is coincident with a N.E. or N.W. wind. The wind now changes and the
barometer begins to fall, and continues falling until the edge _a a_
coincides with the line of country on which _b b_ first impinged.
During this process we have all the phænomena exhibited by an
atmospheric wave: when the edge _b b_ passes a line of country the
barometer is at a _minimum_, and this minimum has been termed the
_anterior trough_. During the period the stratum _b' b' b b_ transits,
the barometer rises, and this rise has been called the _anterior slope_.
When the conterminous edges of the strata _a' a' b' b'_ pass, a
barometric _maximum_ extends along the line of country formerly occupied
by the anterior trough, and this maximum has been designated the
_crest_. During the transit of the stratum _a' a' a a_ the barometer
_falls_, and this fall has been characterised as the _posterior slope_;
and when the edge _a a_ occupies the place of _b b_, the descent of the
mercurial column is completed, another _minimum_ extends in the
direction of the former, and this minimum has been termed the _posterior
trough_.

It will be readily seen that the lateral passage of the N.W. and S.E.
currents towards the N.E. presents precisely the same barometric and
anemonal phænomena as the rotatory storms when moving in the same
direction. If the observer, when the barometer is at a _maximum_ with a
N.W. wind, place himself in the same position with regard to the
laterally advancing current as he did with regard to the advancing
storm, _i. e._ with his face _towards_ the quarter from which it is
advancing--S.W., he will find that with a _falling barometer and S.E.
wind the current passes him from the left to the right hand_; but if at
a barometric _minimum_ he place himself in the same position with his
face directed to the quarter from which the N.W. current is advancing
laterally, also S.W., he will find that with a rising barometer _and