114 Southern's Experiments on the Density of' Steam, 



security against breaking, without any unnecessary weight. An 

 experimental inquiry into the properties of steam was neces- 

 sarily a part of such a course, and Mr. Southern stated the 

 results he had obtained in a letter to Mr. Watt, which was 

 printed by Dr. Brewster in liis new edition of the articles 

 written by Dr. Robison for the Encydopccdia Britannica. 

 The article Steam-engine in that edition was revised by Mr. 

 Watt, in 1814, and he added some notes and an appendix, 

 which contains Mr. Southern's letter. 



I am, dear Sir, 

 67 Guildford Street, Russell Square, Yours very sincerely, 



London, Jan. 16, 1847. JoHN FarEY. 



Letter from Mr, Southern to Mr. Watt*. 



The experiments, of which the particular circumstances are 

 hereafter related, were made in 1803, with the view of ascer- 

 taining, chiefly, the density of steam raised from water under 

 different pressures above that of the atmosphere, an apparatus 

 having then been made for a different purpose, which seemed 

 pretty well adapted to this object, as it did equally so to that 

 of ascertaining the latent heat of steam. 



It may be premised, that the thermometers employed in all 

 the experiments which will be now related, were made and 

 graduated with the greatest care, the tubes having been accu- 

 rately measured as to the proportional capacity of their dif- 

 ferent parts, the boiling-point of each ascertained, according 

 to the rules prescribed by a committee of the Royal Society, in 

 1777, (viz. the bulbs and tubes being in steam when the baro- 

 meter stood at 29*8 inches, this degree of temperature being 

 called 212°,) and in all cases the bulb and the tube, as high as 

 the mercury ascended in it, were kept in the steam or the 

 water whose temperature was to be noted. This latter cir- 

 cumstance was effected in the case of steam, by sliding the 

 tube of the thermometer through a stuffing-box, or collar, 

 made tight, till the mercury in it could just be seen above it. 

 The tube had known marks on it, from which measurements 

 were taken to the mercury, and thence the temperature 

 known. 



The quantity of steam was measured by filling a cylinder 

 with it (inclosed in the steam), whose diameter was about 3' 16 

 inches, and driving it out by the motion of a piston, which had 

 18 inches stroke regulated by the rotation of a crank. The 

 solid contents of the piston-rod, wliich was 0*86 inch dia- 

 meter, diminished the contents of the cylinder, leaving the 

 quantity discharged each stroke by the motion of the piston, 



* In all the experiments, of which an account is given in this letter, Mr. 

 Southern was assisted by Mr. William Creighton. 



I 



