Southern's Formula fen- the Elasticity of Steam. 123 



The table will stand as follows, supposing the thermometer 

 had been graduated for 212^ to correspond with 30 inches of 

 the barometer : 



- I remain, with the greatest esteem and respect, 



Dear Sir, your vei'y obedient Servant, 

 Oakhill, 26th March, 1814. JoHN SOUTHERN. 



To James Watt, Esq., Heaihfteld. 



P.S. Some circumstances which occurred in the perform- 

 ance of the experiments (made in 1797 and 1798) of which 

 the results are last related, suggested the trials of a mixture 

 of air with the steam ; and I made a few, not indeed widi the 

 greatest nicety, but as they furnished a strong probability that 

 the following law of elasticity of a given mixture was either 

 nearly or accurately correct, it may be of use to say, that the 

 apparatus used in the steam experiments being prepared as if 

 for a repetition of them, and as perfectly exhausted of air as 

 for them, a known measure of common air was sent up the 

 tube through the mercury and water, and look its place in the 

 ball; the water surrounding which was heated, and its tempe- 

 rature observed at different periods as before ; and indeed the 

 process was precisely the same as the former, with the addi- 

 tional notice of the space in the ball occupied by the expanded 

 air and steam jointly. This process was repeated three or 

 four times with different quantities of air, but the notes not 

 being preserved, I can only now mention the conchision they 

 induced me to form as to the law above-mentioned, viz. that 

 whatever the elastic force of the air admiited would be in its 

 expanded state, supposing it dry and to occupy the whole 



• These are inserted from numerous experiments made by Mr. W. 

 Creighton. Mr. W. Creighton pubhshed a theorem for the elasticity of 

 steam at different temperatures in the Philosophical Magazine, 1819, 

 vol. liii. p. 266.— Ed. 



K2 



