4.8S Mr. W. R. Bii-t on Atmospheric Waves. 



may here inquire, how are these alternate aerial currents related 

 to the waves before alluded to ? It is one of the objects of the 

 discussion to exhibit this relation, which may be thus briefly ex- 

 pressed, at least in so far as the examination of the observations has 

 yet extended*. Let tiie strata a a a' a', b' b' b b, fig. 2, represent two 

 parallel aerial currents, a a «' a' being from S.W. and b' b' b b from 

 N.E., and conceive them both to advance from the N.W. in the 

 direction of the large arrow, that is the strata themselves will advance 

 with a lateral motion. Now conceive the barometer to commence 

 rising just as the edge b b passes any line of country, and to continue 

 rising until the edge h' b' arrives at that line, when the maximum is 

 attained. The wind now changes and the barometer immediately 

 begins to fall, and continues to fall until the edge a a coincides 

 with the line of country on which b b first impinged. During this 

 process we have all tiic phasnomena exhibited by an atmospheric 

 wave ; when the edge h b, fig. 2, passes the line of country, the point 

 «, fig. 1, of the wave (the anterior trough) transits that line of coun- 

 try and the barometer begins to rise with a N.E.wind. During the 

 period the stratum b' b' b b, fig. 2, transits the line the anterior slope 

 W a, fig. I, passes; when the conterminous edges of the strata a' a' 

 b' V, fig. 2, pass, the crest W, fig. 1, extends in the direction of the 

 preceding trough : the barometer now begins to fall, and when the 

 edge a a, fig. 2, occupies the place of b Z>, also fig. 2, the descent of 

 the mercurial column is completed; the posterior slope W w, fig. 1, 

 has passed, and the posterior trough w, fig. 1, now occupies the line 

 in which the anterior trough extended. 



This arrangement of the aerial currents in connexion with a baro- 

 metric wave may be termed the air-motion of the wave. In contem- 

 plating the transference of the barometric maxima and minima, we 

 regard only the loave-motion. In contemplating the direction and 

 force of the wind, we regard also the air-nwtion. The wave-motion 

 consists in the kiteral transference of the parallel beds of aerial cur- 

 rents ; the air-motion being these oppositely directed parallel currents 

 themselves. 



From these considerations, we readily see that the tvave is a con- 

 venient method of representing the barometric fluctuations. We have 

 seen that the rise is due to the anterior slope and the fall to the 

 posterior ; and we now further learn that the direction of the aerial 

 current on the anterior slope is at right angles to the axis of transla- 

 tion directed towards the left-hand, while on the posterior slope it is 

 the reverse ; still at right angles to the axis of translation, but directed 

 towards the right-hand. 



Having thus noticed the wave-motion with its accomijanying «/»•- 

 7)iotion, these interesting questions suggest themselves: — How are 

 the forces of this air-motion arranged ? Do all the particles move 

 with the same velocity ? Are there different velocities in ditt'erent 

 parts of tiie wave? Our anemometers will answer these questions. 

 In the troughs, the edges b b a a, the forces are strongest; as the 

 barometer rises, the force gradually subsides; when the crest passes, 

 * For this knowledge I am indebted to Mr. Brown's plates. 



