ON ATMOSPHERIC WAVES. 137 
fluence took place. These phenomena, however, could only occur upon the 
impinging of currents; upon M. Dove’s theory of parallel currents in oppo- 
site directions, it does not appear likely that they can exist. M. Dove has 
suggested that these parallel currents 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 
opposite and superposed equatorial and polar currents, will also give rise to 
the same opposite dué parallel currents in the temperate zone, 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 wind changing the 
barometer commences rising ; the wind however continues to blow with about 
the same force as it did with the previous falling barometer. 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. current 
would rush with the greatest force to supply the ascending column of heated 
air*. This N.E. current would be compensated by a S.W. current of nearly 
or quite the same force, situated just to the S.E. Fig. 3. 
of it, as in fig. 3, in which let a point of rare- 
faction, @ for instance, exist in any locality, 
so that a N.E. current may be established to ¥ 
supply the ascending column; suppose the 
greatest force to exist along the line of crossed | 
arrows 6 6, the air would be drawn from the 
end of this line to fill up the vacuum at a, | 
and a compensating S.W. current, ¢ c; esta- 
blished. This S.W. current would be established | 
partly by the descent of the overflowing current 
at a, and partly by the rush to supply the air | \ 
constantly drawn off to feed the ascending co- 
lumn. hen however it is once established, \ 
the velocity of the line of S.W. current nearest 
to the N.E. would probably be equal, or nearly \ a 
80, to that of the N.E. current itself. <a 
Tn this way it is easy to conceive that a complete barometric wave may be 
produced ; the lines of greatest velocity of the parallel currents will indicate 
the trough; the rapidity with which the currents pass in opposite directions 
greatly diminishes the pressure, and according to this view somewhere near 
_ the direction of the trough and to the S.W. of it, we ought to find the point 
_ of greatest rarefaction: the velocity decreases on each side this trough, 
_ and with this decrease of velocity the pressure increases, so that we have a 
_ * Tn attributing the greatest force to the N.E. current, I do not by any means wish to put 
_ forward or support any hypothesis that would at all interfere with the well-known fact, that 
_ the greatest force is usually manifested by $.W. winds. The point to which I wish more par- 
ticularly to solicit the attention of the Association is this, the cause which induces the south~ 
westerly current itse(f. This must reside in or near the torrid zone. Here we have a suffi- 
cient cause; we are presented with phenomena fully adequate to explain an influx of cool 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, 1845; p. 61), and we shall at once find the reason why S,W. winds manifest by means 
_ of our instruments the greatest force. The N.E. currents are narrower, and the force soon 
 abates as they pass over towards the S.E.; while on the other hand the same station is not 
~ only oftener, but longer 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 lineof greatest 
_ force soon passes the station, so that upon a mean of numerous observations the south-westerly 
; wind exhibits the greatest force. . 
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