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



constantly drawn off to feed the ascending column. When however 

 it is once established, the velocity of the line of S.W. current near- 

 est to the N.E. would probably be equal, or nearly so, to that of the 

 N.E. current itself. 



In this way it is easy to conceive that a complete barometric wave 

 may be pi-oduced ; the lines of greatest velocity of the parallel cur- 

 rents will indicate the trough ; the rapidity with which the currents 

 . ]iass in opposite directions greatly diminishes the pressure, and ac- 

 cording 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 de- 

 crease of velocity the pressure increases, so that M'e have a distri- 

 bution of pressure of a wave form gradually rising on each side 

 the trough, the pressure being dependent on the velocity of the pa- 

 rallel currents. 



The constant ascent of air at the point of rarefaction would con- 

 tinually draw off a quantity of air from the S.E. side of the line of 

 greatest velocity b — b, fig. 3, and this would be attended with two 

 results ; first, there would be a real hollow or trough formed in the 

 line of junction of the parallel currents; and secondly, this line 

 would gradually advance towards the S.E,; for as more air would be 

 drawn off from that side, the whole body of air would advance in 

 that direction to supply the deficiency ; and should the rarefying 

 process cease, we can readily conceive that not only will the wave- 

 form be continued, but also tvave-motion. The establishment of the 

 parallel cun-ents will give the air-motion ; the diminution of pres- 

 sure towards the lines of greatest velocity will give the wave-form ; 

 and the drawing-off of air from the S.E. will induce t\\eivuve-motion. 

 The wave thus generated is negative ; it consists of a hollow pro- 

 duced by the ascending current of heated air carrying off a con- 

 siderable portion of air set in motion by this ascending column, 

 and its direction of motion is determined by more air being drawn 

 off from the S.E. slope than the N.W\ 



Cross Waves. 



The examination of the transit of a single wave by means of 

 barometric and anemonal observations, would be comparatively 

 easy, but it seldom happens, from the operation of natural causes, 

 that an isolated or solitary wave is produced. In almost every 

 instance (except in those in which the generating power is very 

 much greater than any which occasions the production of smaller 

 waves) the wave is contemporaneous with others of equal, if not of 

 greater magnitude, so that different systems are in motion at the 

 same time, each individual pursuing its own course, and although 

 perfectly Independent of every other, yet greatly modifying the re- 

 sulting phcenomena as exhibited by the barometer and anemometer. 

 When therefore we proceed with the examination of certain baro- 

 metric and anemonal phaeuomena in the manner above alluded to, 

 we arc speedily perjilexed with the barometric and anemonal effects 



