104 THE NATURE AND PROPERTIES OF STEAM. [SECT. n. 



that is, steam at 212 and thirty inches pressure, the equation becomes negative, 

 and steam rises only by the difference between this negative quantity and the 

 buoyancy ; leading us to the beautiful theory of evaporation. 



An equivalent rule is given in my book ' On Warming and Ventilating ;' but 

 though it is from the same principles, it is not there so directly nor so generally 

 derived. 1 



1 Treatise on Warming, &c. p. 148. 



