6. Also the cause why the sound of a pipe, which is made by blowing into it, is nevertheless clear, is the same with that of the sound which is made by collision. For if the breath, when it is blown into a pipe, do only rake its concave superficies, or fall upon it with a very sharp angle of incidence, the sound will not be clear, but hoarse. But if the angle be great enough, the percussion, which is made against one of the hollow sides, will be reverberated to the opposite side; and so successive repercussions will be made from side to side, till at last the whole concave superficies of the pipe be put into motion; which motion will be reciprocated, as it is in collision; and this reciprocation being propagated to the organ, from the reaction of the organ will arise a clear sound, such as is made by collision, or by breaking asunder of hard bodies.
In the same manner it is with the sound of a man's voice. For when the breath passeth out without interruption, and doth but lightly touch the cavities through which it is sent, the sound it maketh is a hoarse sound. But if in going out it strike strongly upon the larynx, then a clear sound is made, as in a pipe. And the same breath, as it comes in divers manners to the palate, the tongue, the lips, the teeth, and other organs of speech, so the sounds into which it is articulated become different from one another.
Of reflected sound.
7. I call that primary sound, which is generated by motion from the sounding body to the organ in a strait line without reflection; and I call that reflected sound, which is generated by one or more reflections, being the same with that we call echo, and is iterated as often as there are reflections made from the object to the ear. And these reflections are made by hills, walls, and other resisting bodies, so placed as that they make more or fewer reflections of the motion, according as they are themselves more or fewer in number; and they make them more or less frequently, according as they are more or less distant from one another. Now the cause of both these things is to be sought for in the situation of the reflecting bodies, as is usually done in sight. For the laws of reflection are the same in both, namely, that the angles of incidence and reflection be equal to one another. If, therefore, in a hollow elliptic body, whose inside is well polished, or in two right parabolical solids, which are joined together by one common base, there be placed a sounding body in one of the burning points, and the ear in the other, there will be heard a sound by many degrees greater than in the open air; and both this, and the burning of such combustible things, as being put in the same places are set on fire by the sun-beams, are effects of one and the same cause. But, as when the visible object is placed in one of the burning points, it is not distinctly seen in the other, because every part of the object being seen in every line, which is reflected from the concave superficies to the eye, makes a confusion in the sight; so neither is sound heard articulately and distinctly when it comes to the ear in all those reflected lines. And this may be the reason why in churches which have arched roofs, though they be neither elliptical nor parabolical, yet because their figure is not much different from these, the voice from the pulpit will not be heard so articulately as it would be, if there were no vaulting at all.
From whence it is that sound is uniform and lasting.
8. Concerning the uniformity and duration of sounds, both which have one common cause, we may observe, that such bodies as being stricken yield an unequal or harsh sound, are very heterogeneous, that is to say, they consist of parts which are very unlike both in figure and hardness, such as are wood, stones, and others not a few. When these are stricken, there follows a concussion of their internal particles, and a restitution of them again. But they are neither moved alike, nor have they the same action upon one another; some of them recoiling from the stroke, whilst others which have already finished their recoilings are now returning; by which means they hinder and stop one another. And from hence it is that their motions are not only unequal and harsh, but also that their reciprocations come to be quickly extinguished. Whensoever, therefore, this motion is propagated to the ear, the sound it makes is unequal and of small duration. On the contrary, if a body that is stricken be not only sufficiently hard, but have also the particles of which it consisteth like to one another both in hardness and figure, such as are the particles of glass and metals, which being first melted do afterwards settle and harden; the sound it yieldeth will, because the motions of its parts and their reciprocations are like and uniform, be uniform and pleasant, and be more or less lasting, according as the body stricken hath greater or less magnitude. The possible cause, therefore, of sounds uniform and harsh, and of their longer or shorter duration, may be one and the same likeness and unlikeness of the internal parts of the sounding body, in respect both of their figure and hardness.
Besides, if two plane bodies of the same matter and of equal thickness, do both yield an uniform sound, the sound of that body, which hath the greatest extent of length, will be the longest heard. For the motion, which in both of them hath its beginning from the point of percussion, is to be propagated in the greater body through a greater space, and consequently that propagation will require more time; and therefore also the parts which are moved, will require more time for their return. Wherefore all the reciprocations cannot be finished but in longer time; and being carried to the ear, will make the sound last the longer. And from hence it is manifest, that of hard bodies which yield an uniform sound, the sound lasteth longer which comes from those that are round and hollow, than from those that are plane, if they be like in all other respects. For in circular lines the action, which begins at any point, hath not from the figure any end of its propagation, because the line in which it is propagated returns again to its beginning; so that the figure hinders not but that the motion may have infinite progression. Whereas in a plane, every line hath its magnitude finite, beyond which the action cannot proceed. If, therefore, the matter be the same, the motion of the parts of that body whose figure is round and hollow, will last longer than of that which is plane.
Also, if a string which is stretched be fastened at both ends to a hollow body, and be stricken, the sound will last longer than if it were not so fastened; because the trembling or reciprocation which it receives from the stroke, is by reason of the connection communicated to the hollow body; and this trembling, if the hollow body be great, will last the longer by reason of that greatness. Wherefore also, for the reason above mentioned, the sound will last the longer.
How sound may be helped and hindered by the wind.
9. In hearing it happens, otherwise than in seeing, that the action of the medium is made stronger by the wind when it blows the same way, and weaker when it blows the contrary way. The cause whereof cannot proceed from anything but the different generation of sound and light. For in the generation of light, none of the parts of the medium between the object and the eye are moved from their own places to other places sensibly distant; but the action is propagated in spaces imperceptible; so that no contrary wind can diminish, nor favourable wind encrease the light, unless it be so strong as to remove the object further off or bring it nearer to the eye. For the wind, that is to say the air moved, doth not by its interposition between the object and the eye work otherwise than it would do, if it were still and calm. For, where the pressure is perpetual, one part of the air is no sooner carried away, but another, by succeeding it, receives the same impression, which the part carried away had received before. But in the generation of sound, the first collision or breaking asunder beateth away and driveth out of its place the nearest part of the air, and that to a considerable distance, and with considerable velocity; and as the circles grow by their remoteness wider and wider, so the air being more and more dissipated, hath also its motion more and more weakened. Whensoever therefore the air is so stricken as to cause sound, if the wind fall upon it, it will move it all nearer to the ear, if it blow that way, and further from it if it blow the contrary way; so that according as it blows from or towards the object, so the sound which is heard will seem to come from a nearer or remoter place; and the reaction, by reason of the unequal distances, be strengthened or debilitated.