on the Formula for Measuring Heights by the Barometer. S59 
To determine the coefficients A and B in this expression^ in 
which, from the nature of the case, the negative sign of the ra- 
dical part is only admissible, two conditional equations are ne- 
cessary, in which the values of r,y*and ^ are known. I have 
thus found A =34.75, and B = 3.11, so that by the substitu- 
tion of these numerical values of A and B we obtain 
/= F + 5.586 (1 — ^/ 1 + .0103 ^) 
To illustrate this by am example, let it be required to deter- 
mine the actual elasticity of atmospheric vapour when the tem- 
perature of the air is 60°, and a thermometer, having its bulb 
covered with moistened paper, stands ^.t 51 1 
'Here by Mr Dalton’s table of the elasticity of vapour at dif- 
ferent temperatures, the maximum elasticity of vapour at 60®, is 
*524 inches, — F and ^ = 60° — 5l|°= 8|o. 
Hence/= *524 -f 5*586 (1 — s!i + *0103 x 8*5) = ' 
= •524 — *239 = *285. 
According to this result, the relative tension of the atmospheric 
vapour, at the time of observation, or the ratio of the elasticity 
of the vapour actually existing in the air, to the maximum elas- 
ticity for the temperature, is expressed by the fraction |||, or 
• 5439 . To compare this result with the tension of vapour de- 
duced from the experiments of Dulong* for the degrees of 
De Luc’s hygrometer, I employed one of these instruments 
which I had recently adjusted, and observed that it stood at 35°, 
reckoning from absolute dryness towards perfect humidity, 
which was marked, as usual, 100. Now according to the expe- 
riments of Dulong, whieh seem to have been made with great 
care, it appears that *4874 of the tension of vapour, corresponds 
with 31® *8 of De Luc’s hygrometer, while *591^ corresponds 
with 37 ® '5. From these data, the tension of vapour ‘answering 
to 35® is, by interpolation, *5456, which differs from the result 
by my formula *0017. Not having one of Saussure’s hygrome- 
ters, I can only compare that result with the relation which 
Biot has deduced between the tension of vapour and the de- 
grees of that instrument, by taking the degrees of Saussure’s 
hygrometer, corresponding with those of De Luc’s, from the 
See Biot’s Traite cle Physique, tom. ii. p. 208, 
