Mr. W. R. Birt on Atmospheric Waves. 491 



of cross leaves ; the flowing of one set of waves in a certain direc- 

 tion is apparently interrupted and interfered with by another in a 

 different direction, and before the first set can be exliibited with its 

 proper proportions, and the true altitudes, amplitudes, velocities, and 

 directions of its individual waves assigned, all the phaenoniena of 

 the other set must be carefully disentangled and separated from the 

 aggregate phaenoniena presented by the contemporaneous systems. 

 The barometric curve, including a complete rise and fall at any one 

 station, is 7iot the curve resulting from the transit of any one wave; 

 it does not represent the form of anj reality in nature ; but it does 

 represent, and is an exponent of the effects resulting from the con- 

 temporaneous transits of waves, or systems of waves, such as have 

 been described. 



The discussion of Mr. Brown's observations has clearly brought to 

 light a set of pai-allel and opposite currents at right angles to those we 

 have been contemplating, namely, from N.W. and S.E. with a wave- 

 motion towards the N.E., producing the cross waves which occasion 

 the complexity before alluded to. The late Professor Daniell has re- 

 marked that the curves increase in range towards the N.W., and in 

 general the neighbourhood of Jt'ater presents curves remarkable forthe 

 boldness of their contour and the large extent of their range. In ven- 

 turing a speculation on these cross waves from the S.W. with parallel 

 and opposite currents from N.W. and S.E., I should be incbned to 

 attribute them to the effect of the solar influence on the terrestrial 

 surface, extending from Cape Verd in Africa to the extreme north of 

 Lapland in Europe. This surface extends from S.W. to N.E., or 

 somewhat in that direction. It may be remarked, that to the north- 

 east of Cape Verd is situated the Sahara or Great Desert of Africa, 

 and here we have a great rarefying surface. To the north-west or 

 west-north-west of this extensive rarefying surface, the broadest part 

 of the Atlantic ocean is situated. The relative positions of the Great 

 Desert and the broadest extent of the Atlantic will produce a great 

 indraught of cool air from the ocean ; the direction of this wind will 

 be W.N.W. or N.W. To the north-east of this current, probably 

 in the neighbourhood of Morocco, Fez, Algiers, Spain and Portugal, 

 and the north-west portions of the Mediterranean sea, we ought 

 to find the counter current from the S.E. or E.S.E., the two por- 

 tions in juxtaposition moving with the greatest velocity. Some- 

 where in the Atlantic the turning-point of these oppositely directed 

 currents should exist. The line of junction of these parallel cur- 

 rents will determine the trough of the wave, and as before shovvu, in 

 consequence of the air being drawn off from the north-cast to supply 

 the ascending current, the wave will progress towards that (juarter; 

 the barometer first (lescending with the S.E. wind as the trough 

 approaches stations to the N.E., and rising with the N.W. as thu 

 current produced by the rarefaction approaches, until the crest 

 passes, when the new counter current or slope of the next wave 

 would set in *. 



• In the above suggestion I have considered tlie northern portion of the African 



