The problem of the relation between microscopic and macroscopic reality in the generally covariant theories is first considered, and it is argued that a sensible definition of the macroscopic averages imposes a restriction of the allowed transformations of coordinates to suitably defined macroscopic transformations. Spacetime averages of the geometric objects of a generally covariant theory are then defined, and the reconstruction of some features of macroscopic reality from hypothetic microscopic structures through such averages is attempted in the case of the geometric objects of Einstein's unified field theory. It is shown with particular examples how a fluctuating microscopic structure of the metric field can rule the constitutive relation for macroscopic electromagnetism both in vacuo and in nondispersive material media. Moreover, if both the metric and the skew field a ik that represents the electric displacement and the magnetic field are assumed to possess a wavy microscopic behaviour, nonvanishing average generalized force densities < T m k;m > are found to occur in the continuum, that originate from a resonance process, in which at least three waves need to be involved. The previously required limitation of covariance to the macroscopic transformations ensures meaning to the notion of a periodic microscopic disturbance, for which a wave four-vector can be defined. Let k A m and k B m represent the wave four-vectors of two plane wave disturbances displayed by a ik , while k C m is the wave four-vector for a plane wave perturbation of the metric; it is found that < T m k;m > can be nonvanishing only if the three-wave resonance condition k A m ± k B m ± k C m = 0, so ubiquitous in quantum physics, is satisfied. A particular example of resonant process is provided, in which < T m k;m > is actually nonvanishing. The wavy behaviour of the metric is essential for the occurrence of this resonance phenomenon.RÉSUMÉ. On examine d'abord le problème de la relation entre la réalité microscopique et la réalité macroscopique dans les théories covariantes générales, et il est montré qu'une bonne définition des moyennes macroscopiques impose une restriction aux transformations de coordonnées permises pour le cas macroscopique. On définit ensuite les moyennes dans l'espace-temps des objects géométriques d'une théorie covariante générale. La reconstitution de certaines propriétés de la * Annales de la Fondation Louis de Broglie, Volume 21, 11-38 (1996). 1 réalité macroscopiqueà partir de structures microscopiques supposées est ensuite tentée dans le cas des objects géométriques de la théorie du champ unifié d'Einstein. Il est montré par des exemples, comment une structure microscopique fluctuante du champ de la métrique peut régir la relation constitutive de l'électromagnétisme macroscopique, dans le vide comme dans des milieux matériels non-dispersifs. De plus, si la métrique, et le champ antisymétrique a ik , qui représente le déplacementélectrique et le champ magnétique, sont supposés avoir un comportement microscopiqu...