two of a sort, one corresponding to the other.

Double Children, Double Cats, Double Pearls, &c. Instances of these are frequent in the Philosophical Transact. and elsewhere. See Monster.

Sir John Floyer, in the same Transactions, giving an account of a double turkey, furnishes some reflections on the production of double animals in general. Two turkeys, he relates, were taken out of an egg of the common size, when the rest were well hatched, which grew together by the flesh of the breast-bone, but in all other parts were distinct. They seemed less than the ordinary size, as wanting bulk, nutriment, and room for their growth; which latter, too, was apparently the occasion of their cohesion. For, having two distinct cavities in their bodies, and two hearts, they must have arisen from two cicatrices; and, consequently, the egg had two yolks; which is an uncommon accident. He mentions a dried double chicken in his possession, which, though it had four legs, four wings, &c. had but one cavity in the body, one heart, and one head; and, consequently, was produced from one cicatrix.

So, Parcus mentions a double infant, with only one heart: in which case, the original or flamen of the infant was one; and the vessels regular; only, the nerves and arteries towards the extremities dividing into more branches than ordinary, produced double parts. The same is the case in the double flowers of plants, occasioned by the richness of the soil. So it is in the eggs of quadrupeds, &c.

There are, therefore, two reasons of duplicity in embryos: 1. The conjoint or connexion of two perfect animals; and, 2. An extraordinary division and ramification of the original vessels, nerves, arteries, &c.

Double Employment, in music, a name given by M. Rameau to the two different manners in which the chord of the sub-dominant may be regarded and treated, viz., as the fundamental chord of the sixth superadded, or as the chord of the great sixth, inverted from a fundamental chord of the seventh. In reality, the chords carry exactly the same notes, are figured in the same manner, are employed upon the same chord of the tone, in such a manner, that frequently we cannot discern which of the two chords the author employs, but by the assistance of the subsequent chord, which resolves it, and which is different in these different cases.

To make this distinction, we must consider the diatonic progress of the two notes which form the fifth and the sixth, and which, constituting between them the interval of a second, must one or the other constitute the dissonance of the chord. Now, this progress is determined by the motion of the bass. Of these two notes, then, if the superior be the dissonance, it will rise by one graduation into the subsequent chord, the lower note will keep its place, and the higher note will be a superadded sixth. If the lower be the dissonance, it will descend into the subsequent chord, the higher will remain in its place, and the chord will be that of the great sixth. See the two cases of the double employment in Rousseau's Musical Dictionary, Plate D, fig. 12.

With respect to the composer, the use which he may make of the double-employment, is to consider the chord in its different points of view, that from thence he may know how to make his entrance to it, and his exit from it; so that having arrived, for instance, at the chord of the superadded sixth, he may resolve it as a chord of the great sixth, and reciprocally.

M. D'Alembert has shewn, that one of the chief uses of the double-employment is, that we be able to carry the diatonic succession of the gamut even to an octave, without changing the mode, at least whilst we rise; for in descending we must change it. Of this gamut and its fundamental bass, an example will be found in Rousseau's Musical Dictionary, Plate D, fig. 13. It is evident, according to the system of M. Rameau, that all the harmonic successions which result from it, are in the same tone: for, in strictness, no other chords are there employed but three, that of the tonic, that of the dominant, and that of the sub-dominant; as this last, in the double-employment, constitutes the seventh from the second note, which is employed upon the fifth.

With respect to what M. D'Alembert adds in his Elements of Music, p. 85, and which he repeats in the Encyclopédie, article Double-emplai, viz., that the chord of the seventh re fa la ut, though we should even regard it only as an inversion of fa la ut re, cannot be followed by the chord ut mi sol ut; "I cannot (says Rousseau) be of his opinion in this point.

"The proof which he gives for it is, that the dissonance ut of the first chord cannot be resolved in the second; and this is true, since it remains in its place: but in this chord of the seventh re fa la ut, inverted from this chord of the superadded sixth fa la ut re, it is not the ut, but the re, which is the dissonance; which, of consequence, ought to be resolved in ascending upon mi, as it really does in the subsequent chord; so that this procedure in the bass itself is forced, which, from re, cannot without an error return to ut, but ought to ascend to mi, in order to resolve the dissonance.

"M. D'Alembert afterwards shews, that this chord re fa la ut, when preceded and followed by that of the tonic, cannot be authorised by the double-employment; and this is likewise very true; because this chord, tho' figured with a 7, is not treated as a chord of the seventh, neither when we make our entrance to it, nor our exit from it; or at least that it is not necessary to treat it as such, but simply as an inversion of the superadded sixth, of which the dissonance is the bass: in which case we ought by no means to forget, that this dissonance is never prepared. Thus, though in such a transition the double-employment is not in question, though the chord of the seventh be no more than apparent, and impossible to be resolved by the rules, this does not hinder the transition from being proper and regular, as I have just proved to theorists, and as I shall immediately prove to practical artists, by an instance of this transition; which certainly will not be condemned by any one of them, nor justified by any other fundamental bass except my own. (See the Musical Dictionary, Plate D, fig. 14.)

"I acknowledge, that this inversion of the chord of the sixth superadded, which transfers the dissonance to the bass, has been censured by M. Rameau. This author, taking for a fundamental chord the chord of the seventh, which results from it, rather chose to make the fundamental bass descend diatonically, and resolve one seventh by another, than to unfold this seventh by an inversion. I had dissipated this error, and many others, in some papers which long ago had passed into the hands of M. D'Alembert, when he was composing his Elements of Music; so that it is not his sentiment which I attack, but my own opinion which I defend."

For what remains, the double-employment cannot be used with too much reserve, and the greatest masters are the most temperate in putting it in practice.

Double Fiche, or Fiche, in heraldry, the denomination of a cross, when the extremity has two points; in contradistinction to fiche, where the extremity is sharpened away to one point.

Double Octave, in music, an interval composed of fifteen notes in diatonic progression; and which, for that reason, is called a fifteenth. "It is (says Rousseau) an interval composed of two octaves, called by the Greeks diddiapason.

It deserves however to be remarked, that in intervals less distant and compounded, as in the third, the fifth, the simple octave, &c. the lowest and highest extremes are included in the number from whence the interval takes its name. But, in the double octave, when termed a fifteenth, the simple number of which it is composed gives the name. This is by no means analogical, and may occasion some confusion. We should rather choose, therefore, to run any hazard which might occur from uniformly including all the terms of which the component intervals consist, and call the double octave a fifteenth, according to the general analogy. See INTERVAL.