644 Lieutenant Kenny on the Constants of 



represents the ratio of the specific gravity, not of dry air, but of an union of 

 dry air with vapour of water, having the elastic force belonging to the freezing- 

 point, to that of quicksilver, in a manner analogous to that of obtaining 

 the constant, 18404"9 metres of my principal formula, from the equation 



C=^v — 7- metres. The new constant C" = 18451'5 metres. The manner of cal- 

 if xZ* 



culating it is given in the Appendix, pp. 559 and 660. Now, following Poisson's 

 recommendation of adding 57 metres, in order to remove z from the right- 

 hand side of the formula, with a view to simplicity, we have, for the new value 

 of C", 18508'5 metres. But this last constant, viz., 18508-5 metres, supposes 

 the air to be in a state of saturation with vapour of water, which is seldom if ever 

 the case. We have, therefore, to make a suitable deduction in consequence of 

 the fraction of saturation, generally (if not always) accompanying the barome- 

 tric observations. In order to do so, let us refer to Table I. of page 630, 

 where we find that the increase in the calculated height of the Convent of Saint 

 Bernard above the Observatory of Geneva, due to vapour of water in the atmo- 

 sphere, amounts to 6-00 metres, very nearly. But it appears, in referring to 

 M. Plantamour's quarto, entitled " Eesume," &c., &c., at page 51, that the 

 mean fraction of saturation for heights, calculated in Table I. of page 630 of 

 this paper, is 0'8 ; if, therefore, the decimal fraction 0'8 cause an increase of 

 height — 6'00 metres, we should have an increase of 7*5 metres, Lf the atmos- 

 phere were in a state of saturation represented by unity, the difference between 

 7'5 and 6'0 being 1-5 metres, shows the diminution to be made on total height, 

 in consequence of the fraction of saturation ; but 1-5 being compared with the 

 total height, 2066'35, gives a fraction y^Vu ^^^ suitable reduction of the constant 

 last obtained, in order to obtain correct allowance for fraction of saturation. 

 The same kind of reasoning being employed in reference to my observations 

 near Montreux, gives for diminution of constant the fraction g-^ nearly, being 

 greater than the first fraction jJfjo, because of the greater temperature of obser- 

 vations near to Montreux. I propose, as a medium between these two values, 

 the fraction yoots^ which is simple, and easily applied; therefore, taking the one- 

 thousandth part of 1850C-5 metres, viz., lC-5 metres ; and subtracting tlie same, 

 we have for the final value of constant C = 18490-0 metres = 60664-0 Enghsh 

 feet, very nearly. 



