56 
MR. J. J. WATERSTON ON THE PHYSICS OF MEDIA COMPOSED OF 
then 
p _ JVt + 461 - 19-736] ( 
S 10422 
(t + 461), 
H = (log B — log b) . - 
1 
1250801 
0-376 
2P 
I) + h 
/ 461 + t \ 
\9958 + t) 
is the correct theoretical formula, in which 
H = height in feet between the two stations. 
B = height of barometer at lower station. 
b = height of barometer at higher station brought to the same temperature as B. 
Addition to Notes B. and H. of the Paper, “ On the Physics of Certain Media ” 
recently submitted to the Royal Society. 
Received January 27, 1846. 
In Note B it was shown that the formula of a vapour might be obtained from two 
experiments on its tension, and in Note H, that the function which defines the law 
of density in vapours is analogous to what defines the law of tension in ascending the 
atmosphere, thereby enabling us to construct a rule for measuring heights by the 
thermometer. It may, perhaps, be useful to add what relates to the law of the tension 
of mixtures of air and vapour. 
In some cases it seems impossible to clear vapours entirely of permanently elastic 
matter, and it will be allowed to be very desirable, in a practical point of view, that 
we should be able to deduce the necessary constants from experiments made upon 
them in their usual state of commixture. It will be found, I believe, that this may be 
accomplished by means of the data afforded by not less than three experiments if the 
volume occupied by the gas and vapour remains constant, or if the proportionate changes 
in it are capable of being accurately determined. We do not require to know anything 
of the quantity of air enclosed with the vapour : this forms one of the three unknown 
quantities involved in the three equations afforded by the experiments ; the other two 
being the constants G and H that develop the law of density of the pure vapour. 
In the accompanying chart (Plate 2), which is drawn on the same scale as the general 
chart of vapours given in Note B, it may be remarked how the straight lines of vapour 
are transformed into a high order of hyperbolas when any permanently elastic matter is 
allowed to contribute its effect of tension. The mode of laying off the points is simply 
as follows. Suppose we wish to know the effect that air of y^tlis of an inch of tension 
at 51° has upon the chart line of aqueous vapour ; we have F G £ = F 6 (51 -f- 461) = 0'06. 
From this we obtain the value of F G , which we employ in the general equation for such 
= e, or /y/.F G + 
mixtures, vi 2 ;.,F°i + t 
\/t — G 
H 
yt - g \8 
\ H 
the ordinate 
