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 phaenomena 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 N.W. wind the current passes him from right to left_. Now the two classes of phaenomena are identical, and it would not be difficult to show that, had we an instance of a rotatory storm in the northern hemisphere moving from N.W. to S.E., it would present precisely the same phaenomena as to the direction of currents passing from left to right and from right to left with falling and rising barometers, increase and decrease in the force of the wind, &c., as the oppositely directed aerial currents do which pass over western central Europe.