  The soft-gamma repeater SGR 1900+14 became active again on June 1998 after a
long period of quiescence; it remained at a low state of activity until August
1998, when it emitted a series of extraordinarily intense outbursts. We have
observed the source with RXTE twice, during the onset of each active episode.
We confirm the pulsations at the 5.16 s period reported earlier (Hurley et al.
1998b, Hurley et al. 1998 e) from SGR 1900+14. Here we report the detection of
a secular spindown of the pulse period at an average rate of 1.1*10^{-10} s/s.
In view of the strong similarities between SGRs, we attribute the spindown of
SGR 1900+14 to magnetic dipole radiation, possibly accelerated by a quiescent
flux, as in the case of SGR 1806-20 (Kouveliotou et al. 1998a). This allows an
estimate of the pulsar dipolar magnetic field, which is 2-8*10^{14} G. Our
results confirm that SGRs are magnetars.
