120 Southern's Experiments on the Elasticity of Steam. 



changes of elasticity corresponding to those of temperature, 

 like as common air may be, without limitation of temperature, 

 as far as is known. This, however, is a view of the subject 

 which has been totally excluded. 



Besides the experiments first related, in which the tempe- 

 rature of steam raised under high pressures was observed, 

 others had been made some years before (in 1797 and 1798) for 

 that purpose only ; and as they were made with the greatest 

 circumspection, both the manner of making them and their 

 results may be here described, as may also the results of other 

 experiments, made with equal care, to ascertain the tempera- 

 lure of steam raised under lo-w pressures. 



The instrument used in the former was a Papin's digester, 

 similar to what you had used in your original experiments, 

 and to that described in the Encyclojxcdia Britannica, art. 

 Steam, No. 22, the leading differences being in adapting a 

 metallic tube to it to contain the thermometer, or rather as 

 much of it as contained mercury, in the manner mentioned in 

 the beginning of this letter, and instead of a valve, by the load 

 on which to measure the elasticity of the contained steam, a 

 nicely-bored cylinder was applied, with a piston fitting it, so 

 as to have very little friction, and to the rod of this was ap- 

 plied a lever, constructed to work on edges like those of a 

 scale-beam, by which the resistance against the elastic force 

 of the steam could be accurately determined ; and at your 

 suggestion, to be assured that no inaccuracy had crept into 

 the calculation, by which this resistance through the medium 

 of the lever was ascertained, an actual column of mercury of 

 30 inches high was substituted, and the correspondence was 

 found to be within yi^j of an inch. 



The observations at each of the points of pressure noted 

 were continued some minutes, the temperature at each being 

 alternately raised and lowered, so as to make the pressure of 

 the steam on the under side of the piston alternately too much 

 and too little for the weight with which it was loaded ; and 

 thence a mean temperature was adopted, the extremes of which 

 were, as well as I now recollect, not more than half a degree 

 on each side of it. The load on the piston, including its own 

 weight, &c., was calculated to be successively just equal to 1, 

 2, 4, and 8 atmospheres of 29-8 inches of mercury each, and 

 the temperature of the steam was varied as above till that of 

 each point was determined : the results were thus — 



