40 Mr. Nixon on the [Jan. 



a formidable list of calculations merely preparatory. Indepen- 

 dent of the objectionable labour, the reductions themselves will 

 ever be of questionable accuracy ; for should the variation of 

 the pressure, considered as constant, have been subject to irre- 

 gularities in the interval of the observations, the interpolations 

 will most probably be incorrect. This remark, it is true, is 

 inapplicable to the register of the thermometer, yet when the 

 altitude has been computed, in default of such register, with 

 twice the temperature at the upper station considered as the 

 sum of the thermometers, the given correction will be exact 

 only when the temperature declines 1° for an ascent of 250 feet. 

 Hence the probable inutility on the one hand of introducing 

 minute corrections in the calculations, and the necessity on the 

 other of forming tables, so constructed after approximative for- 

 mulae, as to render the computations simple, brief, and easy. 



The barometrical observations of Shuckburgh, Roy, and 

 Ramond, made at ordinary temperatures on mountains, having 

 on the average an elevation of 3000 feet, coincide in determining 

 the coefficient or multiplier of the logarithmic difference of the 

 pressures to be about 64096 at 60° F. If we suppose the baro- 

 meter at the base to be 30-000 inches, and the temperature of 

 the air 65°, we shall have the pressure at the summit equal to 

 26940 inches, the thermometer indicating about 55°. The 

 naean of nearly 300 afternoon observations, made by Mr. Daniell 

 within a thermometric range of from 60° to 70° (mean 65°) give 

 the corresponding dew point at 53°. The point of condensation 

 at the summit may be estimated, according to the experiments 

 of Mr. Dalton, at 48°. The mean density of the intercepted 

 column of air would consequently be equal to that of perfectly 

 dry air of the temperature of 62"7.* Hence the coefficient at the 



30*7 



level of the sea would be equal to 60095 + — — = 63939 ; or, 



corrected for the vertical diminution of gravity, 64119. The 

 result differs so little from 64096, that we are warranted in con- 

 cluding the coefficient of dry air at 32° F. to be, as inferred from 

 the experiments of Arago and Biot on the relative weights of 

 mercury and air, very nearly 60095. 



The mean temperature of the stratum of air being 32°, we may 

 estimate the corresponding dew point at 22°, the density being 

 in such case equal to dry air at 333, for which the coefficient is 

 60258 at the level of the sea, or 60418 at an altitude of 3000 

 feet. In the formula of Laplace, it is given at 60345 : Roy and 



• Lower station .SO-00 in. pressure, 65° air, .53° dew point, 2-8 equation. 

 Upper station 26-94 65 4S 8'6 



Mean 60 Mean 2*7 



Equation 2-7 



62-7 (See Table I. vol. x. p. 177.) 



