  We consider here a new statistical measure for cosmic shear, the aperture
mass Map, which is defined as a spatially filtered projected density field and
which can be measured directly from the image distortions of high-redshift
galaxies. By selecting an appropriate spatial filter function, the dispersion
of the aperture mass is a convolution of the power spectrum of the projected
density field with a narrow kernel, so that <Map**2(theta)> provides a well
localized estimate of the power spectrum at wavenumbers s~5/theta. We calculate
<Map**2(theta)> for various cosmological models, using the fully non-linear
power spectrum of the cosmic density fluctuations. The non-linear evolution
yields a significant increase of <Map**2> relative to the linear growth on
scales below ~0.5 degree. The third-order moment of Map can be used to define a
skewness, which is a measure of the non-Gaussianity of the density field. We
present the first calculation of the skewness of the shear in the frame of
quasi-linear theory of structure growth. We show that it yields a sensitive
measure of the cosmological model; in particular, it is independent of the
normalization of the power spectrum. Several practical estimates for <Map**2>
are constructed and their dispersions calculated. We show that one sq.deg. of a
high-quality image is sufficient to detect the cosmic shear with the
Map-statistics on scales below ~10 arcmin, and to estimate its amplitude with
an accuracy of ~30% on scales below ~5 arcmin. (abbreviated)
