extending over the whole range of the space-time _sichel_. We shall
decompose the _sichel_ into elementary space-time filaments, and every
one of these filaments in small elements _d_τ of its proper-time, which
are however large compared to the linear dimensions of the normal
cross-section; let us assume that the mass of such a filament
ν_d_J_{_n_} = _dm_ and write τ⁰, τ^l for the ‘Proper-time’ of the upper
and lower boundary of the _sichel_.