Of BAROMETRICAL MEASUREMENTS. 127 



more elevated above it than the third, becaufe of the greater 

 cold fuppofed to prevail in the region where this laft barometer 

 is placed. Here, therefore, are two different determinations of 

 the height of the third ftation above the firft, neither of which 

 has any claim to be preferred to the other. It is evident, there- 

 fore, that, in barometrical meafurements, there is always a de- 

 gree of uncertainty introduced by the horizontal diflance be- 

 tween the two ftations, and that, befide thofe accidental errors, 

 which are of the lefs confequence, that, in a number of obfer- 

 vations, they may nearly compenfate for one another. 



IT mufl be confefled, too, that we have not at prefent the 

 means of removing this uncertainty, nor even of afcertaining 

 its limits with tolerable exactnefs. Thefe depend on a problem 

 which is no longer to be refolved by the principles of flaticks, 

 but requires the motions of an elaftic fluid, under various de- 

 grees of compreffion and rarefaction, to be determined. The 

 folution, therefore, is extremely difficult ; and no refult, fuffi- 

 ciently {imple to be of ufe in thefe computations, is ever likely 

 to be obtained from it. 



IT would, however, be of confequence to determine, by obfer- 

 vation, the mean height of the barometer at the level of the fea 

 in the different regions of the earth. That mean height is not 

 every where the fame. Under the line, it appears, from the ob- 

 fervarions of M. BOUGUER, to be 29.852 inches, reducing the 

 mercury to the temperature of 55 ; and in Britain, it is 30.04, 

 reducing the mercury to the fame temperature. The mean 

 temperature of the air, as well as its mean weight in different 

 climates, will alfo require to be determined before the art of 

 levelling extenfive tracts by the barometer can be brought to- 

 perfection. 



42. THERE is another caufe of error which, had the effects 

 of it been fufficiently known, ought, no doubt, to have entered 

 into this invefhigation. Moifture, when chemically united to 

 air, or difTolved in it, fo as to compofe a part of the fame ho- 

 mogeneous 



