the Barometric Formula;, ^-c. 643 



relative to the recommendation of Poisson to increase the constant of Laplace 

 by 57 metres, in order to remove from the right-hand side of the formula the 

 unknown quantity s, being the required height of one station of observation 

 above tlie other. The mistake to which I allude is this: I expressed my belief 

 that the recommendation of Poisson was not good, because it occasioned errors 

 greater than tlie unavoidable errors of observations, — in extreme cases it does 

 so, but more often it does not. I now believe Poisson's advice to be good, for 

 I have ascertained that in omitting the unknown quantity for height) c, from 

 the riglit-hand side of the formula applied to the height of the Convent of Saint 

 Bernard above the Observatory of Geneva, an error of 6-17 metres in defect 

 is occasioned, and that by increasing the constant of the formula by 57 metres, 

 according to Poisson's recommendation, there is an increase of height to the 

 amount of 6-39 metres, — the difference of 6-39 and 6-17 (being only 0-22 

 metres) is the total error caused by Poisson's recommendation. This error of 

 0-22 metres (less than 9-0 English inches), in a height of 2070-34 metres, is 

 very much less than the errors of observation. 



Moreover, in reference to the height C, viz., 1015-5 English feet, near 

 Montreux, the error caused by Poisson's recommendation is only 0-36 Eng- 

 lish feet, being much less than the unavoidable error of observation. In gene- 

 ral, therefore, Poisson's recommendation is excellent, seeing that it very much 

 simplifies the calculations for height by the barometer. But the recommenda- 

 tion is not to be followed when the lower station is very much elevated above 

 the level of the sea, such as the height of the Convent of Saint Bernard compared 

 with the height of Mont Blanc. In such instances, when the height of tlie 

 lower station (as the convent) is upwards of 8000 English feet above the level 

 of the sea, the recommendation of Poisson is not applicable. But in the 

 British Isles, and even in most parts of Switzerland, the recommendation sliould 

 be followed. I purpose, in the formula which I am now about to propose, to 

 combine Poisson's useful recommendation with my own contrivance, thereby 

 producing a formula extremely simple, and which makes correct allowance for 

 the hygrometric state of the atmosphere on ordinary occasions. My method is 

 as follows : — 



I calculate a constant from the equation C — 7> — ,— metres, in which D' 



M X D ' 



