overcome by the power of the smaller gravity FA, which is impossible. Thus, the external force of compression of the air or of any other fluid could not achieve this spherical bulging which we observe in drops of mercury and water if these drops were deprived of gravity and were set at the centre of an elementary region.

It remains to demonstrate the same phenomenon in the drops of water present at the surface of our earth. Let the drop of water ABCD be suspended by a thread GA so as not to touch the floor VX (Table 5.​2, Fig. 8). The suspended drop is supposed to be squeezed from everywhere by the ocean of air RS and constricted as if it were by a clamp, from above by the columns of air GA, from the sides by the cylinders GH and SD, and from below by the columns of air RVB and SXI. I claim that the drop ABCD is not rounded up spherically by the ocean of air. The parts AHCD of the drop of water are completely deprived of a motive force which would carry them to the centre of the drop since the upper part A is pulled upwards by the thread GA, the lower part C strives downwards because it is heavy, the two opposite parts A and C shun each other and, therefore, attempt rather at receding from the centre of the drop than at moving to it and uniting with it. The adjacent parts H and D either exert a force of gravity or they do not. They never strive instinctively towards the centre of the drop in a horizontal movement. Thus if the centre of the drop ABCD is conceived as if it were the centre of an elementary system, the parts of the drop could be considered as not heavy. With this supposition, an elevated eminence or tit of water H is imagined as being added to a side of the drop. Then the ocean of air RS will compress with an equal energy not only the above surface A, but also the lateral H, D and lower aspects B, C, by way of its gravity and of its elastic virtue. Consequently, we have a case similar to the one supposed in the previous proposition, i.e. the drop ABCD the parts of which do not attempt to unite and are not displaced spontaneously towards the centre of the drop, is compressed by the air from everywhere with equal forces. Therefore, the eminence H cannot possibly be squeezed which, as said above, requires a greater compressive force at H than at D. Thus, this drop cannot be rounded up exactly nor acquire a spherical bulge.

The same is verified in droplets of water resting on the floor. At least their lateral parts H and D lack a horizontal motive force which would carry them towards the centre of the drop. The eminence H cannot be squeezed by the air GH since the force of GH is not greater than the compressive force of the air SD. From this it is concluded that the compression by the fluid air RSXV in no way creates the sphericity of the droplets of water. Therefore, it must be admitted that this sphericity results from a by far different cause.

It must be seen whether these droplets of mercury or of water are suspended and rounded up by an intrinsic and natural force.

Fluid drops can be located and formed in different places. Consequently, their component particles are forced to strive towards one area or towards another, and to move inasmuch as the centre or the suspension of a droplet can be located in, or moved to, different places. If they are deprived of mind, one of course is led to wonder by what messenger they are warned and shown where the centre of a drop among many others is and where it is carried, and by what sense they are able to reach this centre, by what appetite they are provided so that they wish to embrace it. Thus same force must be assumed which, by a blind necessity carries, retains and agglutinates the particles of water round the centre of the hanging droplet. This motive force, since it is not restricted to a certain area, will of course be vague and undetermined. It will, however, have a certain degree of impetus and, therefore, an equal force to move upwards, downwards and sideways. Thus, the particles of water or mercury in the hanging and round drops should be agitated by all these motions. And indeed some modern authors boldly assert this. Their theory, if I am right, like the previous one, can be refuted easily. If every part of the fluid in a drop moves with an equal force and energy, once the rotundity of the drop is altered and disturbed, i.e. when some eminence bulges from the fluid composing the drop, this could not acquire again its previous sphericity because that part more remote from the centre could not come closer to the centre of the drop, without expelling further from the centre other parts present in the depressions of the drop and these could not leave their place since they have the same energy and force as those which are present at the top of the tit. If actually opposite and diverse movements were considered, inasmuch as one portion is lowered to the bottom of the drop whereas another is raised and others are moved sideways, who could then understand that the parts of the fluid moving irregularly could result in a globular and spherical shape? Imagine in this yard an abundant rain of falling grains of wheat and, at the same time, the fallen grains are repelled upwards and sideways from below by some violence. In such perpetual and confuse agitation, how could these falling and ascending grains form a spherical figure rather than any other irregular figure, as experience shows?

Resorting to a deflection of the particles of mercury or water which, like eels, gather in a ball and unite, thus forming hanging and spherical droplets seems completely absurd, as we suggested above.

We shall now consider what others assert. According to them, quicksilver or water shun air and air shuns water and this rather conglomerates and unites to itself spontaneously, as a result of a lack of analogy between the mercury or the water and the ambient air. This does not result from a spontaneous choice in a perception of what is useful but from some necessity which forces fluid parts to move and be agitated perpetually whereas in air they cannot move and they withdraw into themselves, thus forming the spherical droplets. These authors then assign two causes which result in the diverse and heterogeneous nature of fluids. The first is a diversity of movements, i.e. the very small particles of water are agitated in ways different from the movements of the particles of the ambient air. As a consequence, they say, the particles of water are unable to exert their motive force in air and, therefore, they are forced, in a movement of reaction, to run into the depths of the droplet of water. These reaction movements result, they say, in the spherical shape of the drop. The second cause is the asymmetry of the pores. They say indeed that the pores of air have a shape such that the particles of water can in no way be introduced and pass through the incongruent pores of air. Consequently, a droplet of water is constrained from everywhere by air as if it were a marble vault.

