Mr. W. R. Birt on Atmospheric Waves, 131 



Third zone on S.W. above trough o/S.W. wave, anterior slope. 





Barom. 



Direction. 



Force, 



Greencastle 



. 29-70 + 0-17 



N.E. 



+ 6 



Pembroke . 



. 29-43-0-19 



N.E. 



+ 6 



Scilly . . . 



. 29-30-0-31 



N.E. 



+ 6 



Plymouth . 



. 29-31 -0-36 



E. 



+ 5 



L' Orient . 



. 29-21 -0-47 



E.N.E. 



+ 4 



Rochefort . 



. 29'26-0-28 





5 



Fourth zone on S.W. from trough o/S.W. wave, anter^ior slope. 



Roche's Point . 29-52 + 0-06 N.E. +5 



Biarritz . . 29-37-003 S. +5 



Fifth zone on S.W. from trough o/S.W. wave, anterior slope. 



Valencia . . 29-60 + 0-01 N.N.E. 7 

 Corunna . . 29-40-0-27 S.W. -6 



In casting the eye over the differences of fall along the line 

 of trough^ it will be seen that the greatest diminution of pressure 

 occurred at London, Dover, and Cape Gris Nez ; and on com- 

 paring the heights of the barometer at these stations (29-21 to 

 29*1 5) with the lowest point in the cyclone of the 20th in the 

 S. of Ireland (29-46), it will be seen that not only did the cy- 

 clonic depression travel froin the S. of Ireland to the N.W. 

 of France, but the diminution of elasticity increased as it pro- 

 gressed } so that the decrease of nearly 0*60 in the S. of Ireland 

 from a pressure of about 30-00 between April 19 and 20 ap- 

 peared in the neighbourhood of Dover on the 21st, reducing the 

 pressure there by about the same amount from about 29-71 or 

 29*72 the readings of the 20th. Changes of elasticity of this 

 nature modify, but do not destroy the wave-forms, which ordi- 

 narily continue to travel in their normal directions. 



The line from Valencia to Christiansund (barometer from 

 29-60 to 29'86) represented the crest of a slope of which the 

 trough extended along the English Channel. This trough in- 

 tersected the trough from Thurso to Paris at the stations Dover 

 and Cape Gris Nez; and near this point of intersection a semi- 

 cyclone was established. The winds of this semicyclone are 

 marked with an asterisk (*). 



The anterior slope between the crest, extending from Valencia 

 to Christiansund and the trough along the English Channel, did 

 not pass onward towards the S.E., but was apparently broken up 

 over the British Isles by a still further reduction of pressure. It. 

 is the study of this phenomenon of decreasing (or its opposite 

 of increasing) pressure, irrespective of the transference of masses 

 of air either of augmented or diminished elasticity from one lo- 



K2 



