As for the concent of sounds, it is to be considered that the reciprocation or vibration of the air, by which sound is made, after it hath reached the drum of the ear, imprinteth a like vibration upon the air that is inclosed within it; by which means the sides of the drum within are stricken alternately. Now the concent of two sounds consists in this, that the tympanum receives its sounding stroke from both the sounding bodies in equal and equally frequent spaces of time; so that when two strings make their vibrations in the same times, the concent they produce is the most exquisite of all other. For the sides of the tympanum, that is to say of the organ of hearing, will be stricken by both those vibrations together at once, on one side or other. For example, if the two equal strings A B and C D be stricken together, and the latitudes of their vibrations E F and G H be also equal, and the points E, G, F and H be in the concave superficies of the tympanum, so that it receive strokes from both the strings together in E and G, and again together in F and H, the sound, which is made by the vibrations of each string, will be so like, that it may be taken for the same sound, and is called _unison_; which is the greatest concord. Again, the string A B retaining still its former vibration E F, let the string C D be stretched till its vibration have double the swiftness it had before, and let E F be divided equally in I. In what time therefore the string C D makes one part of its vibration from G to H, in the same time the string A B will make one part of its vibration from E to I; and in what time the string C D hath made the other part of its vibration back from H to G, in the same time another part of the vibration of the string A B will be made from I to F. But the points F and G are both in the sides of the organ, and therefore they will strike the organ both together, not at every stroke, but at every other stroke. And this is the nearest concord to unison, and makes that sound which is called an _eighth_. Again, the vibration of the string A B remaining still the same it was, let C D be stretched till its vibration be swifter than that of the string A B in the proportion of 3 to 2, and let E F be divided into three equal parts in K and L. In what time therefore the string C D makes one third part of its vibration, which third part is from G to H, the string A B will make one third part of its vibration, that is to say, two-thirds of E F, namely, E L. And in what time the string C D makes another third part of its vibration, namely H G, the string A B will make another third part of its vibration, namely from L to F, and back again from F to L. Lastly, whilst the string C D makes the last third part of its vibration, that is from G to H, the string A B will make the last third part of its vibration from L to E. But the points E and H are both in the sides of the organ. Wherefore, at every third