As far as the diversity of movements is concerned, it must be noticed that it is not true that the movement of a body is completely impeded by the different movement of another body. This indeed occurs when the movements are opposite and are carried out over the same straight line with equal forces and velocities. If they are not opposite but are both carried out in the same direction, the movement of the other body is not impeded at all. It is only altered either in direction or in velocity. Since the opponents suppose motions of the parts of water and of air to be vague and varied, upwards, downwards and sideways, it is absolutely impossible that the movements of the particles of water always oppose the motions agitating the parts of air. If this is true, the movement of the parts of water must necessarily be impeded partly and sometimes by the ambient air but, more often, and nearly always, this would not hinder the movement of water. Then they penetrate each other and, therefore, the drops of water do not unite spherically. But this is not what occurs.

Moreover, if air is considerably expanded and rarer than water, of such a poor and weak consistency that it can be easily dissipated and displaced from its position by any small force, it is likely that the denser and consistent parts of water can, while moving, expel easily the particles of air from their positions and thus penetrate through its substance. Of course this seems to be confirmed by experience. Indeed we see that water vapours exhaling from the seas and ponds penetrate through air very easily. These vapours are nothing else than a conglomerate of very small particles of water which are blown from the water in a placid and quiet movement, in the winter, without the help of fire or of any other quick violence. Of course pure air can never be found without some mixture of very small parts of water as appears from experiments performed at our Accademia del Cimento. Thus, as all the minute particles of water forming vapour are neither slowed down nor impeded by varied agitation of air without being able to penetrate air freely and harmlessly, similarly the particles of a hanging drop will be able to penetrate the consistency of the ambient air. Therefore, the diverse motions of the parts of air do not impede outflow and vague motion of the parts of water. But if one ponders this matter carefully, he will understand that these motions are impeded by the particles of water themselves rather than by the external air. Firstly, they are equally consistent and voluminous. Thus, they cannot displace and expel each other from their positions. Moreover, since their movements are vague and disordered, all cannot be sent in the same direction. Thus, one part collides with another in opposite movements and they impede each other in their progression. Besides, where air is absent, the cause impeding the motions of the particles of water is missing. Therefore, where there is no air, its movement cannot be an obstacle to the agitation of the parts of water. This occurs in the vacuum of Torricelli where the motions of the particles of water would in no way be impeded. They could be more easily dispersed and dissipated through an almost empty space. Therefore, they would not be forced to return inside these drops in a movement of reaction, to be agitated and condensed. Thus, the cause and necessity as a result of which droplets of water in vacuum or in very rare air would have to acquire a spherical bulging would disappear. But this does not agree with experience. In vacuum indeed the droplets do not become less rotund than in open air.

If now we consider the incongruity of the pores, the assertion of the opponents appears not to be true when they say that the motions of the parts of water are impeded by air because the orifices of the pores of air are too narrow for the particles of water to be able to enter them and flow through them. From this indeed it would be permitted to infer that the particles of water cannot move and run through water itself either. It is easily understood that in water the pores cannot be large enough to enable the passage of particles of water itself through them. They must be much smaller like the interstices in a heap of grains of wheat or of bird-seeds are always smaller than the grains of wheat or the bird-seeds. Otherwise, after squeezing the heap, the grains would press and intermingle with each other more, with grains being introduced into the large interstices. Consequently, the particles of water can move and be agitated as uneasily through water itself as through air since progression of water through water is as difficult as through air, if it is true that in a fluid pores of such a size as to accommodate particles of water are required to enable these to move through the fluid. Since the narrow pores of water do not impede the movement of particles of water through water itself, the narrowness of the pores of air will not impede the movement of parts of water through the air either.

From this it is deduced that the actual cause why water can easily penetrate and flow through water is not the size of its pores but the fact that the parts of water can be expelled easily from their positions to make place for other particles of water which must be introduced there. If the former particles of water were not expelled from their positions, other parts could never arrive there and flow. Consequently, if this is true, we understand that the particles of water can also penetrate air and flow through it, although air has pores so narrow that particles of water cannot enter them. It is sufficient that the particles of air can be expelled from their positions so that water can be introduced there in the same way as occurs in water. Actually, this can occur more easily in air than in water. This results from the fact that the particles of air are rarer and more expanded. They are, therefore, less resistant than the parts of water. Consequently, it will not be difficult for the parts of water more solid than air to expel particles of air from their positions and water will thus move through them easily. Moreover, experience shows that particles of water are perpetually introduced into air, as was mentioned above about vapours. Actually, air absolutely dry without any addition of water can never be found. It is like a sponge.