Influnece of Atmospheric Currents upon the Barometer. 475 



second case, except that the motion will be retrograde : and 

 the interval of time between the occurrence of the positions 

 will be diminished by the time of describing the small quan- 

 tity just mentioned, instead of increased thereby. 



It is clear, from the nature of the phaenomenon, that unless 

 the excess or deficiency be very small, the continuous image 

 will not be distinct, but will have a flickering and irregular 

 appearance. 

 Exeter, March 4, 1842. 



LXVII. On the Iiifluence of Atmospheric Currents upon the 

 Height of the Barometer. By William Brown, Jun.* 



IN this essay I have endeavoured to explain the cause of 

 the oscillations in the height of the barometer. It is a 

 question which has claimed the attention of philosophers from 

 the time of the discovery of Torricelli, and has not hitherto 

 received a solution ; it is therefore with much diffidence that 

 I attempt to give an explanation of this phaenomenon. I think, 

 however, that in the opposition, given to that current of the at- 

 mosphere which is caused by the greater specific gravity of 

 cold air than of warmer, and by which the equality of its 

 pressure can only be maintained, by descending particles of 

 an upper current flowing in the contrary direction, there ex- 

 ists a cause fully adequate to the production of the principal 

 variations in the atmospheric pressure, as indicated by the 

 barometer. 



Notwithstanding the great differences of temperature which 

 exist between the poles and the equator, and the consequent 

 differences in the density of the air, the mean height of the 

 barometer is nearly the same at everyplace on the surface of the 

 globe. This equality of pressure, which is the necessary con- 

 sequence of the properties of a fluid, can therefore only be 

 maintained by the greater height of the columns of least den- 

 sity ; their height being in inverse proportion to their specific 

 gravity. 



The condition in which an atmosphere surrounding a 

 sphere decreasing in temperature from the equator to the 

 poles is placed, is thus given by Professor Daniellf: — "The 

 elasticity of the air as measured by the barometer remaining 

 the same, its specific gravity is very much greater at the poles 

 than at the equator; and hence it is clear that the atmo- 

 spherical column must be proportionally shorter at the former 

 than at the latter point. 



" The further conclusion follows, that this heavier fluid 



• Communicated by the Author. •[■ Meteorological Essays, p. 19, 



