XLV

Among Prerogative Instances I will put in the twenty-first place Instances of the Rod or Rule, which I also call Instances of Range or of Limitation. For the powers and motions of things act and take effect at distances not indefinite or accidental, but finite and fixed; so that to ascertain and observe these distances in the investigation of the several natures is of the greatest advantage to practice, not only to prevent its failure but also to extend and increase its power. For we are sometimes enabled to extend the range of powers and, as it were, to diminish distances, as for instance by the use of telescopes.

Most of these powers act and take effect only by manifest contact, as in the impact of two bodies, where the one does not move the other from its place unless they touch each other. Also medicines that are applied externally, as ointments or plasters, do not exert their virtues without touching the body. Finally, the objects of the taste and touch do not strike those senses unless they be contiguous to the organs.

There are also powers which act at a distance, though a very small one; and of these only a few have been hitherto observed, albeit there are many more than men suspect; as (to take common examples) when amber or jet attracts straws; bubbles dissolve bubbles on being brought together; certain purgative medicines draw humors downward, and the like. So, too, the magnetic power by which iron and a magnet, or two magnets, are made to meet, operates within a fixed but narrow sphere of action; but if there be any magnetic virtue flowing from the earth (a little below the surface), and acting on a steel needle in respect of its polarity, the action operates at a great distance.

Again, if there be any magnetic power which operates by consent between the globe of the earth and heavy bodies, or between the globe of the moon and the waters of the sea (as seems highly probable in the semimenstrual ebbs and floods), or between the starry sphere and the planets whereby the latter are attracted to their apogees, all these must operate at very great distances. There are found also certain materials which catch fire a long way off, as we are told the naphtha of Babylon does. Heat also insinuates itself at great distances, as also does cold; insomuch that by the inhabitants of Canada the masses of ice that break loose and float about the northern ocean and are borne through the Atlantic toward that coast are perceived at a great distance by the cold they give out. Perfumes also (though in these there appears to be always a certain corporeal discharge) act at remarkable distances, as those find who sail along the coasts of Florida or some parts of Spain, where there are whole woods of lemon and orange and like odoriferous trees, or thickets of rosemary, marjoram, and the like. Lastly, the radiations of light and impressions of sound operate at vast distances.

But whether the distances at which these powers act be great or small, it is certain that they are all finite and fixed in the nature of things, so that there is a certain limit never exceeded, and a limit which depends either on the mass or quantity of matter in the bodies acted on; or on the strength or weakness of the powers acting; or on the helps or hindrances presented by the media in which they act — all which things should be observed and brought to computation. Moreover, the measurements of violent motions (as they are called), as of projectiles, guns, wheels, and the like, since these also have manifestly their fixed limits, should be observed and computed.

There are found also certain motions and virtues of a contrary nature to those which operate by contact and not at a distance, namely, those which operate at a distance and not by contact; and again those which operate more feebly at a lesser distance, and more powerfully at a greater. The act of sight for instance is not well performed in contact but requires a medium and a distance. Yet I remember being assured by a person of veracity that he himself under an operation for the cataract, when a small silver needle was inserted within the first coat of the eye in order to remove the pellicle of the cataract and push it into a corner, saw most distinctly the needle passing over the very pupil. But though this may be true, it is manifest that large bodies are not well or distinctly seen except at the vertex of a cone, the rays from the object converging at a certain distance from it. Moreover, old people see objects better at a little distance than if quite close. In projectiles, too, it is certain that the impact is not so violent at too small a distance as it is a little further off. These, therefore, and like things should be observed in the measurements of motions with regard to distances.

There is also another kind of local measurement of motions which must not be omitted. This has to do with motions not progressive, but spherical, that is, with the expansion of bodies into a greater sphere or their contraction into a less. For among our measurements of motions we must inquire what degree of compression or extension bodies (according to their nature) easily and freely endure, and at what point they begin to resist, till at last they will bear no more. Thus, when a blown bladder is compressed, it allows a certain compression of the air, but if the compression be increased the air does not endure it and the bladder bursts.

But this same thing I have tested more accurately by a subtle experiment. I took a small bell of metal, light and thin, such as is used for holding salt, and plunged it into a basin of water so that it carried down with it the air contained in its cavity to the bottom of the basin, where I had previously placed a small globe, on which the bell was to light. I found then that if the globe was small enough in proportion to the cavity, the air contracted itself into a less space and was simply squeezed together, not squeezed out. But if it was too large for the air to yield freely, then the air, impatient of greater pressure, raised the bell on one side and rose to the surface in bubbles.

Again, to test the extension as well as compression of which air was susceptible, I had recourse to the following device. I took a glass egg with a small hole at one end of it, and, having drawn out the air through the hole by violent suction, I immediately stopped up the hole with my finger and plunged the egg into water, and then took away my finger. The air, having been extended by the suction and dilated beyond its natural dimensions, and therefore struggling to contract itself again (so that if the egg had not been plunged into the water it would have drawn in air with a hissing sound), now drew in water in sufficient quantities to allow the air to recover its old sphere or dimension.

Now it is certain that the rarer bodies (such as air) allow a considerable degree of contraction, as has been stated, but that tangible bodies (such as water) suffer compression with much greater difficulty and to a lesser extent. How far they do suffer it I have investigated in the following experiment. I had a hollow globe of lead made, capable of holding about two pints, and sufficiently thick to bear considerable force. Having made a hole in it, I filled it with water and then stopped up the hole with melted lead, so that the globe became quite solid. I then flattened two opposite sides of the globe with a heavy hammer, by which the water was necessarily contracted into less space, a sphere being the figure of largest capacity. And when the hammering had no more effect in making the water shrink, I made use of a mill or press, till the water, impatient of further pressure, exuded through the solid lead like a fine dew. I then computed the space lost by the compression and concluded that this was the extent of compression which the water had suffered, but only when constrained by great violence.

But the compression or extension endured by more solid, dry, or more compact bodies, such as wood, stones and metals, is still less than this, and scarcely perceptible. For they free themselves either by breaking, or by moving forward, or by other efforts, as is apparent in the bending of wood or metal, in clocks moving by springs, in projectiles, hammerings, and numberless other motions. And all these things with their measures should in the investigation of nature be explored and set down, either in their certitude, or by estimate, or by comparison, as the case will admit.