1823.] the Baroketer to the Measurement of Heights. IH 



An attention to the foregoing considerations in conducting a 

 series of meteorological observations will be accompanied by 

 the following advantages. 



1. The mean height of the barometer at noon, at the same 

 time that it has the property of expressing the mean pressure of 

 the atmosphere, disengaged from the diurnal varieltion, possesses 

 also exclusively among all other means, the qualities required 

 for the determination of differences of elevation. The coefficient 

 of a barometric formula can never be exact but in reference to St 

 fixed hour. Now the coefficient of M. de Laplace's formula iS 

 appropriated precisely to the hour of noon : it is a truly fortunate 

 coincidence that we thus are enabled to determine the elevation 

 of places, by the use of the same barometric means which have 

 served to determine the respective pressures. 



2. The morning, afternoon, and evening observations made at 

 the critical hours of the daily variation, after having been of 

 daily utility for foreseeing changes of weather, have besides 

 the advantage of fixing, for each climate, the extetit and circum- 

 stances of the variation : and each series separately reduced to 

 its mean expression, being employed in the calculation of differ- 

 etices of level, instead of the mean of noon, will give the measure 

 of the error arising from the hour ; and consequently the correct- 

 tion which the coefficient requires in order to become applicable 

 to that hour. 



I will conclude with one considerationof which we must nevei: 

 lose sight. Barometric means cannot be employed to deter- 

 mine the elevation of distant places above oue another, except 

 so far as their respective climates continue the same. The cli- 

 mate has a powerful influence on the variable ratio which sub- 

 sists between the weight and the pressure of the column of air. 

 These two quantities, perhaps, attain equality at the mean 

 parallel where all meteorological influences are in a state of 

 equilibrium ; and in this case it will be true for us, that the mean 

 height of the mercury expresses exactly the mean weight of the 

 atmosphere ; but at the same time that we find the temperate 

 regions enjoying this advantage, it also follows that no others 

 partake of it. The pressure diminishes in proportion as we 

 approach the equator ; and on the shores of the south sea, the 

 barometer stands lower than on our western coasts. This same 

 pressure increases as we approach the pole, and the barometer, 

 tautens paribus, would stand higher on the shores of the arctic 

 sfia than in our latitudes : even between the northern and south- 

 ern parts of France, the differences may become sensible ; and 

 though Geneva is not far from the Mediterranean, the difference 

 of climate is such that the absolute elevation of its lake would 

 be but iU established if it rested only on the comparison with 

 observations made at Marseilles. 



