  Space-time--time is a natural hybrid of Kaluza's five-dimensional geometry
and Weyl's conformal space-time geometry. Translations along the secondary time
dimension produce the electromagnetic gauge transformations of Kaluza--Klein
theory and the metric gauge transformations of Weyl theory, related as Weyl
postulated. Geometrically, this phenomenon resides in an exponential-expansion
producing ``conformality constraint'', which replaces Kaluza's ``cylinder
condition''. The curvature tensors exhibit a wealth of ``interactions'' among
geometrical entities with physical interpretations. Unique to the conformally
constrained geometry is a sectionally isotropic, ultralocally determined
``residual curvature'', useful in construction of an action density for field
equations. A space-time--time geodesic describes a test particle whose rest
mass m and electric charge q evolve according to definite laws. The particls's
motion is governed by four apparent forces: the Einstein gravitational force,
the Lorentz electromagnetic force, a force proportional to the electromagnetic
potential, and a force proportional to a gradient d(ln phi), where the scalar
field phi is essentially the space-time--time residual radius of curvature. The
particle appears suddenly at an event E1 with q = -phi(E1) and vanishes at an
event E2 with q = phi(E2). At E1 and E2 the phi-force infinitely dominates the
others, causing E1 and E2 to tend to occur near where phi has an extreme value;
application to the modeling of orbital transitions of atomic electrons suggests
itself. The equivalence of a test particle's inertial mass and its passive
gravitational mass follows from the gravitational force's proportionality to m.
No connection is apparent between m and active gravitational mass or between q
and active electric charge, nor does the theory seem to require any.
