56 REPORT — 1844, 



Prague and Toronto furnish the materials for an interesting comparison 

 of their respective mean gaseous pressures. I have exhibited this comparison 

 in the subjoined table. After the proper corrections have been applied for 

 the reduction to an invariable scale of pressure, and of the pressure itself to 

 a common elevation above the sea, the residual difference in the pressure is 

 about four hundredths of an inch. This is within the amount of difference 

 that might reasonably be expected in so indirect a comparison. The infer- 

 ence therefore at present must be, that no unaccounted for difference of 

 pressure exists, or at least next to none, at these two stations in Europe 

 and America. 



Inches. 

 Pressure of the dry atmosphere at Toronto .... SQ'S^g 

 Pressure of the dry atmosphere at Prague .... 29'019 



Difference 0-330 



Reduction to an invariable scale of pressure. . . . 0*017 



True difference of pressure 0"313 



Difference of elevation equivalent to 0'273 



DiflFerence of pressure unaccounted for O'Oi 



Modern researches have shown that the height of the barometer at dif- 

 ferent points of the earth's surface is not only disturbed by self-adjusting 

 causes which produce temporary displacements, but that there are causes in 

 action which effect persistent differences in the mean height of the barometer 

 in different localities, strictly at the level of the sea ; so that, to use the 

 words of Bessel, the mean atmospheric pressure depends on the geographical 

 co-ordinates of a station in latitude and longitude as well as in elevation. 

 This remark of Bessel's is founded chiefly on Erman's observations * ; and 

 Erman himself, who has considered the effect of the vapour pressure upon his 

 barometrical heights, concludes that the pressures which the air would have 

 exerted without the presence of aqueous vapour, indicate also persistent dif- 

 ferences of mean gaseous pressure depending on geographical position. The 

 instance quoted by Professor Forbes from Captain Kingf , who found the 

 mean height of t!ie barometer 29'462 in observations repeated five times a 

 day in five consecutive months of summer at Port Famine, is an example of 

 an atmospheric valley, as it lias been called, in the former sense, but not in 

 the latter. When allowance is made for the probable vapour pressure, the 

 gaseous pressure at Port Famine will be found greater than its ordinary 

 amount at the Equator ; where indeed other observations liave indicated a 

 gaseous pressure lower than in the adjacent extra-tropical latitudes. 



Assumed equatorial barometer 29*95 



Deduct vapour pressure (assumed dew point 74°) 0*83 



Mean pressure of the gaseous atmosphere at the Equator 29*12 



Barometer at Port Famine in five summer months 29*462 



Deduct vapour pressure (assumed dew point 38°) '230 



Pressure of the gaseous atmosphere at Port Famine .... 29*23 : 



* Erman, Met. Beob. bei einer Seereise iim die Erde. 



t Forbes, Reports of the Brit. Assoc., 1832. 



X This is of course only an approximate comparison ; to render it more exact, it would be 



