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



Sir John Herschel's report were projected. The result of the 

 examination has been the apprehension of several large atmo- 

 spheric waves of so distinct a character, that the barometric 

 movements over an area included by the angular points, the 

 Orkneys, St. Petersburgh, Geneva and Cork, have been fully 

 explained, especially during the earlier portion of the month. 

 In order clearly to place these barometric waves before the 

 readers of the Philosophical Magazine, I shall at once proceed 

 to make various extracts from the report bearing immediately 

 upon the subject, commencing with the 



Definition and Phcpnomena of an Atmospheric Wave. 



When a nutnberof barometric observations arc projected on paper 

 according to a suitable scale, and continued for montlis and years, 

 the eye on contemplating them will recognise a variety of curved 

 forms, some of large and some of small amplitude; some rising to a 

 considerable altitude, others sinking far below the level, representing 

 the mean barometric pressure at the station of observation. At first 

 tiiere appears but little regularity in these curvilinear records of the 

 ever-shifting state of our atmosphere, but here and there the atten- 

 tive observer will notice some similarity existing between two or 

 more individual curves, and he may notice some which possess a 

 certain symmetrical arrangement of the ascents and descents. In 

 consequence of this similarity and symmetrical arrangement, he 

 examines more carefully the records of barometric pressure, and not 

 onlj^ discusses the observations at one station, but compares those 

 observations with others made at various stations ; and here again 

 he finds apparent irregularity and confusion. The curves to a cer- 

 tain extent agree, but in many minor points they differ often very 

 considerably, in some cases rising at one station while falling at an- 

 other; this induces a still more minute and careful investigation: 

 the distribution of pressure over the largest area he can command is 

 carefully examined ; and whether his stations are few or many at 

 any given time, he finds on this area a point of maximum pressure 

 and a point of minimum pressure ; between tliese points he finds 

 various pressures, generally increasing from the point of least pres- 

 sure to the point of greatest pressure. On some occasions he finds 

 a line of high pressure, stretching quite across the area, and on others 

 a line of loiu pressure. By continuing his inquiries for successive 

 epochs, he finds these lines of high and low pressure move across 

 the area, or in other words, the high pressure or low pressure is 

 gradually transferred from one point to another. He also finds at 

 still more remote epochs other lines of high and low pressure, some 

 having the same direction with the lines originally noticed, and others 

 crossing the direction of the original lines at various angles. 



The questions which now suggest themselves are the following : — 

 What are these movements? How can they be represented? In 

 what manner can they be explained ? A simple consideration of 

 the curves suggests the idea of waves as explanatory of the phamo- 



