Mr. W. R. Birt on Atmospheric Waves. 489 



it IS zero ; and as the barometer falls, it increases until the trough 

 passes, when it is again strongest. 



M. Dove has suggested that the parallel currents, by which the 

 equipoise is maintained in the temperate zones, may be shifting ones, 

 and we have supposed that the parallel currents of N.E. and S.W. 

 winds may advance from the N.W. with a lateral motion. The same 

 cause that produces the well-known opposite and superposed equato- 

 rial and polar currents, will also give rise to the same opposite hut- 

 parallel currents in the temperate zones, namely, the ascending column 

 of heated and consequently rarefied air. Now it is well known that 

 in stormy weather, when the wind is blowing with great force, the 

 barometer being nearly at its minimum, upon the toind changing 

 the barometer commences rising ; the wind however continues to blow 

 with about the same force as it did with the previous falling baro- 

 meter. Upon M. Dove's view of parallel and opposite currents, 

 somewhere in or near the line forming the boundary between the 

 currents, towards or in the torrid zone, we ought to find the point 

 of rarefaction, and to this point the N.E. p- „ 



current would rush with the greatest ^' " ' 



force to supply the ascending column of ''^Tv^ 

 heated air*. This N.E. current would be •/ X \ v 

 compensated by a S.W. current of nearly J- ^v 



or quite the same force, situated just to i I t 

 the S.E. of it, as in fig. 3, in which let a 

 point of rarefaction, a for instance, exist 

 in any locality, so that a N.E. current 



lift 



111 any luuctiiuy, su mai a. rt.iz,. cuneiii ' i 



may be established to supply the ascend- I" f 1 



ing column ; suppose the greatest force i ' V I ' 



ing column ; suppose the greatest force 

 to exist along the line of crossed 



b b, tlie air would be drawn from the \ ' t ' ' 



of this line to fill up the vacuum at cr, V i u>t ^^■ 



and a compensating S.W. current, c c, \\\ t- '"^ — .^ 



established. This S.W. current would be x^' '*Q^ — •< < — 



established partly by the descent of the -^ 



overflowing current at a, and partly by the rush to supply the air 



* III attributing the greatest force to the N.E. current, I do not by any means 

 wisli to put forward or support any hypothesis tliat would at all interfere with tlie 

 well-known fact, tliat the greatest force is usually manifested by S.W. winds. Tlie 

 point to which I wish more particularly to solicit the attention of the Association is 

 lliis, tlic cause which induces tlie south-wesferl;/ current itself. Tliis must reside in 

 or near the torrid zone. Here we have a sufficient cause; we are presented with 

 plixnomena fully adequate to explain an influx o( coul air from the N.E. This is 

 the current that must first be established, and in the first instance its force will be 

 greatest. We have however only to turn to Prof. Dove's letter to Col. Sabine 

 (Report of the British Association, 1845, p. 01 ), and we shall at once find the reason 

 why S.W. winds manifest by means of our insi rumen Is the greiitcst force. The N.E. 

 currents are narrower, and the force soun abates as they pass over towards the 

 S.E. ; while on the other hand the same station is not only o/lener, but lonfi^er in 

 the S.W. currents, and as the line of greatest force approaches, the force increases, 

 on some occasions very rapidly, until the wind changes. The line of greatest force 

 soon passes the station, so that upon a mean of numerous observations the south- 

 westerly wind exiiibits the greatest force. 



