1825.] Barometrical Measurement of Heights. 91 



be reduced -g— . Lastly, as the weights of equal (volumes or) 



heights of air, if so small as to be sensibly of uniform density, 

 are directly as the pressures they sustain, the value of the cor- 

 rection in feet, the mean temperature of the air being 32° F. and 

 containing its usual quantity of moisture, will be constant, and 

 equal to 13*25 feet. 



Having proved the correction to be directly in the ratio of the 

 altitude without regard to the elevations of the barometers 

 above the level of the sea, and equal at a temperature of the air 



13'25 



of32°F.tol3-25ft.permile, or to gggo^, we may strictly dis- 

 pense with any reduction of the observed heights of the baro- 

 meters, or subsequent augmentation of the approximate altitude, 

 if we make our calculations in the first instance on the suppo- 

 sition that 1000 + 528000 ^^^^' ^^ ^^^^ correctly 1000 H-gggi 

 = 1002*51 vertical feet of dry air at 32° F. and under the pres- 

 sure of 26*0988 inches, will counterpoise one inch of mercury in 

 the latitude of 45°, or more conveniently if we admit that the 

 inch of mercury will balance 1000 feet of the dry air under a 

 pressure of 26*1643 inches. ' r/c ,-: • j -.i:; 



When the mean temperature differs from 32° F. the value in 

 feet of T^zu of the height of the barometer will vary, and we 



must augment (or diminish) the 1 3*25 feet at the rate of — per 



degree of the difference. To meet this slight correction, we 

 have but to consider the dilatation in volume of dry air as 



;j^^ in lieu of— per degree. 



To prove that the view we have taken of the subject is quite 

 correct, a table is subjoined of the value of the correction at dif- 

 ferent elevations, and temperatures of the air, strictly calculated 

 on the supposition of the lower barometer being placed at; the 

 level of the sea. , ..^ 



Elevations of the Upper Barometer above the Lower one placed at 



'- if; ,^., the Level of the Sea. 



J x\x|3i9in ao .irnjd o Temperature of thcAm^j suj ." aorrax: ; in itHl^tY.m 



1 mile 12-3 . . . . 13-2 . . . ; IS^P ^Z. .14*4 ..i. 15*0 feei. 



.^'Uhi^m:w.^/m:^... 41*6 .... 43*;3 ...;4s^.^!f [ 



Gravity dnmhiShmg as we ascend above the surface of. the 

 eartli, JL]t Ipllows that a pendulum clock taken from the level of 

 the sea I9 iiie summit of a mountain would haye it's rate of going 

 retarded. Some experiments of this nature undertakeu in 



