40 Mr. W. R. Birt 07i the Great Sj/mmetrical 



luinimiim, being to a great extent similar to those marking 

 the barometric rise to the central apex. From this it follows 

 that great difficulty must exist in the observation of the wave. 

 If, for instance, any particular curve is expected to return, 

 the observer in most cases will be disappointed; he will also 

 meet with disappointment if he expects that on certain days 

 the barometer will attain certain altitudes. The surest way 

 is to prepare a paper of engraved squares, and for a few days 

 before the expected return of the wave, to project the altitudes 

 of the barometer in a curve. While this is going on, uncer- 

 tainty will often attach to the proceeding until the wave is just 

 approaching its termination, when the symmetry will become 

 apparent, provided the observer is situated near the locality 

 of greatest symmetry. Should he be removed even a short 

 distance in some directions from the area or line of greatest 

 symmetry, the curve will not heftdly referable to the foregoing 

 type : one portion may be thrown higher thaif^another, with 

 whicii at the point of greatest symmetry it is of equal altitude. 

 It is very desirable to mark out distinctly the localities of these 

 departures from symmetry, and to determine the directions in 

 which the greatest excursions take place ; especially as another 

 important feature of the wave appears to be intimately con- 

 nected with them, viz. the barometric range from the lowest 

 minimum to the highest maximum, which increases in some 

 directions very considerably. In order therefore to observe 

 the wave satisfactorily, and to be certain of its return, three 

 features require to be fully apprehended : — the symmetrical 

 character of the curve, as expressed in the foregoing type, with 

 ihe modification alluded to ; the departure from this symmetry 

 in certain localities, which may be noticed in barometric move- 

 ments, evidently referable, by the nature of the curve and the 

 period elapsing between the initial and terminal minima, to 

 the great wave ; and the barometric range during this period, 

 which will be found to vary in certain directions. 



The reader will readily see from this enunciation of phse- 

 nomena appertaining to the great symmetrical barometric 

 wave, that the j)hases of the wave, as exhibited at any given 

 locality, bear a considerable analogy to certain phaenomena 

 })resented during a solar eclipse. The line of terrestrial sur- 

 face on which the barometric wave presents the most perfect 

 symmetry, corresponds to the line of country on which the 

 eclipse is central ; and the spot at which the wave is observed 

 to be most strongly developed, may be regarded as correspond- 

 ing to the spot at which the sun is eclipsed centrally on the 

 meridian. This line of greatest symmetry, referring to the 

 barometric wave, extends a considerable distance over central 

 iMuope; on each side the syunnetry ?4- departed from, som& 



