, 1 825.] Bammetrical Measurement of Heights. ,^1 



add the corresponding equation (5°) to the temperature of the 

 air (90^^); and their sum (95°) will denote the temperature 

 required. Jn the extreme case before us the discrepancy falls 

 short of a quarter of a degree,— a quantity ,WMc|),,jni?^i,9fl,jt<|. the 

 probable error of observation. ,; , , ,y ^ f , , . [| jj , , J u c^^^<^ ^ j ,^ , , 



In case tlie construction of the hygrometer shoma be such as 

 to indicate merely the degree of saturation, find by the table 

 the equation for saturated air at the observed temperature, and 

 reduce the quantity in proportion. The correction at 60° for air 

 two-thirds saturated with moisture, and supporting a pressure of 

 30 inches, would be equal to f of 3*6°, or to 2-4°. (Seethe 

 tables given at the end of the first volume of the Traitt de Phy 

 sique by M. Biot, to reduce the degrees of saturation of the hair 

 hygrometer of Saussure to the degree of tension of the vapour 

 existing in the atmosphere.) 



In the barometrical table of Mr. Daniell before alluded to are 

 given the densities of saturated air at different temperatures 

 under the pressure of thijiy inches ; but as no allusion is made 

 to any correction for difference of pressure, and as the calculation 

 illustrating the table is worked without introducing one, we 

 must necessarily conclude that Mr. Daniell conceives the den- 

 sity of saturated air of any given temperature, supporting the 

 pressure of 30 inches, to be specifically hghter than dry air of 

 that pressure in the same ratio that saturated air of the same 

 temperature under any other pressure is specifically lighter thaa 

 dry air supporting that other pressure. To prove the incorrect- 

 ness of the idea, let us find the density of a stratum of saturated 

 air supporting a pressure of 30 inches, and that of another stra- 

 tum under the pressure of 15 inches, the temperature of both 

 being 90° F. 



n il Density of dry air at 30 inches ,...,,. ^Hl*Oooc)i>.>vi 

 ,. Ditto at 16 inches*^^e^*jg.,b^nijrtfi^«^»i?r«^?,:jMu^qd ' I 



' > Density of saturated air at 30 in . . 0-876245 

 ivvHv Ditto at 15 inches * ♦ . 0-430149 



Had the ratio been constant, the density of the saturated air 

 at 15 inches would have been 0*4381225. It is evident that as 

 the stratum under the lesser pressure contains a greater propor- 

 tion of the lighter fluid, it must be specifically lighter than dry 

 air in a greater ratio than the stratum supporting^ the heavier 



pressure. :>{r:.Sr,vo<> ^i -J^ipiun ^prLbMfV 



When the force of the vapour rising from the surface! oi a liquid 



freely exposed to the atmosphere equals the pressure of the 

 ; latter, ebullition ensues. Consequently if we note the tempera- 

 j,ture of the liquid, or that of the vapour immediately abave its 



surface when the ebullition is perfect, we may find by the 

 . tables giving the force of aqueous vapour iitdiil'er^n^^teuf^eta 



