arid on Astronomical Refractions. 



513 



Pressure. 



Temperature. 



Error of 

 calculated 



Pressure. 



Temperature. 



j 



Error of 

 calculated 













Observed. 



Calculated. 



temp. 





Observed. 



Calculated. 



temp. 



Inch. 









Inch. 









•52 



62 



59-5 



-2-0 



4-68 



132 



131-4 



--6 



•73 



72 



69^3 



-2-7 



6-06 



142 



141-3 



--7 



1^02 



82 



79^3 



-•27 



7-85 



152 



151-6 



--4 



1^42 



92 



89^9 



-2-1 



9-99 



162 



161-7 



-■3 



\U 



102 



100-2 



-1-8 



12-64 



J72 



171-8 



— -2 



2-65 



112 



110-7 



-1-3 



15-9] 



182 



182-0 







3^57 



122 



121-3 



- 7 



29-80 



212 



212- 





The formula deviates slightly from the observations at very low 

 pressures. Dalton says that it is next to impossible to free any li- 

 quid entirely from air; of course if any air enter, it unites its force 

 to that of the vapour. — Manchester Memoirs, vol. v. p. 570. it 

 must be recollected that according to theory the constants y and 

 S are the same only as long as the chemical constitution of the 

 vapour remains the same, and they vary for different substances. 



With regard to the nature of the accurate expression which con- 

 nects the pressure with the temperature, opinions have hitherto been 

 various. According to Dr. Robison, Mr, Watt found that water 

 would distil z'w vacuo when of the temperature of 70°, and that in 

 this case the latent heat of the steam appeared to be about 100°; 

 and some other experiments made him suppose that the sum of the 

 sensible and latent heats is a constant quantity. This, Dr. Robi- 

 son says, is a curious and not improbable circumstance. Southern, 

 on the contrary, concluded from experiments on the latent heat of 

 steam at high temperatures that the latent heat is a constant quantity, 

 instead of the latent heat + sensible heat being so. M. de Pambour, 

 in speaking of Southern's view, says, " Cette opinion a paru plus 

 rationelle a quelques auteurs, mais le premiere nous semble mise 

 hors de doute par les observations que nous ailons rapporter." It 

 appears to me by no means clear that Watt entertained the opinion 

 here attributed to him, for in a note in the Appentlix to Sir David 

 Brewster's edition of Robison's Mechanical Philosophy, vol. ii. p. 

 167, he professes to agree in the opinion there delivered by Southern. 

 In p. 166 Southern records three experiments, from which he ob- 

 tained 1 171^ 1212°, and 1245°, for the sums of the latent-}- sensible 

 heat corresponding to the temperatures or sensible heat 229°, 270°, 

 295°. If we take the two extreme observations, we find a difference 

 in the sum of the latent + sensible heat of 74 degrees, corresponding 

 to a difference in the sensible heat of 66 degrees. 



If the conditions under which Laplace obtained the equation 



J_ 



Phil. Ma<r. S. 3. Vol. 16. No. 105. Jmie 1840. 2 M 



