IS23J] the Barometer to the Meamrement of Heights, 261 



ing to this sort of observation ; to replace in short whatever 

 observations we possess of a doubtful character, by such as are 

 certain, and the circumstances of which have been judiciously- 

 appreciated. It will be time enough to dispute, if there be 

 occasion for dispute, when the propositions in question shall be 

 clear, and the facts free from ambiguity. 



Thus far I have spoken of the influence which the configura- 

 tion of the surface of the earth exercises on the variations of the 

 instruments. The irregular modifications of the atmosphere are 

 another source of errors against which we ought to be on our 

 guard. The theory of barometrical measurements supposes the 

 air in a state of perfect equilibrium ; its strata superposed in the 

 order of their density ; and the decrease of temperature uniform 

 and regular. It is ordinarily so on fine days and in calm wea- 

 ther; but if the air be agitated and divided between opposing 

 winds, this order is disturbed ; strata of different densities are 

 intermixed,^ and succeed each other in a different order from 

 that of their respective densities ; the thermometric mean no 

 longer expresses the mean temperature of the intercepted column 

 of air; the difference of the heights of the barometer ceases to 

 maintain its relation to the difference of elevation ; and no for- 

 mula can satisfy the exactness of mensuration, in a state of 

 things thus opposed to the fundamental supposition. 



When this deviation from regularity is manifest, no one needs 

 to be told that this is not the time to obtain exact measure- 

 ments ; and every one will distrust observations made during a 

 storm, in the midst of tempests, and while violent winds are 

 raging in the atmosphere. But this state of disorder may in 

 some cases be perfectly real without being so apparent ; and 

 the intermixture t of winds of different densities is a very usual 

 phenomenon, which, however, frequently escapes the attention, 

 and is the origin of a great number of errors from which the 

 most experienced observer does not easily preserve himself. If 

 we have not been able to avoid them, we must endeavour to 

 beware of their existence, in order that we may not repose in an 

 operation a degree of confidence which it does not deserve. 



I have treated elsewhere of the influence of the wind on 

 barometrical measurements ; and I invite beginners to profit by 

 my experience, and more advanced observers to correct or 

 extend my first outlines. Whatever judgment they may pass, 

 they will probably agree with me in thinking that there are few 

 subjects of research more interesting, and that we cannot have 

 any just idea of the value of an observation if we neglect, in 

 examining it, considerations of such importance. 



I have hitherto always found that northern winds tend to raise 

 the mercury, and southern to depress it. 



♦ " S'intcrcalent." f " L'intercalation." 



